在onChange事件中设置后，AVPlayer的currentItem始终为nil的问题排查

我尝试播放从onChange事件中设置的流媒体URL，闭包内打印的URL正常（非nil），但在View的body渲染阶段，AVPlayer的currentItem却始终为nil。相关代码如下：

struct VideoPlayerContainerView: View {
    @EnvironmentObject private var store: AppStore
    private var player = AVPlayer()
    @State private var urlToPlay: URL?
    @State private var playingUrl: URL?
    
    var body: some View {

            let currentPlayerUrl = ((player.currentItem?.asset) as? AVURLAsset)?.url
            print("asset loaded in the VideoPlayer ⬇")
            print(currentPlayerUrl ?? "no asset!") // 始终为nil
        
        return ZStack {
            VideoPlayerView(player: player)
        }
        .onChange(of: self.store.state.videos.playingVideo?.stream?.best.url) {
            [oldValue = self.store.state.videos.playingVideo?.stream?.best.url] newValue in

            print("- - - - - - - - - - - - ")
            
            self.urlToPlay = newValue
            if (self.urlToPlay != self.playingUrl) {
                let newPlayerItem = self.urlToPlay != nil ? AVPlayerItem(url: self.urlToPlay!) : nil
                player.replaceCurrentItem(with: newPlayerItem)
                self.playingUrl = self.urlToPlay
                
                print("newPlayerItem ⬇")
                print(newPlayerItem ?? "nil")
                print("url that should play now ⬇")
                let currentPlayerUrl = ((player.currentItem?.asset) as? AVURLAsset)?.url
                print(currentPlayerUrl ?? "nil") // 不为nil
            }
        }
    }
}

struct VideoPlayerView: View {
    let player: AVPlayer
    var body: some View {
        VideoPlayer(player: player)
    }
}

原因分析 #

核心问题在于你声明的private var player = AVPlayer()是普通存储属性。在SwiftUI的View生命周期里，每次View因为状态变化重新渲染时，这个普通属性都会被重新初始化——也就是说，你在onChange闭包里给旧player设置的currentItem，到了渲染阶段已经变成了一个全新的player实例，自然没有任何currentItem，所以打印结果始终是nil。

拆解来看：

1. onChange触发时，你操作的是当前View实例的player，设置完currentItem后打印正常；
2. 但onChange执行后，playingUrl或urlToPlay的更新会触发View重新计算body，此时新的player实例被创建，旧的player被丢弃，渲染阶段打印的是新player的状态，所以显示nil。

解决方案 #

要解决这个问题，关键是让player实例在View重新渲染时保持不变，有两种常用实现方式：

方法一：用@StateObject包装AVPlayer #

因为AVPlayer本身不是ObservableObject，所以需要先做一层包装：

class PlayerWrapper: ObservableObject {
    let player = AVPlayer()
}

struct VideoPlayerContainerView: View {
    @EnvironmentObject private var store: AppStore
    @StateObject private var playerWrapper = PlayerWrapper() // @StateObject保证实例不被重置
    @State private var playingUrl: URL?
    
    var body: some View {
        let currentPlayerUrl = ((playerWrapper.player.currentItem?.asset) as? AVURLAsset)?.url
        print("asset loaded in the VideoPlayer ⬇")
        print(currentPlayerUrl ?? "no asset!")
        
        return ZStack {
            VideoPlayerView(player: playerWrapper.player)
        }
        .onChange(of: store.state.videos.playingVideo?.stream?.best.url) { newValue in
            guard let newValue = newValue, newValue != playingUrl else { return }
            
            let newPlayerItem = AVPlayerItem(url: newValue)
            playerWrapper.player.replaceCurrentItem(with: newPlayerItem)
            playingUrl = newValue
            
            print("- - - - - - - - - - - - ")
            print("newPlayerItem ⬇")
            print(newPlayerItem)
            print("url that should play now ⬇")
            let currentPlayerUrl = ((playerWrapper.player.currentItem?.asset) as? AVURLAsset)?.url
            print(currentPlayerUrl ?? "nil")
        }
    }
}

方法二：直接用@State持有AVPlayer #

利用@State可以在View生命周期中保持引用类型实例的特性，直接把player声明为@State属性：

struct VideoPlayerContainerView: View {
    @EnvironmentObject private var store: AppStore
    @State private var player = AVPlayer() // @State保证实例不被重置
    @State private var playingUrl: URL?
    
    var body: some View {
        let currentPlayerUrl = ((player.currentItem?.asset) as? AVURLAsset)?.url
        print("asset loaded in the VideoPlayer ⬇")
        print(currentPlayerUrl ?? "no asset!")
        
        return ZStack {
            VideoPlayerView(player: player)
        }
        .onChange(of: store.state.videos.playingVideo?.stream?.best.url) { newValue in
            guard let newValue = newValue, newValue != playingUrl else { return }
            
            let newPlayerItem = AVPlayerItem(url: newValue)
            player.replaceCurrentItem(with: newPlayerItem)
            playingUrl = newValue
            
            print("- - - - - - - - - - - - ")
            print("newPlayerItem ⬇")
            print(newPlayerItem)
            print("url that should play now ⬇")
            let currentPlayerUrl = ((player.currentItem?.asset) as? AVURLAsset)?.url
            print(currentPlayerUrl ?? "nil")
        }
    }
}

内容的提问来源于stack exchange，提问作者zakdances