objective c assignment to readonly property

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Readonly properties in objective c

Introduction.

Objective-C is a programming language that is widely used for developing applications on Apple’s platforms, including iOS and macOS. One of the key features of Objective-C is its support for properties, which allow developers to define and access instance variables in a controlled manner. In this article, we will explore the concept of readonly properties in Objective-C and discuss how they can be used effectively in your code.

Understanding Readonly Properties

In Objective-C, a property is a way to encapsulate the state of an object and provide controlled access to it. By default, properties are readwrite, which means that they can be both read and modified. However, there are situations where you may want to restrict the modification of a property to ensure data integrity or prevent unintended changes. This is where readonly properties come into play.

A readonly property, as the name suggests, allows you to define a property that can only be read and not modified. Once the value of a readonly property is set, it cannot be changed. Readonly properties are particularly useful when you want to expose certain information to other parts of your code without allowing them to modify it.

Declaring Readonly Properties

To declare a readonly property in Objective-C, you need to use the readonly keyword in the property declaration. Here’s an example:

In this example, we have declared a readonly property called name of type NSString . This property can only be read and not modified once it is set.

Initializing Readonly Properties

Readonly properties can be initialized in two ways:

1. Initialization in the Class Implementation: Readonly properties can be initialized in the class implementation using the designated initializer or custom initialization methods. Once the value is set, it cannot be changed. Here’s an example:

In this example, we have defined a custom initializer that takes a name parameter and sets the value of the name property. Once the name property is set, it cannot be modified.

2. Initialization in the Property Declaration: Readonly properties can also be initialized directly in the property declaration using the = operator. Here’s an example:

In this example, we have initialized the name property with the value "John" . Once the property is initialized, it cannot be modified.

Using Readonly Properties

Once you have declared and initialized a readonly property, you can access its value just like any other property. However, you cannot modify its value. Here’s an example:

In this example, we are accessing the value of the name property using the dot notation and assigning it to the personName variable. We can then use the personName variable to perform any necessary operations.

Readonly properties in Objective-C provide a way to define properties that can only be read and not modified. They are useful when you want to expose certain information without allowing it to be changed. By declaring and initializing readonly properties, you can ensure data integrity and prevent unintended modifications in your code.

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Objective-C Properties: Getters, Setters, and Beyond

Objective-C is a powerful and versatile programming language, especially when it comes to iOS and macOS app development. One of the fundamental concepts in Objective-C is properties. Properties allow you to define the interface for accessing and manipulating object attributes, encapsulating data and providing controlled access to it. In this comprehensive guide, we will delve into Objective-C properties, covering everything from basic getters and setters to advanced techniques that will help you become a more proficient iOS developer.

1. Understanding Properties in Objective-C

Properties are an essential part of object-oriented programming, providing a way to manage the state of objects. In Objective-C, properties are used to define the attributes of a class that can be accessed and modified from outside the class. These attributes can be anything from simple data types like integers and strings to more complex objects.

1.1 Declaring Properties

To declare a property in Objective-C, you use the @property keyword followed by the property’s data type and name. Here’s a basic example:

In this example, we declare a property named name of type NSString. The (nonatomic, strong) part is an attribute that specifies the memory management behavior for this property. The nonatomic attribute means that the property is not thread-safe, while strong indicates that the property retains the assigned value.

1.2 Synthesizing Properties

After declaring a property, you need to synthesize it in your implementation file (.m) using the @synthesize keyword:

This step generates the getter and setter methods for your property, allowing you to access and modify its value.

2. Getters and Setters

Getters and setters are the two essential methods associated with properties. They are used to read and write property values, respectively.

2.1 Getters

A getter method retrieves the value of a property. In Objective-C, getter methods are automatically generated when you declare a property. For our name property, the getter method would be named name. Here’s how you would use it:

Alternatively, you can use dot notation to access the property:

2.2 Setters

A setter method assigns a new value to a property. Like getters, setter methods are automatically generated for properties. For our name property, the setter method would be named setName:. Here’s how you would use it:

Or using dot notation:

2.3 Custom Getters and Setters

While Objective-C generates default getters and setters for properties, you can also customize them to add additional logic. For example, you might want to validate input values or perform some actions when setting or getting a property. Here’s how you can define custom getters and setters:

Custom getters and setters allow you to encapsulate complex behavior within your class while still providing a simple and consistent interface to the outside world.

3. Property Attributes

In addition to the basic nonatomic and strong attributes mentioned earlier, Objective-C provides several other property attributes that allow you to fine-tune the behavior of your properties.

3.1 Readonly and Readwrite

By default, properties are readwrite, meaning they can be both read and written. However, you can make a property readonly by using the readonly attribute. Readonly properties can only be accessed through their getter method, and attempts to set them will result in a compilation error.

3.2 Atomic and Nonatomic

The atomic attribute makes property access thread-safe, ensuring that multiple threads can’t interfere with each other when accessing or modifying the property’s value. However, it comes at a performance cost. The nonatomic attribute, as mentioned earlier, makes property access non-thread-safe but is faster.

The copy attribute is used when you want to ensure that a property holds an immutable copy of the assigned value. This is commonly used for properties that are NSString or other mutable objects to prevent them from being modified inadvertently.

3.4 Weak and Strong

The weak attribute is used for properties that should not retain the assigned object. It is commonly used to avoid strong reference cycles (retain cycles) in delegate relationships and other situations where two objects reference each other. The strong attribute, as mentioned earlier, retains the assigned object.

The assign attribute is used for properties that hold primitive data types like NSInteger or CGFloat. It does not retain or copy the assigned value and is commonly used for properties that don’t own their values.

The retain attribute is used as an alternative to strong and is mainly used in Objective-C code written before the introduction of Automatic Reference Counting (ARC). It indicates that the property should retain the assigned object.

3.6 Getter and Setter Methods

You can also customize the names of the generated getter and setter methods using the getter and setter attributes:

In this example, the getter method will be named getFullName, and the setter method will be named setFullName:.

4. Advanced Property Techniques

Objective-C properties offer more than just simple getters and setters. Here are some advanced techniques that can help you write cleaner and more efficient code:

4.1 Lazy Initialization

Lazy initialization is a technique where you delay the creation of an object until it’s actually needed. This can be useful for optimizing memory usage in your app. You can implement lazy initialization in your getter method like this:

With lazy initialization, the _name property is only created when it’s accessed for the first time.

