How to add initializers in extensions to existing UIKit classes such as UIColor?

Question

The Swift documentation says that adding initializers in an extension is possible, and the example in the document is about adding an initializer to a struct. Xcode doe

Answer 1

Well, if you really, really, really want to override an initialiser, there is a way.

Before you read further: never do this to change UIKit behaviour. Why? It could confuse the heck out of someone that can't figure out why a UIColor initialiser isn't doing what it normally does. Only do it to fix a UIKit bug, or add functionality, etc.

I have used the following to patch several iOS bugs.

Code

extension UIColor {

    private static var needsToOverrideInit = true

    override open class func initialize() {

        // Only run once - otherwise subclasses will call this too. Not obvious.

        if needsToOverrideInit {
            let defaultInit = class_getInstanceMethod(UIColor.self, #selector(UIColor.init(red:green:blue:alpha:)))
            let ourInit = class_getInstanceMethod(UIViewController.self, #selector(UIColor.init(_red:_green:_blue:_alpha:)))
            method_exchangeImplementations(defaultInit, ourInit)
            needsToOverrideInit = false
        }

    }

    convenience init(_red: CGFloat, _green: CGFloat, _blue: CGFloat, _alpha: CGFloat) {

        // This is trippy. We swapped implementations... won't recurse.
        self.init(red: _red, green: _green, blue: _blue, alpha: _alpha)

        /////////////////////////// 
        // Add custom logic here // 
        ///////////////////////////         

    }

}

Explanation

This is using the dynamic nature of Objective-C, called from Swift, to swap method definition pointers at runtime. If you don't know what this means, or implications of it, it is probably a good idea to read up on the topic before you use this code.