CODEWITHSUNDEEP
iosswift
Ninja
Ninja

Reputation: 358

Why can i use self when I initialize property with a closure?

Official docs (Swift 4.1) says:

If you use a closure to initialize a property, remember that the rest of the instance has not yet been initialized at the point that the closure is executed. This means that you cannot access any other property values from within your closure, even if those properties have default values. You also cannot use the implicit self property, or call any of the instance’s methods.

So I guess i need to use lazy var,today I created a button 

let loginRegisterButton: UIButton = {
    let button = UIButton(type: .system)
    button.backgroundColor = UIColor.rgb(red: 80, green: 101, blue: 161)
    button.setTitle("Register", for: .normal)
    button.translatesAutoresizingMaskIntoConstraints = false
    button.setTitleColor(UIColor.white, for: .normal)
    button.titleLabel?.font = UIFont.boldSystemFont(ofSize: 16)
    button.addTarget(self , action: #selector(handleRegister), for: .touchUpInside)
    return button
}()

so I just used self in addTarget method and XCode did not seem to bother even more program went without any errors. Everything worked out well.So Why can i use self when I initialize property with a closure? Is there some changes or maybe I missed something?

Upvotes: 4

Views: 1335

Answers (2)

rob mayoff
rob mayoff

Reputation: 385580

Weirdly—perhaps buggily—the self in your closure evaluates to a function object. (Hamish explained in a comment that the function is a curried form of NSObjectProtocol.self.) However, when you later ask the button for its target, it reports +[NSNull null]:

let loginRegisterButton: UIButton = {
    let button = UIButton(type: .system)
    button.backgroundColor = .red
    button.setTitle("Register", for: .normal)
    button.translatesAutoresizingMaskIntoConstraints = false
    button.setTitleColor(UIColor.white, for: .normal)
    button.titleLabel?.font = UIFont.boldSystemFont(ofSize: 16)

    print(self)
    // Output: (Function)

    button.addTarget(self, action: #selector(handleRegister), for: .touchUpInside)

    let target = button.allTargets.first!.base
    print(target, type(of: target))
    // Output: <null> NSNull

    return button
}()

(Tested using Xcode 9.3.1.)

The function gets turned into NSNull as follows: the function object is allocated just before the call to addTarget, to be passed as the target argument. The loginRegisterButton-creating closure holds a strong reference to the newly-allocated function.

The button stores the target as a zeroing weak reference.

After addTarget returns, the closure releases its strong reference to the function. This was the only strong reference to the function, so Swift deallocates the function. This sets the button's weak reference to the function to nil.

When we then ask the button for allTargets, it constructs an NSSet. Since NSSet cannot hold nil directly, the button puts +[NSNull null] in the set instead.

Upvotes: 4

Alladinian
Alladinian

Reputation: 35616

The target in your code is probably nil, the reason that this might work is that:

...If you specify nil, UIKit searches the responder chain for an object that responds to the specified action message and delivers the message to that object

(from the documentation of the method)

You can verify that the target is actually nil by setting a breakpoint during or after your controller's init and by inspecting the _targetActions array (in the debugger's variables view) of the button (you can see that target's address is 0x0).

In your example, loginRegisterButton is set during the controller's initialization which means that there is no self yet (in other words you cannot guarantee that all instance variables are initialized at this point). The way lazy solves this is that the actual assignment is deferred on first access.

Upvotes: 4

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