4.2 Computed Properties

Computed properties are properties that don’t have a backing instance variable but instead calculate their value based on other properties or data. You can use custom getter methods to implement computed properties. Here’s an example:

In this example, the fullName property is computed based on the firstName and lastName properties.

4.3 Key-Value Coding (KVC)

Objective-C provides Key-Value Coding (KVC), which allows you to access and modify properties using string-based keys. This can be particularly handy when working with dictionaries and dynamic data structures. To enable KVC for a property, you need to use the @synthesize keyword with the property name.

With this setup, you can access and modify the name property using strings:

4.4 Property Observers

Property observers allow you to monitor changes to a property and take action when the property’s value is set or modified. You can use the willChangeValueForKey: and didChangeValueForKey: methods to implement property observers:

Property observers are particularly useful for implementing features like automatic UI updates when a property changes.

Objective-C properties are a fundamental concept in iOS and macOS app development. They provide a clean and efficient way to manage object attributes, encapsulate data, and control access to it. Understanding how to declare, synthesize, and customize properties, as well as how to use advanced techniques like lazy initialization and computed properties, is essential for becoming a proficient Objective-C developer.

In this guide, we’ve covered the basics of Objective-C properties, including getters and setters, and explored various property attributes that allow you to fine-tune their behavior. We’ve also introduced advanced property techniques like lazy initialization, computed properties, Key-Value Coding, and property observers. Armed with this knowledge, you’re well-equipped to make the most of properties in your Objective-C projects, creating more robust and maintainable code. Happy coding!

objective c assignment to readonly property

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An object’s properties let other objects inspect or change its state. But, in a well-designed object-oriented program, it’s not possible to directly access the internal state of an object. Instead, accessor methods (getters and setters) are used as an abstraction for interacting with the object’s underlying data.

The goal of the @property directive is to make it easy to create and configure properties by automatically generating these accessor methods. It allows you to specify the behavior of a public property on a semantic level, and it takes care of the implementation details for you.

This module surveys the various attributes that let you alter getter and setter behavior. Some of these attributes determine how properties handle their underlying memory, so this module also serves as a practical introduction to memory management in Objective-C. For a more detailed discussion, please refer to Memory Management .

The @property Directive

First, let’s take a look at what’s going on under the hood when we use the @property directive. Consider the following interface for a simple Car class and its corresponding implementation.

The compiler generates a getter and a setter for the running property. The default naming convention is to use the property itself as the getter, prefix it with set for the setter, and prefix it with an underscore for the instance variable, like so:

After declaring the property with the @property directive, you can call these methods as if they were included in your class’s interface and implementation files. You can also override them in Car.m to supply custom getter/setters, but this makes the @synthesize directive mandatory. However, you should rarely need custom accessors, since @property attributes let you do this on an abstract level.

Properties accessed via dot-notation get translated to the above accessor methods behind the scenes, so the following honda.running code actually calls setRunning: when you assign a value to it and the running method when you read a value from it:

To change the behavior of the generated accessors, you can specify attributes in parentheses after the @property directive. The rest of this module introduces the available attributes.

The getter= and setter= Attributes

If you don’t like @property ’s default naming conventions, you can change the getter/setter method names with the getter= and setter= attributes. A common use case for this is Boolean properties, whose getters are conventionally prefixed with is . Try changing the property declaration in Car.h to the following.

The generated accessors are now called isRunning and setRunning . Note that the public property is still called running , and this is what you should use for dot-notation:

These are the only attributes that take an argument (the accessor method name)—all of the others are Boolean flags.

The readonly Attribute

The readonly attribute is an easy way to make a property read-only. It omits the setter method and prevents assignment via dot-notation, but the getter is unaffected. As an example, let’s change our Car interface to the following. Notice how you can specify multiple attributes by separating them with a comma.

Instead of letting other objects change the running property, we’ll set it internally via the startEngine and stopEngine methods. The corresponding implementation can be found below.

Remember that @property also generates an instance variable for us, which is why we can access _running without declaring it anywhere (we can’t use self.running here because the property is read-only). Let’s test this new Car class by adding the following snippet to main.m .

Up until this point, properties have really just been convenient shortcuts that let us avoid writing boilerplate getter and setter methods. This will not be the case for the remaining attributes, which significantly alter the behavior of their properties. They also only apply to properties that store Objective-C objects (opposed to primitive C data types).

The nonatomic Attribute

Atomicity has to do with how properties behave in a threaded environment. When you have more than one thread, it’s possible for the setter and the getter to be called at the same time. This means that the getter/setter can be interrupted by another operation, possibly resulting in corrupted data.

Atomic properties lock the underlying object to prevent this from happening, guaranteeing that the get or set operation is working with a complete value. However, it’s important to understand that this is only one aspect of thread-safety—using atomic properties does not necessarily mean that your code is thread-safe.

Properties declared with @property are atomic by default, and this does incur some overhead. So, if you’re not in a multi-threaded environment (or you’re implementing your own thread-safety), you’ll want to override this behavior with the nonatomic attribute, like so:

There is also a small, practical caveat with atomic properties. Accessors for atomic properties must both be either generated or user-defined. Only non-atomic properties let you mix-and-match synthesized accessors with custom ones. You can see this by removing nonatomic from the above code and adding a custom getter in Car.m .

Memory Management

In any OOP language, objects reside in the computer’s memory, and—especially on mobile devices—this is a scarce resource. The goal of a memory management system is to make sure that programs don’t take up any more space than they need to by creating and destroying objects in an efficient manner.

Many languages accomplish this through garbage collection, but Objective-C uses a more efficient alternative called object ownership . When you start interacting with an object, you’re said to own that object, which means that it’s guaranteed to exist as long as you’re using it. When you’re done with it, you relinquish ownership, and—if the object has no other owners—the operating system destroys the object and frees up the underlying memory.

With the advent of Automatic Reference Counting , the compiler manages all of your object ownership automatically. For the most part, this means that you’ll never to worry about how the memory management system actually works. But, you do have to understand the strong , weak and copy attributes of @property , since they tell the compiler what kind of relationship objects should have.

