Methods Swizzling in Swift

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Methods Swizzling is very powerful, dangerous technique and at the same time very easy to use.
Swift is static typed and safe programming language, and I it 😍😘❤️. But sometimes you git a wall where you need a bit of dynamic behaviour.

As an example I want to override description method of a UIColor. I can't do that because methods overriding doesn't work in Objc categories :(

So the solution is to use Method Swizzling. This doesn't sound like a Swifty approach, but actually it is. Here is the solution.

public extension UIColor {

  func colorDescription() -> String {
    let color = "Some Custom text Here"
    return color
  }

  public class func swizzleDesription() {
    let instance: UIColor = UIColor.redColor()
    let aClass: AnyClass! = object_getClass(instance)

    let originalMethod = class_getInstanceMethod(aClass, "description")
    let swizzledMethod = class_getInstanceMethod(aClass, "colorDescription")
    method_exchangeImplementations(originalMethod, swizzledMethod)
  }
}

UIColor.swizzleDesription()
print(UIColor.redColor())

It just works 🙌🎉.
P.S. - The class has to inherit from NSObject and has dynamic runtime behaviour

But we can do better. The swizzling technique is not unique to just UIColor class, but it's a generic and can be used with any NSObject subclass. Let's do refactoring baby re-🏭-ing ))).


// Swizzling
extension NSObject {

  class func swizzleMethods(origSelector: Selector, withSelector: Selector, 
    forClass: AnyClass) {

    let originalMethod = class_getInstanceMethod(forClass, origSelector)
    let swizzledMethod = class_getInstanceMethod(forClass, withSelector)
    method_exchangeImplementations(originalMethod, swizzledMethod)
  }

  func swizzleMethods(origSelector: Selector, withSelector: Selector) {
    let aClass: AnyClass! = object_getClass(self)
    NSObject.swizzleMethods(origSelector, withSelector: withSelector, forClass: aClass)
  }
}

// Use Swizzling
public extension UIColor {

  func colorDescription() -> String {
    return "Some Custom text Here"
  }

  public class func swizzleDesription() {
    let instance: UIColor = UIColor.redColor()
   instance.swizzleMethods("colorDescription", withSelector: "description")
  }
}

UIColor.swizzleDesription()
print(UIColor.redColor())

That's it, Thanks :) 🙇