I have a method that changes the audio track played by my app's AVPlayer
and also sets MPNowPlayingInfoCenter.defaultCenter().nowPlayingInfo
for the new track:
func setTrackNumber(trackNum: Int) {
self.trackNum = trackNum
player.replaceCurrentItemWithPlayerItem(tracks[trackNum])
var nowPlayingInfo: [String: AnyObject] = [ : ]
nowPlayingInfo[MPMediaItemPropertyAlbumTitle] = tracks[trackNum].albumTitle
nowPlayingInfo[MPMediaItemPropertyTitle] = "Track \(trackNum)"
...
MPNowPlayingInfoCenter.defaultCenter().nowPlayingInfo = nowPlayingInfo
print("Now playing local: \(nowPlayingInfo)")
print("Now playing lock screen: \(MPNowPlayingInfoCenter.defaultCenter().nowPlayingInfo)")
}
I call this method when the user explicitly selects an album or track and when a track ends and the next one automatically starts. The lock screen correctly shows the track metadata when the user sets an album or track but NOT when a track ends and the next one is automatically set.
I added print statements to make sure I was correctly populating the nowPlayingInfo
dictionary. As expected, the two print statements print the same dictionary content when this method is called for a user-initiated change of album or track. However, in the case when the method is called after an automatic track change, the local nowPlayingInfo
variable shows the new trackNum
whereas MPNowPlayingInfoCenter.defaultCenter().nowPlayingInfo
shows the previous trackNum
:
Now playing local: ["title": Track 9, "albumTitle": Test Album, ...]
Now playing set: Optional(["title": Track 8, "albumTitle": Test Album, ...]
I discovered that when I set a breakpoint on the line that sets MPNowPlayingInfoCenter.defaultCenter().nowPlayingInfo
to nowPlayingInfo
, then the track number is correctly updated on the lock screen. Adding sleep(1)
right after that line also ensures that the track on the lock screen is correctly updated.
I have verified that nowPlayingInfo
is always set from the main queue. I've tried explicitly running this code in the main queue or in a different queue with no change in behavior.
What is preventing my change to MPNowPlayingInfoCenter.defaultCenter().nowPlayingInfo
? How can I make sure that setting MPNowPlayingInfoCenter.defaultCenter().nowPlayingInfo
always updates the lock screen info?
EDIT
After going through the code for the Nth time thinking "concurrency", I've found the culprit. I don't know why I didn't get suspicious about this earlier:
func playerTimeJumped() {
let currentTime = currentItem().currentTime()
dispatch_async(dispatch_get_main_queue()) {
MPNowPlayingInfoCenter.defaultCenter().nowPlayingInfo?[MPNowPlayingInfoPropertyElapsedPlaybackTime] = CMTimeGetSeconds(currentTime)
}
}
NSNotificationCenter.defaultCenter().addObserver(
self,
selector: "playerTimeJumped",
name: AVPlayerItemTimeJumpedNotification,
object: nil)
This code updates the lock screen's time elapsed when the user scrubs or skips forward/back. If I comment it out, the nowPlayingInfo
update from setTrackNumber
works as expected under any condition.
Revised questions: how are these two pieces of code interacting when they're both run on the main queue? Is there any way I can do a nowPlayingInfo
update on AVPlayerItemTimeJumpedNotification
given that there will be a jump when there's a call on setTrackNumber
?
The problem is that nowPlayingInfo
is updated in two places at the same time when the track automatically changes: in the setTrackNumber
method which is triggered by AVPlayerItemDidPlayToEndTimeNotification
and in the playerTimeJumped
method which is triggered by AVPlayerItemTimeJumpedNotification
.
This causes a race condition. More details are provided by an Apple staff member here.
The problem can be solved by keeping a local nowPlayingInfo
dictionary that gets updated as needed and always setting MPNowPlayingInfoCenter.defaultCenter().nowPlayingInfo
from that instead of setting individual values.
For background info update, my colleague suggests some implementations are necessary. Maybe you can check and verify some of these requirements in your view controller:
//1: Set true for canBecomeFirstResponder func
override func canBecomeFirstResponder() -> Bool {
return true
}
//2: Set view controller becomeFirstResponder & allow to receive remote control events
override func viewDidLoad() {
super.viewDidLoad()
self.becomeFirstResponder()
UIApplication.sharedApplication().beginReceivingRemoteControlEvents()
....
}
//3: Implement actions after did receive events from remote control
override func remoteControlReceivedWithEvent(event: UIEvent?) {
guard let event = event else {
return
}
switch event.subtype {
case .RemoteControlPlay:
....
break
case .RemoteControlPause:
....
break
case .RemoteControlStop:
....
break
default:
print("default action")
}
}
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With