The strong Attribute

The strong attribute creates an owning relationship to whatever object is assigned to the property. This is the implicit behavior for all object properties, which is a safe default because it makes sure the value exists as long as it’s assigned to the property.

Let’s take a look at how this works by creating another class called Person . It’s interface just declares a name property:

The implementation is shown below. It uses the default accessors generated by @property . It also overrides NSObject ’s description method, which returns the string representation of the object.

Next, let’s add a Person property to the Car class. Change Car.h to the following.

Then, consider the following iteration of main.m :

Since driver is a strong relationship, the honda object takes ownership of john . This ensures that it will be valid as long as honda needs it.

The weak Attribute

Most of the time, the strong attribute is intuitively what you want for object properties. However, strong references pose a problem if, for example, we need a reference from driver back to the Car object he’s driving. First, let’s add a car property to Person.h :

The @class Car line is a forward declaration of the Car class. It’s like telling the compiler, “Trust me, the Car class exists, so don’t try to find it right now.” We have to do this instead of our usual #import statement because Car also imports Person.h , and we would have an endless loop of imports. (Compilers don’t like endless loops.)

Next, add the following line to main.m right after the honda.driver assignment:

We now have an owning relationship from honda to john and another owning relationship from john to honda . This means that both objects will always be owned by another object, so the memory management system won’t be able to destroy them even if they’re no longer needed.

This is called a retain cycle , which is a form of memory leak, and memory leaks are bad. Fortunately, it’s very easy to fix this problem—just tell one of the properties to maintain a weak reference to the other object. In Person.h , change the car declaration to the following:

The weak attribute creates a non-owning relationship to car . This allows john to have a reference to honda while avoiding a retain cycle. But, this also means that there is a possibility that honda will be destroyed while john still has a reference to it. Should this happen, the weak attribute will conveniently set car to nil in order to avoid a dangling pointer.

A common use case for the weak attribute is parent-child data structures. By convention, the parent object should maintain a strong reference with it’s children, and the children should store a weak reference back to the parent. Weak references are also an inherent part of the delegate design pattern.

The point to take away is that two objects should never have strong references to each other. The weak attribute makes it possible to maintain a cyclical relationship without creating a retain cycle.

The copy Attribute

The copy attribute is an alternative to strong . Instead of taking ownership of the existing object, it creates a copy of whatever you assign to the property, then takes ownership of that. Only objects that conform to the NSCopying protocol can use this attribute.

Properties that represent values (opposed to connections or relationships) are good candidates for copying. For example, developers usually copy NSString properties instead of strongly reference them:

Now, Car will store a brand new instance of whatever value we assign to model . If you’re working with mutable values, this has the added perk of freezing the object at whatever value it had when it was assigned. This is demonstrated below:

NSMutableString is a subclass of NSString that can be edited in-place. If the model property didn’t create a copy of the original instance, we would be able to see the altered string ( Nissan Versa ) in the second NSLog() output.

Other Attributes

The above @property attributes are all you should need for modern Objective-C applications (iOS 5+), but there are a few others that you may encounter in older libraries or documentation.

The retain Attribute

The retain attribute is the Manual Retain Release version of strong , and it has the exact same effect: claiming ownership of assigned values. You shouldn’t use this in an Automatic Reference Counted environment.

The unsafe_unretained Attribute

Properties with the unsafe_unretained attribute behave similar to weak properties, but they don’t automatically set their value to nil if the referenced object is destroyed. The only reason you should need to use unsafe_unretained is to make your class compatible with code that doesn’t support the weak property.

The assign Attribute

The assign attribute doesn’t perform any kind of memory-management call when assigning a new value to the property. This is the default behavior for primitive data types, and it used to be a way to implement weak references before iOS 5. Like retain , you shouldn’t ever need to explicitly use this in modern applications.

This module presented the entire selection of @property attributes, and we hope that you’re feeling relatively comfortable modifying the behavior of generated accessor methods. Remember that the goal of all these attributes is to help you focus on what data needs to be recorded by letting the compiler automatically determine how it’s represented. They are summarized below.

Now that we’ve got properties out of the way, we can take an in-depth look at the other half of Objective-C classes: methods. We’ll explore everything from the quirks behind their naming conventions to dynamic method calls.

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Properties in Objective-C

Operators in Objective-C
Pointers in Objective-C
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Variables in Objective-C
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Strings in Objective-C
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Classes & Objects in Objective-C
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Composite Objects in Objective-C
Properties Class in Java
NSArray and NSMutableArray in Objective-C
NSDictionary and NSMutableDictionary in Objective-C
Interface in Objective-C
Return Array From a Function in Objective-C
Log Handling in Objective-C
Extensions in Objective-C
Memory Management in Objective-C
Constructors in Objective-C

The object-oriented programming language Objective-C is largely used to create applications for Apple’s macOS and iOS platforms. It incorporates all of the characteristics of C while also providing more features for object-oriented programming, making it a superset of the C programming language. The use of properties is one of Objective-key C’s features. By encapsulating data and activity within an object, properties give other objects a regulated way to access and interact with that data. In other programming languages, instance variables are comparable to properties, but properties offer further features like automatic memory management and the capacity to specify unique getter and setter methods.

Syntax of Properties in Objective-C

Properties are used in Objective-C to declare instance variables and give users a means to access and change their values. The following is the syntax for declaring properties in Objective-C:

In this case, a property is declared using @property, and details about the property are provided in the parenthesis.

Attributes of Properties in Objective-C

Properties in Objective-C can be specified with a variety of attributes to change how they behave or manage memory. The following are the most typical attributes:

Nonatomic : This characteristic indicates that the property should be accessed non-atomically, allowing for simultaneous access from many threads. Although it is not thread-safe, this can boost performance.
Atomic : This characteristic indicates that the property should only be accessed once by a single thread, or in other words, atomically. Although it guarantees thread safety and is the default property, it may have an impact on performance.
Strong : This attribute indicates that the property should keep the object and is used for objects. As long as the property is present, the object won’t be deallocated.
Weak : This attribute indicates that the property should not keep the object and is also used for objects. This indicates that even if the property is still present, the object can be deallocated.
Assign : This primitive type attribute instructs the property to merely assign the value to the instance variable. This indicates that memory management is not necessary.
Readonly : This attribute instructs the system not to build a setter method for the property. This indicates that the property can only be set once, during initialization, and cannot be modified afterward.
Read-write : The getter and setter methods for the property should be produced, according to the read-write attribute. The default attribute is this one.
Copy : This attribute, which is used for objects, instructs the property to make a copy of the object instead of just keeping it. This makes sure that outside code cannot change the value of the attribute.
Getter : This attribute is used to define a unique name for the property’s getter method.
Setter : This attribute is used to give the property’s setter method a unique name.

