swiftui - 如何访问符合 UIViewRepresentable 的 SwiftUI 包装器中的属性？

Question

我想控制 AVPlayer 的一个实例，该实例被实例化并分配给类 VideoPlayer 中的一个属性，该类符合 UIView。为了确保清楚我的意图，我在处理 SwiftUI 和 UIViewRepresentable 包装器时将一个已知模式组合在一起，如下所示:

class VideoPlayer: UIView {
    private let playerLayer = AVPlayerLayer()
    private var previewTimer: Timer?
    var previewLength: Double
    var player: AVPlayer?
    
    init(frame: CGRect, url: URL, previewLength:Double) {
        self.previewLength = previewLength
        
        super.init(frame: frame)
        self.player = AVPlayer(url: url)
        self.player?.volume = 0
        self.player?.play()

        NotificationCenter.default.addObserver(forName: .AVPlayerItemDidPlayToEndTime, object: self.player?.currentItem, queue: .main) { [weak self] _ in
            self?.player?.seek(to: CMTime.zero)
            self?.player?.play()
        }
        
        playerLayer.player = player
        playerLayer.videoGravity = .resizeAspectFill
        playerLayer.backgroundColor = UIColor.black.cgColor
        
        previewTimer = Timer.scheduledTimer(withTimeInterval: previewLength, repeats: true, block: { (timer) in
            self.player?.seek(to: CMTime(seconds: 0, preferredTimescale: CMTimeScale(1)))
        })
        
        layer.addSublayer(playerLayer)
    }
    
    required init?(coder: NSCoder) {
        self.previewLength = 15
        super.init(coder: coder)
    }

    override func layoutSubviews() {
        super.layoutSubviews()
        playerLayer.frame = bounds
    }
}

包装器 MyWrapper 将 UIView 暴露给 SwiftUI，并且 wrapper 可以具有公共(public)属性，例如 myState :

struct MyWrapper: UIViewRepresentable {
    var videoURL: URL
    var previewLength: Double
    @Binding var mySate: Bool

    func makeUIView(context: Context) -> UIView {
        let videoPlayer = VideoPlayer(frame: .zero, url: videoURL, previewLength: previewLength)
        return videoPlayer
    }
    
    func updateUIView(_ uiView: UIView, context: Context) {
        if myState {
          // do something
        } else {
          // do something else
        }
    }
}

如果需要控制 VideoPlayer 实例的属性，我们可能会想在 updateUIView 方法中直接从 UIView 对象访问属性>。因此，假设我们想要访问 player 属性:

func updateUIView(_ uiView: UIView, context: Context) {
    if myState {
      uiView.player.play()
    }
}

这失败并出现错误 Value of type 'UIView' has no member 'player'。

为了克服这个问题，在 MyWrapper 中创建了一个代理来引用 AVPlayer 实例，如下所示:

struct MyWrapper: UIViewRepresentable {
    var videoURL: URL
    var previewLength: Double
    @State var player: AVPlayer?
    @Binding var play: Bool

    func makeUIView(context: Context) -> UIView {
        let videoPlayer = VideoPlayer(frame: .zero, url: videoURL, previewLength: previewLength)
        DispatchQueue.main.async {
            self.player = videoPlayer.player
        }
        return videoPlayer
    }
    
    func updateUIView(_ uiView: UIView, context: Context) {
        if play {
            self.player?.play()
        } else {
            self.player?.pause()
            self.player?.rate = 0
        }
    }
}

Obs:我们使用 DispatchQueue.main 绕过警告 在 View 更新期间修改状态，这将导致未定义的行为 将在 makeUIView 中抛出。

这种方法有效，但我没有在 Apple 中找到任何文档来确认在处理 SwiftUI 和 UIViewRepresentable 包装器时这是正确的方法，因为 SwiftUI 是一个黑盒子。

因此，出于这个原因，我想知道如何访问符合 UIViewRepresentable 的 SwiftUI 包装器中的属性？

Answer 1

UIViewRepresentable 推断类型，在这种情况下，从声明的返回/参数类型推断类型，因此只需明确突出显示代表的类型

struct MyWrapper: UIViewRepresentable {
    var videoURL: URL
    var previewLength: Double
    @Binding var mySate: Bool

    func makeUIView(context: Context) -> VideoPlayer {
        let videoPlayer = VideoPlayer(frame: .zero, url: videoURL, previewLength: previewLength)
        return videoPlayer
    }
    
    func updateUIView(_ uiView: VideoPlayer, context: Context) {
        if myState {
          uiView.player.play()    // now it knows that uiView is-a VideoPlayer
        } else {
          // do something else
        }
    }
}