To implement KVC support for a property in Swift 4, you need two things:
Since the current implementation of KVC is written in Objective-C, you need the @objc annotation on your property so that Objective-C can see it. This also means that the property's type needs to be compatible with Objective-C.
In addition to exposing the property to Objective-C, you will need to set up your notifications in order for observers to be notified when the property changes. There are three ways to do this:
For stored properties, the easiest thing to do is to add the dynamic keyword like so:
@objc dynamic var foo: String
This will allow Cocoa to use Objective-C magic to automagically generate the needed notifications for you, and is usually what you want. However, if you need finer control, you can also write the notification code manually:
@objc private static let automaticallyNotifiesObserversOfFoo = false
@objc var foo: String {
willSet { self.willChangeValue(for: \.foo) }
didSet { self.didChangeValue(for: \.foo) }
}
The automaticallyNotifiesObserversOf<property name> property is there to signify to the KVC/KVO system that we are handling the notifications ourselves and that Cocoa shouldn't try to generate them for us.
Finally, if your property is not stored, but rather depends on some other property or properties, you need to implement a keyPathsForValuesAffecting<your property name here> method like so:
@objc dynamic var foo: Int
@objc dynamic var bar: Int
@objc private static let keyPathsForValuesAffectingBaz: Set<String> = [
#keyPath(foo), #keyPath(bar)
]
@objc var baz: Int { return self.foo + self.bar }
In the example above, an observer of the baz property will be notified when the value for foo or the value for bar changes.
@objc. I dont understand how does it work.