swift - AVPlayerItem's canPlaySlowForward property never called

Question

I'm trying to enable slow playback rate for a video I'm playing using AVPlayerLayer. To do so the documentation states it will enable rate in the range of 0.0 - 1.0 if the AVPlayerItem returns true for canPlaySlowForward. This property is readonly, so you need to subclass AVPlayerItem and override this property to accomplish this. I've done that, but the video still does not playing at a slow rate, it's always the 1.0 rate. It never even calls the canPlaySlowForward property. Why is this?

import AVFoundation

class SlowMoPlayerItem: AVPlayerItem {

    override var canPlaySlowForward: Bool {
        return true
    }

}

Playing the video:

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()

The only other thing I do related to video/audio is prevent it from stopping background audio, via:

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

The video file does not include an audio track but this was still necessary to include.

Answer 1

Set the rate after calling play(). No need to subclass 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
}