Performing Key-Value Observing with Combine
Expose KVO changes with a Combine publisher.
Overview
Several frameworks use key-value observing to notify your app of asynchronous changes. By converting your use of KVO from callbacks and closures to Combine, you can make your code more elegant and maintainable.
Monitoring Changes with KVO
In the following example, the type UserInfo supports KVO for its lastLogin property, as described in Using Key-Value Observing in Swift. The viewDidLoad() method uses the observe(_:options:changeHandler:) method to set up a closure that handles any change to the property. The closure receives an NSKeyValueObservedChange object that describes the change event, retrieves the newValue property, and prints it. The viewDidAppear(_:) method changes the value, which calls the closure and prints the message.
class UserInfo: NSObject {
@objc dynamic var lastLogin: Date = Date(timeIntervalSince1970: 0)
}
@objc var userInfo = UserInfo()
var observation: NSKeyValueObservation?
override func viewDidLoad() {
super.viewDidLoad()
observation = observe(\.userInfo.lastLogin, options: [.new]) { object, change in
print ("lastLogin now \(change.newValue!).")
}
}
override func viewDidAppear(_ animated: Bool) {
super.viewDidAppear(animated)
userInfo.lastLogin = Date()
}Converting KVO Code to Use Combine
To convert KVO code to Combine, replace the observe(_:options:changeHandler:) method with an NSObject.KeyValueObservingPublisher. You get an instance of this publisher by calling publisher(for:) on the parent object, as shown in the following example’s viewDidLoad() method:
class UserInfo: NSObject {
@objc dynamic var lastLogin: Date = Date(timeIntervalSince1970: 0)
}
@objc var userInfo = UserInfo()
var cancellable: Cancellable?
override func viewDidLoad() {
super.viewDidLoad()
cancellable = userInfo.publisher(for: \.lastLogin)
.sink() { date in print ("lastLogin now \(date).") }
}
override func viewDidAppear(_ animated: Bool) {
super.viewDidAppear(animated)
userInfo.lastLogin = Date()
}The KVO publisher produces elements of the observed type — in this case, Date — rather than NSKeyValueObservedChange. This saves you a step, because you don’t have to unpack the newValue from the change object, as in the first example.