在 SwiftUI/AVPlayer 中与 NotificationCenter/Combine 斗争

Question

我正在尝试在项目播放完毕后暂停我的 AVPlayer。使用 SwiftUI 执行此操作的最佳方法是什么？我不太了解通知，在哪里声明它们等。有没有办法为此使用组合？示例代码会很棒！提前谢谢你。

更新：

在下面答案的帮助下，我设法创建了一个类，它采用 AVPlayer 并在项目结束时发布通知。您可以通过以下方式订阅通知：

类：

import Combine
import AVFoundation

class PlayerFinishedObserver {

    let publisher = PassthroughSubject<Void, Never>()

    init(player: AVPlayer) {
        let item = player.currentItem

        var cancellable: AnyCancellable?
        cancellable = NotificationCenter.default.publisher(for: .AVPlayerItemDidPlayToEndTime, object: item).sink { [weak self] change in
            self?.publisher.send()
            print("Gotcha")
            cancellable?.cancel()
        }
    }
}

添加到您的结构中：

let finishedObserver: PlayerFinishedObserver

订阅一些观点：

.onReceive(finishedObserver.publisher) {
                print("Gotcha!")
            }

Answer 1

我找到了一个类似问题的解决方案：

1. 我创建了AVPlayer 的新子类；
2. 为currentItem添加了观察者；
3. 覆盖func observeValue，当玩家到达结束时间时为当前项目添加观察者；

这里是简化的例子：

import AVKit // for player
import Combine // for observing and adding as environmentObject

final class AudioPlayer: AVPlayer, ObservableObject {

    var songDidEnd = PassthroughSubject<Void, Never>() // you can use it in some View with .onReceive function

    override init() {
        super.init()
        registerObserves()
    }

    private func registerObserves() {
        self.addObserver(self, forKeyPath: "currentItem", options: [.new], context: nil)
        // example of using 
    }

    override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        // currentItem could be nil in the player. I add observer to exist item
        if keyPath == "currentItem", let item = currentItem {
            NotificationCenter.default.addObserver(self, selector: #selector(playerDidFinishPlaying(_:)), name: NSNotification.Name.AVPlayerItemDidPlayToEndTime, object: item)

            // another way, using Combine
            var cancellable: AnyCancellable?
            cancellable = NotificationCenter.default.publisher(for: .AVPlayerItemDidPlayToEndTime, object: item).sink { [weak self] _ in
                self?.songDidEnd.send()
                cancellable?.cancel()
            }
        }
        // other observers
    }

    @objc private func playerDidFinishPlaying(_ notification: Notification) {
        playNextSong() // my implementation, here you can just write: "self.pause()"
    }

}

更新：使用.onReceive 的简单示例（注意，我在没有 Playground/Xcode 的情况下编写了它，所以它可能会出错）：

struct ContentView: View {

    @EnvironmentObject var audioPlayer: AudioPlayer
    @State private var someText: String = "song is playing"

    var body: some View {
        Text(someText)
            .onReceive(self.audioPlayer.songDidEnd) { // maybe you need "_ in" here
                self.handleSongDidEnd()
        }
    }

    private func handleSongDidEnd() {
        print("song did end")
        withAnimation {
            someText = "song paused"
        }
    }

}

关于Combine 和AVPlayer：你可以看看我的question，在那里你会看到一些观察播放时间的方法和使用SwiftUI 中的滑块倒带时间的功能。

我正在使用AudioPlayer 的一个实例，控制播放/暂停功能或更改currentItem（这意味着设置另一首歌曲），如下所示：

class SceneDelegate: UIResponder, UIWindowSceneDelegate {

    // other staff
    func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
        let homeView = ContentView()
            .environmentObject(AudioPlayer())

        // other staff of SceneDelegate
    }
}