How do I replace target/action with callbacks in Swift?

NSButton uses the target/action pattern: a button : NSButton has two properties button.target and button.action which specify what to call when the button is clicked.

This is rather different to the way I’m used to programming: I’d like to just set button.onClick = { ... do some stuff ... }. In short, this is how to do it:

class CallbackWrapper {
    var callback : () -> ();
    init(callback: @escaping () -> ()) {
        self.callback = callback;
    }
    @objc public func callCallback() {
        self.callback();
    }
}

func setTargetActionCallback(control: NSControl, callback: @escaping () -> ()) {
    let wrapper = CallbackWrapper(callback: callback);
    control.target = wrapper;
    control.action = #selector(CallbackWrapper.callCallback);
    let key = UnsafeMutablePointer<Int8>.allocate(capacity: 1);
    objc_setAssociatedObject(control, key, wrapper, .OBJC_ASSOCIATION_RETAIN);
}

Explanation:

• We wrap the callback in an object, so wrapper.callCallback() is the same as callback()
• We use wrapper as the target and its callCallback method as the action
• We set the wrapper as an associated object of the button. This is important because button.target is a weak reference, for questionable design reasons!

Thanks to Mike Ash and Ham Chapman for finding that post.

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