如果 URL 快速失败，如何为 AVPlayer 重新加载数据

Question

我有 json 文件，其中包含一些单词 URL（带有 .mp3）。有些 URL 无效（或有效，但返回错误，因此我无论如何都无法获取数据）。

我用这个 URL 来播放单词的发音。所以，我分三步走：

1. 查找某个单词的 URL。如果找不到，那么什么也不会发生
2. 使用此 URL 初始化 AVPlayerItem 并准备 AVPlayer。当用户按下时，不仅仅是等待。
3. 当用户按下单词时播放声音

所以，首先，我正在准备我的AVPlayer，以避免播放延迟。

我对多线程有点困惑，我不明白我应该在哪里检查我是否能够播放这个声音，或者我应该使用下一个 URL。

代码：

extension WordCell {

func playPronunciation() {
    player?.play()
    player?.seek(to: .zero)
}

func prepareForPronunciation() {
    if let word = myLabel.text {
        UIApplication.shared.isNetworkActivityIndicatorVisible = true
        DispatchQueue.global(qos: .userInteractive).async { [weak self] in
            let foundURL = self?.findURL(for: word)
            if let url = foundURL {
                let playerItem = AVPlayerItem(url: url)

                //here "playerItem.status" always equals .unknown (cause not initialized yet)
                if playerItem.status == .failed {
                     //self?.giveNextUrl() - will do some code there
                }
                self?.player = AVPlayer(playerItem: playerItem)
                self?.player!.volume = 1.0
            }
            // this is also not correct, because networking continueing
            // but i don't know where to place it
            DispatchQueue.main.async {
                UIApplication.shared.isNetworkActivityIndicatorVisible = false
            }
        }
    }
}

// right now i take URL from file, where there is only one.
// but i will use file with a few URL for one word
private func findURL(for word: String) -> URL? {

    if let path = Bundle.main.path(forResource: "data", ofType: "json") {
        do {
            let data = try Data(contentsOf: URL(fileURLWithPath: path), options: .mappedIfSafe)
            let jsonResult = try JSONSerialization.jsonObject(with: data, options: .mutableLeaves)
            if let jsonResult = jsonResult as? [String: String] {
                if let url = jsonResult[word] {
                    return URL(string: url)
                } else {
                    return nil
                }
            }
        } catch {
            return nil
        }
    }
    return nil
}

}

这是 json 文件，每个单词有几个 URL

"abel": [
    "http://static.sfdict.com/staticrep/dictaudio/A00/A0015900.mp3",
    "http://img2.tfd.com/pron/mp3/en/US/d5/d5djdgdyslht.mp3",
    "http://img2.tfd.com/pron/mp3/en/UK/d5/d5djdgdyslht.mp3",
    "http://www.yourdictionary.com/audio/a/ab/abel.mp3"
],
"abele": [
    "http://www.yourdictionary.com/audio/a/ab/abele.mp3",
    "http://static.sfdict.com/staticrep/dictaudio/A00/A0016300.mp3",
    "http://www.oxforddictionaries.com/media/english/uk_pron/a/abe/abele/abele__gb_2_8.mp3",
    "http://s3.amazonaws.com/audio.vocabulary.com/1.0/us/A/1B3JGI7ALNB2K.mp3",
    "http://www.oxforddictionaries.com/media/english/uk_pron/a/abe/abele/abele__gb_1_8.mp3"
],

所以，我需要获取第一个 URL 并检查它。如果失败，则在 URL 结束时采取另一个并检查...等等，或者找到一些有效的 URL。所有这些事情都必须在 AVPlayer 尝试播放声音之前完成。

如何实施以及在哪里实施？

请用简单的语言讲述和描述解决方案，因为我是快速和多线程的初学者。

Answer 1

我会使用该AVPlayerItem.Status属性来查看它何时失败。在当前代码中，您在创建项目后立即检查状态，这将始终产生与默认情况下AVPlayerItem初始化时相同的结果。statusunknown

AVPlayerItem一旦您与玩家关联，它们就会排队。为了能够跟踪状态变化，您需要设置一个观察者。

文档https://developer.apple.com/documentation/avfoundation/avplayeritem仍然建议使用“旧式”，addObserver但根据您的偏好，我会选择较新的块式。

// make sure to keep a strong reference to the observer (e.g. in your controller) otherwise the observer will be de-initialised and no changes updates will occur
var observerStatus: NSKeyValueObservation?


// in your method where you setup your player item
observerStatus = playerItem.observe(\.status, changeHandler: { (item, value) in
    debugPrint("status: \(item.status.rawValue)")
    if item.status == .failed {
        // enqueue new asset with diff url
    }
})

您也可以在实例上设置类似的观察者AVPlayer。

更新了完整的示例 - 这段代码远非完美，但展示了观察者的好处

import UIKit
import AVFoundation

class ViewController: UIViewController {
    var observerStatus: NSKeyValueObservation?
    var currentTrack = -1
    let urls = [
        "https://sample-videos.com/audio/mp3/crowd-cheerin.mp3", // "https://sample-videos.com/audio/mp3/crowd-cheering.mp3"
        "https://sample-videos.com/audio/mp3/wave.mp3"
    ]
    var player: AVPlayer? {
        didSet {
            guard let p = player else { return debugPrint("no player") }
            debugPrint("status: \(p.currentItem?.status == .unknown)") // this is set before status changes from unknown
        }
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        nextTrack()
    }

    func nextTrack() {
        currentTrack += 1
        guard let url = URL(string: urls[currentTrack]) else { return }
        let item = AVPlayerItem(url: url)

        observerStatus = item.observe(\.status, changeHandler: { [weak self] (item, value) in
            switch item.status {
            case .unknown:
                debugPrint("status: unknown")
            case .readyToPlay:
                debugPrint("status: ready to play")
            case .failed:
                debugPrint("playback failed")
                self?.nextTrack()
            }
        })

        if player == nil {
            player = AVPlayer(playerItem: item)
        } else {
            player?.replaceCurrentItem(with: item)
        }
        player?.play()
    }
}