UIView: how does the appearance() proxy work? - ios

Question

I have created a simple custom UIView:
final class TestView: UIView {
var testColor: UIColor = .white {
didSet {
backgroundColor = testColor
}
}
}
Then I wrote this in my view controller:
import UIKit
class ViewController: UIViewController {
#IBOutlet weak var testView: TestView!
#IBOutlet weak var testView2: TestView!
override func viewDidLoad() {
super.viewDidLoad()
DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + 3) {
TestView.appearance().testColor = .red
}
}
}
By doing this, I get an error:
Could you help me understanding what's wrong here and how to implement the UIAppearance proxy for any custom UIView?
Thank you for your help

Answer 1

You need to make the property #objc and dynamic:
final class TestView: UIView {
#objc dynamic var testColor: UIColor? = .white {
didSet {
backgroundColor = testColor
}
}
}
Worth noting: the UIAppearance proxy does NOT affect views which are already part of the view hierarchy.
So, in your example, adding #objc dynamic to your property will get rid of the crash, but you will not see any change to the two #IBOutlet views.
If you call it as part of viewDidLoad() (instead of DispatchQueue.main.asyncAfter):
override func viewDidLoad() {
super.viewDidLoad()
TestView.appearance().testColor = .red
}
The two #IBOutlet views will get the red background.
Or, if you add a new view to the hierarchy, it will get the red background:
class AppearanceTestViewController: UIViewController {
#IBOutlet weak var testView: TestView!
#IBOutlet weak var testView2: TestView!
override func viewDidLoad() {
super.viewDidLoad()
DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + 3) {
TestView.appearance().testColor = .red
self.addAnotherTestView()
}
}
func addAnotherTestView() -> Void {
let v = TestView()
v.translatesAutoresizingMaskIntoConstraints = false
view.addSubview(v)
NSLayoutConstraint.activate([
v.widthAnchor.constraint(equalToConstant: 240.0),
v.heightAnchor.constraint(equalTo: v.widthAnchor),
v.centerXAnchor.constraint(equalTo: view.centerXAnchor),
v.centerYAnchor.constraint(equalTo: view.centerYAnchor),
])
// this newly added view WILL have a red background
}
}