ios - 为什么 addTarget 在 Swift 中没有被删除？

Question

当我从屏幕导航回来并再次进入屏幕时，nextTrackCommand 的目标被多次调用，即使我在 viewWillDisappear 中删除了目标。我做错了什么？

override func viewDidLoad() {
    super.viewDidLoad()

    UIApplication.shared.beginReceivingRemoteControlEvents()

    MPRemoteCommandCenter.shared().nextTrackCommand.addTarget { [unowned self] (_) -> MPRemoteCommandHandlerStatus in
        print("go to next track")
        return .success
    }
}

override func viewWillDisappear(_ animated: Bool) {
    MPRemoteCommandCenter.shared().nextTrackCommand.removeTarget(self)
}

Answer 1

addTarget 的过载您正在调用的不会将 self 添加为目标。它添加了一个您之前不知道的 NSObject 对象作为目标。 它返回这个对象。所以如果你想删除它，你应该得到它的返回值，把它保存在一个属性中，这样你就可以把它传递给removeTarget。

Call the addTarget(handler:) method to add a block to be called. Remove the handler by calling the removeTarget(_:) method, passing in the object returned by this method.

var target: NSObject?
override func viewDidLoad() {
    super.viewDidLoad()

    UIApplication.shared.beginReceivingRemoteControlEvents()

    target = MPRemoteCommandCenter.shared().nextTrackCommand.addTarget { [unowned self] (_) -> MPRemoteCommandHandlerStatus in
        print("go to next track")
        return .success
    }
}

override func viewWillDisappear(_ animated: Bool) {
    MPRemoteCommandCenter.shared().nextTrackCommand.removeTarget(target)
}

或者，调用 other overload这实际上允许您将 self 添加为目标。