In this example, the strong attribute is used to maintain the NSString object while also declaring the name property with the nonatomic attribute to increase efficiency.

Example of how to use properties in Objective-C

In this example, a “Person” class with the fields “name” and “age” is declared. Age is a “assign” property of type NSInteger, and the name is a “strong” property of type NSString. Also, we declare the instance function “introduction,” which just prints the person’s name and age to the console.

We construct a new “Person” object and set its “name” and “age” attributes in the “main” method. The message is then recorded to the console by calling the “introduction” method on the “Person” object. As you can see, we call the getter and setter methods of the properties using a shortcut called the dot syntax. So, “person.name = @”John”” is the same as “[person setName:@”John”]”.

The make, model, and year attributes of the ‘Car’ class are the three that are declared in this example. Make, Model and Year are “strong” properties of type “NSString” and “assign” properties of type “NSInteger,” respectively. We also create a special initializer called “initWithMake:model: year,” which initializes properties by accepting values for make, model, and year.

Also, we declare the instance function “drive,” which logs the make, model, and year of the vehicle to the console as a message. We use the ‘initWithMake:model: year’ method to create a new Vehicle object in the main function and pass values for the properties make, model, and year ‘. To log the message to the console, we next use the ‘drive’ function on the ‘Car’ object.

Properties are a strong component of Objective-C, offering a practical and effective way to access the information and behavior of an object. You can build more reliable and effective Objective-C code by being aware of its syntax, properties, and usage.

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Write Objective-C Code

If you haven’t programmed for either OS X or iOS, you need to become acquainted with the primary programming language, Objective-C. Objective-C is a not a difficult language, and once you spend some time with it you’ll find it elegant as well. The Objective-C language enables sophisticated object-oriented programming. It extends the standard ANSI C programming language by providing syntax for defining classes and methods. It also promotes dynamic extension of classes and interfaces that any class can adopt.

If you are familiar with ANSI C, the following information should help you learn the basic syntax of Objective-C. And if you have programmed with other object-oriented languages, you’ll find that many of the traditional object-oriented concepts, such as encapsulation, inheritance, and polymorphism, are all present in Objective-C. If you are not familiar with ANSI C, it’s a good idea to at least read an overview of the C language before you attempt to read this article.

The Objective-C language is fully explained in The Objective-C Programming Language .

Objective-C is a Superset of the C Language

The Objective-C language specifies a syntax for defining classes and methods, for calling methods of objects, and for dynamically extending classes and creating programming interfaces adapted to address specific problems. As a superset of the C programming language, Objective-C supports the same basic syntax as C. You get all of the familiar elements, such as primitive types ( int , float , and so on), structures, functions, pointers, and control-flow constructs such as if...else and for statements. You also have access to the standard C library routines, such as those declared in stdlib.h and stdio.h .

Objective-C adds the following syntax and features to ANSI C:

Definition of new classes

Class and instance methods

Method invocation (called messaging )

Declaration of properties (and automatic synthesizing of accessor methods from them)

Static and dynamic typing

Blocks—encapsulated segments of code that can be executed at any time

Extensions to the base language such as protocols and categories

Don’t worry if these aspects of Objective-C are unfamiliar to you now. As you progress through the remainder of this article, you will learn about them. If you’re a procedural programmer new to object-oriented concepts, it might help at first to think of an object as essentially a structure with functions associated with it. This notion is not too far off the reality, particularly in terms of runtime implementation.

In addition to providing most of the abstractions and mechanisms found in other object-oriented languages, Objective-C is a very dynamic language, and that dynamism is its greatest advantage. It is dynamic in that it permits an app’s behavior to be determined when it is running (that is, at runtime) rather than being fixed when the app is built. Thus the dynamism of Objective-C frees a program from constraints imposed when it’s compiled and linked; instead it shifts much of the responsibility for symbol resolution to runtime, when the user is in control.

Classes and Objects

As in most other object-oriented languages, classes in Objective-C support the encapsulation of data and define the actions that operate on that data. An object is a runtime instance of a class. It contains its own in-memory copy of the instance variables declared by its class and pointers to the methods of the class. You create an object in a two-step procedure called allocation and initialization .

The specification of a class in Objective-C requires two distinct pieces: the interface and the implementation. The interface portion contains the class declaration and defines the public interface of the class. As with C code, you define header files and source files to separate public declarations from the implementation details of your code. (You can put other declarations in your implementation file if they are part of the programmatic interfaces but are meant to be private.) These files have the filename extensions listed in the following table.

When you want to include header files in your source code, specify a pound import ( #import ) directive as one of the first lines in a header or source file; a #import directive is like C’s #include directive, except that it makes sure that the same file is never included more than once. If you want to import some or all of the header files of a framework, import the framework’s umbrella header file, which has the same name as the framework, and not individual header files. The syntax for importing the header files of the (hypothetical) Gizmo framework is:

The diagram below shows the syntax for declaring a class called MyClass , which inherits from the base (or root ) class, NSObject . (A root class is one that all other classes directly or indirectly inherit from.) The class declaration begins with the @interface compiler directive and ends with the @end directive. Following the class name (and separated from it by a colon) is the name of the parent class. In Objective-C, a class can have only one parent.

You write declarations of properties and methods between the @interface and @end directives. These declarations form the public interface of the class. (Declared properties are described in “Declared Properties and Accessor Methods.” ) A semicolon marks the end of each property and method declaration. If the class has custom functions, constants, or data types associated with its public interface, put their declarations outside the @interface . . . @end block.

The syntax for a class implementation is similar. It begins with the @implementation compiler directive (followed by the name of the class) and ends with the @end directive. Method implementations go in between. (Function implementations should go outside the @implementation…@end block.) An implementation should always import its interface file as one of the first lines of code.

