Swift AVPlayerItem'；s canPlaySlowForward属性从未调用

我正在尝试为正在使用

AVPlayerLayer

播放的视频启用慢速播放速率。为此，文档说明，如果

AVPlayerItem

返回

true

for

canPlaySlowForward

，它将在0.0-1.0范围内启用

rate

。此属性是只读的，因此您需要子类化

AVPlayerItem

并重写此属性以完成此操作。我已经这样做了，但视频播放速度仍然不慢，始终是1.0。它甚至从不调用

canPlaySlowForward

属性。为什么会这样

import AVFoundation

class SlowMoPlayerItem: AVPlayerItem {

    override var canPlaySlowForward: Bool {
        return true
    }

}

播放视频：

let asset = AVAsset(URL: NSBundle.mainBundle().URLForResource("some-video", withExtension: "mp4")!)
let playerItem = SlowMoPlayerItem(asset: asset)
let player = AVPlayer(playerItem: playerItem)
player.rate = 0.5
player.muted = true

let playerLayer = AVPlayerLayer(player: player)
playerLayer.frame = self.view.layer.bounds
self.view.layer.addSublayer(playerLayer)
player.play()

我所做的另一件与视频/音频相关的事情是通过以下方式防止其停止背景音频：

do {
    try AVAudioSession.sharedInstance().setCategory(AVAudioSessionCategoryAmbient)
}

---

视频文件不包括音频曲目，但这仍然需要包括。

在调用

play（）

后设置

速率。无需子类化
AVPlayerItem

func playVideo() {
    let asset = AVAsset(URL: NSBundle.mainBundle().URLForResource("SampleVideo", withExtension: "mp4")!)
    let playerItem = AVPlayerItem(asset: asset)
    let player = AVPlayer(playerItem: playerItem)
    let playerLayer = AVPlayerLayer(player: player)
    playerLayer.frame = self.view.bounds
    self.view.layer.addSublayer(playerLayer)
    player.play()
    player.rate = 0.5
}