Declarations in extensions cannot override yet error in Swift 4

Question

I have an extension:

public extension UIWindow {
    override public func topMostController()->UIViewController? { ... }
}

but for my

Answer 1

Swift 5

Actually there are few problems in OP code:

1. UIView (which is superclass of UIWindow) doesn't have method topMostController(), that why you can't override it.

2. Apple doesn't encourage override func inside extension:

   Extensions can add new functionality to a type, but they cannot override existing functionality.

3. Incase you still want to override function in extension, there are 2 ways:

[A] Mark your function with @objc dynamic func in parent class:

class Vehicle {
    @objc dynamic func run() { /* do something */ }
}

class Car: Vehicle { }

extension Car {
    override func run() { /* do another thing */ }
}

[B] Override function from build-in classes, which is descendant of NSObject.

 extension UIWindow {
    // UIWindow is a descendant of NSObject, and its superclass UIView has this function then you can override
    override open func becomeFirstResponder() -> Bool {
        ...
        return super.becomeFirstResponder()
    }
 }