What is the @objc attribute and when to use @objc in swift code?
The @objc attribute makes your Swift API available in Objective-C and the Objective-C runtime.
A Swift class or protocol must be marked with the @objc attribute to be accessible and usable in Objective-C. This attribute tells the compiler that this piece of Swift code can be accessed from Objective-C. If your Swift class is a descendant of an Objective-C class, the compiler automatically adds the @objc attribute for you.
By default Swift generates code that is only available to other Swift code, but if you need to interact with the Objective-C runtime – all of UIKit, for example – you need to tell Swift what to do.
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Don’t worry: if you forget to add @objc when it’s needed, your code simply won’t compile – it’s not something you can forget by accident and introduce a bug.
To use Swift’s functions from Objective-C:
Swift’s class should be extended from NSObject
Mark Swift’s:
a. @objcMembers class – to expose all public fields and methods
b. @objc class – to expose public init().
@objc public fields and methods – to expose them selectively
Swift’s method will be available by the next naming:
With::
For example: public func printHelloWorld(arg1: String, arg2:String) is reached through: [someObject printHelloWorldWithArg1: arg2:];
To expose a method to Objective-C, just write @objc before its name like this:
class MyController: UIViewController {
@objc func authenticateUser() {
}
}
That whole class is automatically Objective-C friendly because it inherits from UIViewController, but if you need it you can also explicitly make a class open to Objective-C by marking it @objc.
class ExampleClass: NSObject {
@objc var enabled: Bool {
@objc(isEnabled) get {
// Return the appropriate value
}
}
}
@objc private dynamic var originalProperty: String
@objc private static let keyPathsForValuesAffectingDependentProperty: Set = [
#keyPath(originalProperty)
]
@objc public var dependentProperty: String { return changeItSomehow(self.originalProperty) }