Objective-C supports dynamic typing for variables containing objects, but it also supports static typing. Statically typed variables include the class name in the variable type declaration. Dynamically typed variables use the type id for the object instead. You find dynamically typed variables used in certain situations. For example, a collection object such as an array (where the exact types of the contained objects may be unknown) might use dynamically typed variables. Such variables provide tremendous flexibility and allow for greater dynamism in Objective-C programs.

This example shows statically and dynamically typed variable declarations:

Notice the asterisk ( * ) in the first declaration. In Objective-C, object references must always be pointers. If this requirement doesn’t make complete sense to you, don’t worry—you don’t have to be an expert with pointers to be able to start programming with Objective-C. You just have to remember to put the * in front of the variable names for statically typed object declarations. The id type implies a pointer.

Methods and Messaging

If you’re new to object-oriented programming, it might help to think of a method as a function that is scoped to a specific object. By sending a message to—or messaging —an object, you call a method of that object. There are two kinds of methods in Objective-C: instance methods and class methods.

An instance method is a method whose execution is scoped to a particular instance of the class. In other words, before you call an instance method, you must first create an instance of the class. Instance methods are the most common type of method.

A class method is a method whose execution is scoped to the method’s class. It does not require an instance of an object to be the receiver of a message.

The declaration of a method consists of the method type identifier, a return type, one or more signature keywords, and the parameter type and name information. Here’s the declaration of the insertObject:atIndex: instance method.

For instance methods, the declaration is preceded by a minus sign ( - ); for class methods, the corresponding indicator is a plus sign ( + ). “Class Methods”, below, describes class methods in greater detail.

A method’s actual name ( insertObject:atIndex: ) is a concatenation of all of the signature keywords, including colon characters. The colon characters declare the presence of a parameter. In the above example, the method takes two parameters. If a method has no parameters, you omit the colon after the first (and only) signature keyword.

When you want to call a method, you do so by sending a message to the object that implements the method—or, in other words, by messaging that object. (Although the phrase "sending a message" is commonly used as a synonym for "calling a method,“ the Objective-C runtime does the actual sending.) A message is the method name along with the parameter information the method needs (properly conforming to type). All messages you send to an object are dispatched dynamically, thus facilitating the polymorphic behavior of Objective-C classes. (Polymorphism refers to the ability of different types of objects to respond to the same message.) Sometimes the method invoked is implemented by a superclass of the class of the object receiving the message.

To dispatch a message, the runtime requires a message expression. A message expression encloses with brackets ( [ and ] ) the message itself (along with any required parameters) and, just inside the leftmost bracket, the object receiving the message. For example, to send the insertObject:atIndex: message to an object held by the myArray variable, you would use the following syntax:

To avoid declaring numerous local variables to store temporary results, Objective-C lets you nest message expressions. The return value from each nested expression is used as a parameter or as the receiving object of another message. For example, you could replace any of the variables used in the previous example with messages to retrieve the values. Thus, if you had another object called myAppObject that had methods for accessing the array object and the object to insert into the array, you could write the preceding example to look something like the following:

Objective-C also provides a dot-notation syntax for invoking accessor methods. Accessor methods get and set the state of an object, and thus are key to encapsulation, which is an important feature of all objects. Objects hide, or encapsulate , their state and present an interface common to all instances for accessing that state. Using dot-notation syntax, you could rewrite the previous example as:

You can also use dot-notation syntax for assignment:

This syntax is simply a different way to write [myAppObject setTheArray:aNewArray]; . You cannot use a reference to a dynamically typed object (type of id ) in a dot-notation expression.

You have used dot-notation syntax already for assigning to a variable in Your First Mac App :

Class Methods

Although the preceding examples sent messages to an instance of a class, you can also send messages to the class itself. (A class is an object of type Class created by the runtime.) When messaging a class, the method you specify must be defined as a class method instead of an instance method. Class methods are a feature similar to static class methods in C++.

You often use class methods as factory methods to create new instances of the class or for accessing some piece of shared information associated with the class. The syntax for a class method declaration is identical to that of an instance method except that you use a plus ( + ) sign instead of a minus sign for the method type identifier.

The following example illustrates how you use a class method as a factory method for a class. In this case, the array method is a class method on the NSArray class—and inherited by NSMutableArray —that allocates and initializes a new instance of the class and returns it to your code.

Declared Properties and Accessor Methods

A property in the general sense is some data encapsulated or stored by an object. It is either an attribute—such as a name or a color—or a relationship to one or more other objects. The class of an object defines an interface that enables users of its objects to get and set the values of encapsulated properties. The methods that perform these actions are called accessor methods .

There are two types of accessor methods, and each method must conform to a naming convention. A “getter” accessor method, which returns the value of a property, has the same name as the property. A "setter” accessor method, which sets a new value for a property, has the form set PropertyName : , where the first letter of the property name is capitalized. Properly named accessor methods are a critical element of several technologies of the Cocoa and Cocoa Touch frameworks, including key-value coding (KVC), which is a mechanism for accessing an object’s properties indirectly through their names.

Objective-C offers declared properties as a notational convenience for the declaration and implementation of accessor methods. In Your First Mac App you declared the volume property:

Declared properties eliminate the need to implement a getter and setter method for each property exposed in the class. Instead, you specify the behavior you want using the property declaration. The compiler can then create—or synthesize —actual getter and setter methods based on that declaration. Declared properties reduce the amount of boilerplate code you have to write and, as a result, make your code much cleaner and less error prone. Use declared properties or accessor methods to get and set items of an object’s state.

You include property declarations with the method declarations in your class interface. You declare public properties in the class header files; you declare private properties in a class extension in the source file. (See “Protocols and Categories” for a short description of class extensions along with an example.) Properties of controller objects such as delegates and view controllers should typically be private.

The basic declaration for a property uses the @property compiler directive, followed by the type information and name of the property. You can also configure the property with custom options, which define how the accessor methods behave, whether the property is a weak reference, and whether it is read-only. The options are in parentheses following the @property directive.

The following lines of code illustrate a few more property declarations:

The compiler automatically synthesizes declared properties. In synthesizing a property, it creates accessor methods for it as well as a private instance variable that “backs” the property. The instance variable has the same name as the property but with an underscore prefix ( _ ). Your app should directly access an instance variable (instead of its property) only in methods for object initialization and deallocation.

