将 swift 中的闭包作为参数传递给 function 中的选择器使用

Question

I am trying to create a generic button creation function into which I pass a closure that represents the action that results as a result of clicking on the button. My code is below. However, I get the following error: Argument of #selector cannot refer to property. Any suggestions for a workaround? I don't want to write separate functions for which everything else is the same except for the target action.

 func myButton(textColor tColor:UIColor , title:String, 
                 _ buttonFcn: (UIButton) -> Void,   
                 titleSize:CGFloat=30) -> UIButton {
    let newButton = UIButton(type: .System)
    let bgColor = UIColor(red:204/255, green:204/255, blue:204/255, alpha:1.0)
    newButton.backgroundColor = bgColor

    newButton.setTitle(title, forState: .Normal)
    newButton.setTitleColor(tColor, forState: .Normal)

    newButton.titleLabel?.font = newButton.titleLabel?.font.fontWithSize(titleSize)


    newButton.addTarget(self,  action:#selector(buttonFcn),
                        forControlEvents: 
                           UIControlEvents.TouchUpInside)

    return newButton
}

Answer 1

The problem is that the target-action mechanism is an Objective-C mechanism, and therefore is predicated on the notion that the action selector is a method of an object . You need, therefore, to have some NSObject-based object that has this function as a method , and which can then serve as the target.

Thus, if what differs in every case is the target and the action, what you need to pass is a reference to the target along with the selector string . Swift will squawk at this, but if you know how to form a selector string correctly you can certainly get away with it; you just won't be able to use the #selector syntax, and so you will risk crashing if you form the selector string incorrectly. But it's the kind of thing we used to do all the time in the old Objective-C days, so go right ahead if that's your aim.

Totally artificial but working example:

func buttonMaker(target:NSObject, selectorString:String) -> UIButton {
    let b = UIButton(type:.system)
    b.setTitle("Testing", for: .normal)
    b.addTarget(target, action: Selector(selectorString), for: .touchUpInside)
    b.sizeToFit()
    return b
}

And here's how to call it from a view controller:

func doButton(_ sender:Any) {
    print("ha!")
}

override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view, typically from a nib.
    let b = buttonMaker(target:self, selectorString:"doButton:")
    b.frame.origin = CGPoint(x:100, y:100)
    self.view.addSubview(b)
}

And when we tap the button, we don't crash (rather, we print "ha"), because I know how to make selector strings correctly. But, as you can see, to accomplish this I had to give up the use of #selector altogether, so safety is out the window. If I had written my selector string incorrectly — for instance, if I had spelled it wrong, or omitted the colon — we'd have crashed on the button tap, just like we used to all the time before Swift #selector and Objective-C @selector were invented.

Answer 2

If your deployment target is iOS 14 or later, you can use the addAction method instead of addTarget . The addAction method lets you use a closure instead of a selector:

func myButton(
    textColor: UIColor,
    title: String,
    titleSize: CGFloat = 30,
    _ handler: @escaping (UIButton) -> Void
) -> UIButton {
    let button = UIButton(type: .system)
    button.backgroundColor = UIColor(red: 204/255, green: 204/255, blue: 204/255, alpha: 1.0)
    button.setTitle(title, for: .normal)
    button.setTitleColor(textColor, for: .normal)
    button.titleLabel?.font = button.titleLabel?.font.withSize(titleSize)

    let action = UIAction { action in
        guard let button = action.sender as? UIButton else { return }
        handler(button)
    }
    button.addAction(action, for: .touchUpInside)

    return button
}

iOS 14 was released on 2020-09-16 and supports iPhone 6S and later devices.