ios - 如果 URL 快速失败，如何为 AVPlayer 重新加载数据

Question

我有 json 文件，其中包含一些单词的 URL(带有 .mp3)。某些 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)。

这个 URL 我用来播放单词的发音。所以，我要抛出 3 个步骤:

1. 查找特定单词的 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
}

}

这是一个 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。

如何实现以及在哪里实现？

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

我将使用 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，这将始终产生与初始化 AVPlayerItem 时相同的结果，status 默认情况下未知.

一旦与播放器关联，AVPlayerItem 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 addObserver 的“旧式”，但根据您的偏好，我会选择较新的 block 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 AVPlayer 实例上设置类似的观察者。

---

更新了完整示例- 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()
    }
}