If you want a different name for an instance variable, you can bypass autosynthesis and explicitly synthesize a property. Use the @synthesize compiler directive in the class implementation to ask the compiler to generate the accessor methods along with the specially named instance variable. For example:

Incidentally, when you declare a property you can specify custom names for the accessor methods, typically to make the getter methods of Boolean properties follow a conventional form, as shown here:

Blocks are objects that encapsulate a unit of work—that is, a segment of code—that can be executed at any time. They are essentially portable and anonymous functions that one can pass in as parameters of methods and functions or that can be returned from methods and functions. Blocks themselves have a typed parameter list and may have an inferred or a declared return type. You can also assign a block to a variable and then call it just as you would a function.

A caret ( ^ ) is used as a syntactic marker for blocks. There are other, familiar syntactic conventions for parameters, return values, and body of the block (that is, the executed code). The following figure explicates the syntax, specifically when assigning a block to a variable.

You can then call the block variable as if it were a function:

A block shares data in the local lexical scope. This characteristic of blocks is useful because if you implement a method and that method defines a block, the block has access to the local variables and parameters of the method (including stack variables) as well as to functions and global variables, including instance variables. This access is read-only, but if a variable is declared with the __block modifier, its value can be changed within the block. Even after the method or function enclosing a block has returned and its local scope has been destroyed, the local variables persist as part of the block object as long as there is a reference to the block.

As method or function parameters, blocks can serve as a callback. When invoked, the method or function performs some work and, at the appropriate moments, calls back to the invoking code—via the block—to request additional information or to obtain program-specific behavior from it. Blocks enable the caller to provide the callback code at the point of invocation. Instead of packaging the required data in a “context” structure, blocks capture data from the same lexical scope as the host method or function. Because the block code does not have to be implemented in a separate method or function, your implementation code can be simpler and easier to understand.

Objective-C frameworks have many methods with block parameters. For example, the NSNotificationCenter class of the Foundation framework declares the following method, which has a block parameter:

This method adds an observer to the notification center (notifications are discussed in Streamline Your App with Design Patterns ). When a notification of the specified name is posted, the block is invoked to handle the notification.

Protocols and Categories

A protocol declares methods that can be implemented by any class, even if those classes implementing the protocol don’t have a common superclass. Protocol methods define behavior that is independent of any particular class. Protocols simply define an interface that other classes are responsible for implementing. When your class implements the methods of a protocol, your class is said to conform to that protocol.

From a practical perspective, a protocol defines a list of methods that establishes a contract between objects without requiring them to be instances of any specific class. This contract enables communication between those objects. One object wants to tell another object about the events it’s encountering, or perhaps it wants to ask for advice about those events.

The UIApplication class implements the required behavior of an application. Instead of forcing you to subclass UIApplication to receive simple notifications about the current state of the application, the UIApplication class delivers those notifications by calling specific methods of its assigned delegate object. An object that implements the methods of the UIApplicationDelegate protocol can receive those notifications and provide an appropriate response.

You specify in the interface block that your class conforms to, or adopts, a protocol by putting the name of the protocol in angle brackets ( <...> ) after the name of the class from which it inherits. In Your First Mac App , adoption of the NSApplicationDelegate protocol is specified in the following line:

You do not have to declare the protocol methods you implement.

The declaration of a protocol looks similar to that of a class interface, with the exceptions that protocols do not have a parent class and they do not define instance variables (although they can declare properties). The following example shows a simple protocol declaration with one method:

For many delegate protocols, adopting a protocol is simply a matter of implementing the methods defined by that protocol. There are some protocols that require you to state explicitly that you support the protocol, and protocols can specify both required and optional methods.

When you begin exploring the header files of the Objective-C frameworks, you’ll soon encounter a line similar to this one:

This line declares a category through the syntactical convention of enclosing the name of the category in parentheses. A category is a feature of the Objective-C language that enables you to extend the interface of a class without having to subclass it. The methods in the category become part of the class type (within the scope of your program) and are inherited by all the class’s subclasses. You can send a message to any instance of the class (or its subclasses) to invoke a method defined in the category.

You can use categories as a means for grouping related method declarations within a header file. You can even put different category declarations in different header files. The frameworks use these techniques throughout their header files for clarity.

You can also use an anonymous category known as a class extension to declare private properties and private methods in the implementation ( .m ) file. A class extension looks like a category except there is no text between the parentheses. For example, the following shows a typical class extension:

Defined Types and Coding Strategies

Objective-C has several terms that you should not use as the names of variables because they are reserved for special purposes. Some of these terms are compiler directives that are prefixed with an at sign ( @ , for example, @interface and @end ). Other reserved terms are defined types and the literals that go with those types. Objective-C uses a number of defined types and literals that you won’t find in ANSI C. In some cases, these types and literals replace their ANSI C counterparts. The following table describes a few of the important ones, including the allowable literals for each type:

You often use these defined types and literals in error-checking and control-flow code. In your program’s control-flow statements, you can test the appropriate literal to determine how to proceed. For example:

To paraphrase this code, if the object representing the date of hire is not nil —in other words, it is a valid object—then the logic proceeds in a certain direction. Here’s a shorthand way of doing the same branching:

You can even reduce these lines of code further (assuming you don’t need a reference to the dateOfHire object):

You handle Boolean values in much the same way. In this example, the isEqual: method returns a Boolean value:

You can shorten this code in the same way you can for the code that tests for the absence or presence of nil .

In Objective-C you can send a message to nil with no ill effects. Indeed there is no effect at all, except that the runtime returns nil if the method is supposed to return an object. Return values from messages sent to nil are guaranteed to work as long as what is returned is typed as an object.

Two other important reserved terms in Objective-C are self and super . The first term, self , is a local variable that you can use within a message implementation to refer to the current object; it is equivalent to this in C++. You can substitute the reserved word super for self , but only as the receiver in a message expression. If you send a message to self , the runtime first looks for the method implementation in the current object’s class; if it can’t find the method there, it looks for it in its superclass (and so on). If you send a message to super , the runtime first looks for the method implementation in the superclass.

