How to use KVO in Swift

Issue #554

A class must inherit from NSObject, and we have 3 ways to trigger property change

Use setValue(value: AnyObject?, forKey key: String) from NSKeyValueCoding

class MyObjectToObserve: NSObject {
    var myDate = NSDate()
    func updateDate() {
        setValue(NSDate(), forKey: "myDate")
    }
}

Use willChangeValueForKey and didChangeValueForKey from NSKeyValueObserving

class MyObjectToObserve: NSObject {
    var myDate = NSDate()
    func updateDate() {
        willChangeValueForKey("myDate")
        myDate = NSDate()
        didChangeValueForKey("myDate")
    }
}

Use dynamic. See Swift Type Compatibility

You can also use the dynamic modifier to require that access to members be dynamically dispatched through the Objective-C runtime if you’re using APIs like key–value observing that dynamically replace the implementation of a method.

class MyObjectToObserve: NSObject {
    dynamic var myDate = NSDate()
    func updateDate() {
        myDate = NSDate()
    }
}

And property getter and setter is called when used. You can verify when working with KVO. This is an example of computed property

class MyObjectToObserve: NSObject {
    var backing: NSDate = NSDate()
    dynamic var myDate: NSDate {
        set {
            print("setter is called")
            backing = newValue
        }
        get {
            print("getter is called")
            return backing
        }
    }
}

Read more

• https://stackoverflow.com/questions/24092285/is-key-value-observation-kvo-available-in-swift/34873979#34873979