@objc Attribute in Swift

@objc Attribute in Swift:

By default, Swift generates code that is only accessible to other Swift code. However, when you need to interact with Objective-C code or frameworks (such as UIKit), you use @objc to explicitly instruct Swift to make certain classes, methods, properties, or other entities available to Objective-C as well as Swift code.

The @objc keyword in Swift is used to expose Swift code to Objective-C.

For example, if you want to use a Swift method with a UIBarButtonItem or a Timer, both of which are Objective-C components, you need to mark that method with @objc. This ensures that the method can be recognized and used by both Swift and Objective-C code.

Example with Timer:

let timer = Timer.scheduledTimer(timeInterval: 1.0, target: self, selector: #selector(startTimer), userInfo: nil, repeats: true)

@objc func startTimer() {
    print("Timer started!")
}

Note: Here we have to add the @objc attribute to a function that you want to use with target-action (startTimer() in this example). This will make the function available to Objective-C, which is required because UIControl (Timer, UIButton) and target-action is an Objective-C class.

What, if we not add @objc in above example?

Without @objc this code will lead to issue –

 

Here are some common scenarios when you might use the @objc keyword:

1. Exposing Swift Code to Objective-C:

• If you have Swift code that needs to be accessible and usable from Objective-C code, you can use the @objc attribute on classes, methods, properties, or other elements that you want to expose.

  @objc class MySwiftClass: NSObject {
          @objc func mySwiftMethod() {
                  // Swift method
          }
  
   }

2. Interoperability with Objective-C Frameworks:

• When working with Objective-C frameworks or APIs, you might need to use the @objc attribute to make your Swift code compatible.

@objc func mySwiftFunction() {
    // Swift function
}

3. Selectors and Dynamic Dispatch:

• When you need to use selectors with methods, the @objc attribute allows the Swift method to be referenced by a string-based selector. This is often necessary when working with certain APIs or performing dynamic dispatch.

@objc func mySwiftMethod() {
    // Swift method
}

let selector = #selector(MySwiftClass.mySwiftMethod)

4. Inheriting from Objective-C Classes:

• When inheriting from Objective-C classes or conforming to Objective-C protocols in Swift, you might need to use the @objc attribute.

@objc class MySwiftClass: NSObject, SomeObjectiveCProtocol {
    // Swift class conforming to Objective-C protocol
}

5. Dynamic Behavior and Key-Value Coding (KVC):

• The @objc attribute is required for classes, methods, and properties if you want to use features like Key-Value Observing (KVO) and Key-Value Coding (KVC).

@objc class MyObservableClass: NSObject {
    @objc dynamic var myProperty: Int = 0
}

Important Considerations:

• The @objc attribute comes with some runtime overhead, and its usage should be considered when needed for interoperability rather than as a default practice.
• Swift classes and methods without @objc are not automatically exposed to Objective-C. Only elements marked with @objc can be used from Objective-C code.

In summary, the @objc keyword is used to make Swift code accessible from Objective-C and enables interoperability between Swift and Objective-C in mixed-language projects. Use it selectively based on the specific interoperability requirements of your project.

Read More:

Attributes in Swift

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