Swift - UIButton overriding setSelected

Question

I'm making a UIButton subclass in Swift to perform custom drawing and animation on selection

What would be the equivalent in Swift of overriding - (void)setSelected:(BOOL)selected in ObjC?

I tried

override var selected: Bool

so I could implement an observer but I get

Cannot override with a stored property 'selected'

Answer 1

Like others mentioned you can use willSet to detect changes. In an override, however, you do not need assign the value to super, you are just observing the existing change.

A couple things you can observe from the following playground:

1. Overriding a property for willSet/didSet still calls super for get/set. You can tell because the state changes from .normal to .selected.
2. willSet and didSet are called even when the value is not changing, so you will probably want do the compare the value of selected to either newValue in willSet or oldValue in didSet to determine whether or not to animate.

import UIKit

class MyButton : UIButton {

    override var isSelected: Bool {
        willSet {
            print("changing from \(isSelected) to \(newValue)")
        }

        didSet {
            print("changed from \(oldValue) to \(isSelected)")
        }
    }
}

let button = MyButton()

button.state == .normal
button.isSelected = true // Both events fire on change.
button.state == .selected
button.isSelected = true // Both events still fire.