The primary uses of both self and super have to do with sending messages. You send a message to self when the method to invoke is implemented by the class of self . For example:

self is also used in dot notation to invoke the accessor method synthesized by a declared property. For example:

You often send messages to super in overrides (that is, reimplementations ) of methods inherited from a superclass. In this case, the method invoked has the same signature as the method overridden.

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@property(<atomicity>, <writability>, <setter semantics>)

<atomicity>.

nonatomic – Specifies that accessors are nonatomic. By default, accessors are atomic.

<writability>

readwrite – Indicates that the property should be treated as read/write. This attribute is the default.

readonly – Indicates that the property is read-only.

<setter semantics>

strong – Specifies that there is a strong (owning) relationship to the destination object. (ARC)

weak – Specifies that there is a weak (non-owning) relationship to the destination object. If the destination object is deallocated, the property value is automatically set to nil. (not supported in iOS 4; use assign instead)

copy – Specifies that a copy of the object should be used for assignment, releasing the previous value and copying the incoming value (only valid for objects that implement the NSCopying protocol).

assign – Specifies that the setter uses simple assignment. This attribute is the default. (use for scalar types, weak references in iOS 4, etc.)

retain – Specifies that retain should be invoked on the object upon assignment. The previous value is sent a release message.

via Apple's Developer Documentation

KevinVitale commented Jan 5, 2012

4.1.1. Property declarations

A property of retainable object pointer type may have ownership. If the property's type is ownership-qualified, then the property has that ownership. If the property has one of the following modifiers, then the property has the corresponding ownership. A property is ill-formed if it has conflicting sources of ownership, or if it has redundant ownership modifiers, or if it has __autoreleasing ownership.

assign implies __unsafe_unretained ownership. copy implies __strong ownership, as well as the usual behavior of copy semantics on the setter. retain implies __strong ownership. strong implies __strong ownership. unsafe_unretained implies __unsafe_unretained ownership. weak implies __weak ownership.

Sorry, something went wrong.

# Properties

# custom getters and setters.

The default property getters and setters can be overridden:

This can be useful to provide, for example, lazy initialization (by overriding the getter to set the initial value if it has not yet been set):

You can also make a property that computes its value in the getter:

# What are properties?

Here is an example class which has a couple of instance variables, without using properties:

This is quite a lot of boilerplate code to create a simple instance variable. You have to create the instance variable & create accessor methods which do nothing except set or return the instance variable. So with Objective-C 2.0, Apple introduced properties, which auto-generate some or all of the boilerplate code.

Here is the above class rewritten with properties:

A property is an instance variable paired with auto-generated getters and setters. For a property called someString , the getter and setter are called someString and setSomeString: respectively. The name of the instance variable is, by default, the name of the property prefixed with an underscore (so the instance variable for someString is called _someString , but this can be overridden with an @synthesize directive in the @implementation section:

Properties can be accessed by calling the getters and setters:

They can also be accessed using dot notation:

# Properties that cause updates

This object, Shape has a property image that depends on numberOfSides and sideWidth . If either one of them is set, than the image has to be recalculated. But recalculation is presumably long, and only needs to be done once if both properties are set, so the Shape provides a way to set both properties and only recalculate once. This is done by setting the property ivars directly.

When properties are assigned to (using object.property = value ), the setter method setProperty: is called. This setter, even if provided by @synthesize , can be overridden, as it is in this case for numberOfSides and sideWidth . However, if you set an property's ivar directly (through property if the object is self, or object->property ), it doesn't call the getter or setter, allowing you to do things like multiple property sets that only call one update or bypass side-effects caused by the setter.

@property ( optional_attributes, ... ) type identifier ;
@synthesize identifier = optional_backing_ivar ;
@dynamic identifier ;

# Parameters

← Methods Random Integer →

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Where x=objective-c.

Get the code: LearnObjectiveC.m

Objective-C is the main programming language used by Apple for the macOS and iOS operating systems and their respective frameworks, Cocoa and Cocoa Touch. It is a general-purpose, object-oriented programming language that adds Smalltalk-style messaging to the C programming language.

Learn Objective-C, Objects (Part 2): Properties

Every object is made up of instance variables (iVars) and methods . Our Fraction class, which we really began to build in the last post , contains two iVars, both NSIntegers, called numerator and denominator. Fraction also has two methods, setNumerator: and setDenominator:. These methods will set a value to the corresponding instance variable. In most cases, you would also have a method, with the same name as the instance variable (yes, this is permitted, and in this case, encouraged), which would return the value of the instance variable. These setter and getter methods are referred to as mutator and accessor methods, respectively. These methods are a major concept of object-oriented programming—data encapsulation, the notion that objects hide their instance variables.

At this moment, our Fraction class defines and implements the setter methods (note that we simply have not needed the getter methods so far). However, this would be difficult to do when we start working with large classes, which have many instance variables. This is also not very scalable; the same code to retrieve values for a small block of data stored on an iPhone should probably not be the same code used to retrieve a long string of data from a powerful desktop Mac or even from the internet. Fortunately, there is a very convenient concept that Objective-C provides, and this construct should be used wherever possible.

Properties replace the accessor methods for an object. Quite simply, our Fraction class’s interface now looks like this:

@interface Fraction : NSObject {
NSInteger numerator ;
NSInteger denominator ;
** @property NSInteger numerator ;**
** @property NSInteger denominator ;**
- ( void ) display ;

The general form of the property statement is

@property *type name*

Note that the setter (and if we had them, the getter) methods are no longer needed. We will have the compiler synthesize these methods for us, but placing this line in our implementation, right after the @implementation line:

#import "Fraction.h" @implementation Fraction
@synthesize numerator , denominator
- ( void ) display {
NSString * numeratorString = [[ NSString alloc ] initWithFormat :@ "%d" , self . numerator ];
NSString * denominatorString = [[ NSString alloc ] initWithFormat :@ "%d" , self . denominator ];
NSLog (@ "%@/%@" , numeratorString , denominatorString );
[ denominatorString release ];
[ numeratorString release ];

Of course, the identifiers after the @synthesize have to match the names you declared as properties in the interface. However, the identifiers that you declared after @property do not have the match the instance variables; instance variables will be created at compile time if this happens.

You do not have to change your method calls in your main() routine; the methods still exist. However, there is an easier (and sometimes controversial) way to access these synthesized properties.

