Add a property observer on an existing property in an iOS library class

问题

In my SpriteKit game, I want to add a property observer on the fieldBitMask property for the SKPhysicsBody of my sprites. I want to be informed when the fieldBitMask property changes so that I can take some action.

I overrode SKPhysicsBody but when I tried assigning the overridden class to a sprite node like a normal SKPhysicsBody object I got a bunch of errors. I also thought about making an extension for SKPhysicsBody, but Apple documentation says, “Extensions can add new functionality to a type, but they cannot override existing functionality.” So it would seem that I can't override the fieldBitMask property this way to make it a property observer.

I know how to create a property observer in a new custom class which I create. But what is the best way to add a property observer to an existing property in a class that is part of the Apple library?

回答1:

I think you can use the Key Value Observing. You can add observer to a property changing.
You should implement observeValue(forKeyPath:of:change:context:) method in your observer class.

Here you can find Swift example ("Key-Value Observing" part).

回答2:

You can subclass SKPhysicsBody like that:

class Subclass: SKPhysicsBody {
    override var fieldBitMask: UInt32 {
        didSet {
            print(fieldBitMask)
        }
    }
}

来源：https://stackoverflow.com/questions/46484188/add-a-property-observer-on-an-existing-property-in-an-ios-library-class