The Dot Operator

As previously mentioned, a getter method returns the value of an iVar; a setter sets a value to the iVar.

[ myFraction setNumerator : 2 ] // Set numerator to 2 NSInteger
numeratorValue = [ myFraction numerator ]; // returns the value of the numerator, and set to the new variable called numeratorValue

The dot operator, introduced in Objective-C 2.0 (yes, there was a 1.0, but that was a long time ago), allows you to invoke the getter method by writing

iVar.property

and the setter by

iVar.property = value

Therefore, the above example can be re-written as

myFraction . numerator = 2 ; // Set numerator to 2
NSInteger numeratorValue = myFraction . numerator ; // set value of numerator to numeratorValue

C programmers might recognize this syntax as being used to access members of a struct—because that is exactly what you are doing. Behind the scenes, an object’s instance variables are stored as a struct. And if you have no idea what this line means, don’t worry—structs are a basic C data type that we may cover in a later lesson.

Property Attributes

In a property declaration, you can specify any number of (non-contradictory) attributes:

@property (attribute1, attribute2, etc.) type name;

A list of possible attributes follows (adapted from Apple’s developer documentation, The Objective-C Programming Language : Declared Properties)

Accessor Method Names

The default names for the getter and setter methods associated with a property are propertyName and setPropertyName : respectively—for example, given a property “foo”, the accessors would be foo and setFoo:. The following attributes allow you to specify custom names instead. They are both optional and may appear with any other attribute (except for readonly in the case of setter=).

getter=getterName: Specifies the name of a different (custom) getter method for the property. The method must return a value matching the iVar’s type, and it takes no arguments.
setter=setterName: Specifies the name of a different (custom) setter method for the property. The method must return void, and it takes one argument of the same type as the iVar.If you specify that a property is readonly then also specify a setter with setter=, you will get a compiler warning.

Writability

These attributes specify whether or not a property has an associated set accessor. They are mutually exclusive.

readwrite: This is the default; this indicates that the property will have read and write capabilities. When synthesized, both the getter and setter will be generated.
readonly: This indicates that the property can only be read; no data can be assigned to it (through the property). When synthesized, only a getter will be generated; when you try to assign the property a value using dot syntax, a compiler error will be generated.

Setter Semantics

These attributes specify the semantics of the setter. They are mutually exclusive.

assign: This is the default; it tells the setter to directly assign the value. This is typically delineated with primitive data types (ints, floats, etc.)
retain: Specifies that a retain should be called on the object upon assignment; the previous value is released. This is only valid for objects. This has to do with memory management, a topic that will be fully explored in a later lesson.
copy: Specifies that the argument passed in should be copied, and the copy will be assigned. The previous value is released. This is only valid for objects. It is used in the case where the original property (perhaps stored on a shared server across the network) should not be modified by any other code, but when other code requests the value, for potential modification.

Atomicity means that the value is written to a temporary location in memory first, and then written to the intended location. This ensures that there is always a valid copy of the data somewhere. If an assignment was not atomic, and something happened during the writing period, the data could become fatally corrupt—the user would lose data. By writing data atomically, the data will either be the original, or the new value—never in a partially written state.

nonatomic: Specifies that the object should not be assigned atomically. By default, accessors are atomic (this is more beneficial in a multi-threaded environment, where multiple threads may be reading/writing value to a piece of memory.

CS193P Material

Properties are a very powerful notion, and used throughout the language. If this explanation was not clear enough, please see Lecture 3 of CS193P . The relevant content begins around the 16 minute mark, and ends with a demo around the 45 minute mark. Of course, feel free to view the whole lecture.

The slides for the lecture are available on the CS193P website ; for a direct link, click here .

The code for the Fraction class at this moment can be found here .

This post is part of the Learn Objective-C in 24 Days course.

Author: Feifan Zhou

Objective-C Class Properties

Objective-C

With all the excitement about the new Swift 3 language features it is easy to overlook some of the small improvements still happening to Objective-C. It may be the case that any love Apple is showing Objective-C is to improve Swift interoperability but it is still welcome for developers needing to get work done in Objective-C .

In this post we look at the addition of class properties to Objective-C.

From the Xcode 8 release notes:

Objective-C now supports class properties, which interoperate with Swift type properties. They are declared as: @property (class) NSString *someStringProperty;. They are never synthesized. (23891898)

To experiment let’s create a simple Objective-C class with some class properties. This is what the interface for our User class looks like:

For illustration purposes I have two properties, the first is a read-only integer and the second is a read-write NSUUID with copy semantics. Note the class in the property declarations.

The implementation is simple, we first need some storage for the identifier and userCount class properties. Since these are class level and not instance variables we declare them as static:

Now we need to create the getter and setter methods for the two properties. As mentioned in the release notes these are never synthesised for you so Xcode will warn you if they are missing. First the read-only userCount needs a getter that just returns the value of the count. Note the + to make our getter a class method:

The identifier property needs both a getter and a setter. We create an identifier in the getter if we do not already have one:

We will also create a basic initializer that updates the count property:

The resetIdentifier class method is a convenience method that creates a new identifier:

You access the class properties using normal dot syntax on the class name:

An example of this class in use:

This produces the output:

Note that since this is a feature of the LLVM compiler in Xcode 8 it will work with deployment targets earlier than iOS 10.

Generated Swift interface

It seems that the only enhancements that Objective-C gets these days are to improve interoperability with Swift. The addition of class properties to Objective-C maps to the use of class variables in Swift. Here is the generated Swift interface for our User class:

Note that our identifier property is an implicitly unwrapped optional meaning that we do not ever expect it to be nil. To allow it be nil we would need to add a nullable annotation to the Objective-C property declaration. Our Swift variable would then be an optional. See Using nullable to annotate Objective-C for more details.

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Current cad, assigned to readonly property objective-c -.

i writing unit test test method updates checklist. checklist has these properties:

however, in unit test, trying validate,

checklistitem.description = @"hello"; , error"assignment readonly property"

heres rest of test method:

edited question:

so when change property above readwrite, error in cchecklistitem

`illegal redeclaration of readwrite property in class extension 'cchecklistitem'

your property set readonly seen here:

or if want consistent other properties (though overly explicit, imo):