ios - Loading an MP3 from remote URL and playing, freezing UI - ios

Question

I am building a very simple app that plays a very small (less than 30 seconds of audio) static MP3 file from the Internet. The files are listed in a TableView and clicking on a cell opens a View that in turn loads and plays the remote MP3. Audio is played using the AVAudioPlayer. Please note that I am NOT attempting to stream live, continuous audio - just a brief clip. Here is an example of what I am trying to play - https://s3.amazonaws.com/tevfd-recording/All_Fire_and_EMS_2015-02-0314_38_54_457551.mp3
Initially, I downloaded the data of the MP3 file using [NSData dataWithContentsOfURL:], which worked fine except for the fact that there is a delay between tapping a row and loading the view / playing the audio. One thing that I like about this approach is that the audio stops playing when the user leaves the View (back button press).
NSURL *callAudioURL = [NSURL URLWithString:[self.call objectForKey:#"url"]];
NSData *callAudioData = [NSData dataWithContentsOfURL:callAudioURL];
AVAudioPlayer *audio = [[AVAudioPlayer alloc] initWithData: callAudioData error:nil];
self.player = audio;
// Set slider to change position in audio
self.slider.minimumValue = 0;
self.slider.maximumValue = self.player.duration;
[self.player prepareToPlay];
[[self player] play];
self.updateTimer = [NSTimer scheduledTimerWithTimeInterval:0.1 target:self selector:#selector(updateSeekBar) userInfo:nil repeats:YES];
In an attempt to eliminate the delay between the views, I wrapped the loading of the file in a call to `dispatch_async' -
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
NSURL *callAudioURL = [NSURL URLWithString:[self.call objectForKey:#"url"]];
NSData *callAudioData = [NSData dataWithContentsOfURL:callAudioURL];
dispatch_async(dispatch_get_main_queue(), ^{
AVAudioPlayer *audio = [[AVAudioPlayer alloc] initWithData: callAudioData error:nil];
self.player = audio;
self.slider.minimumValue = 0;
self.slider.maximumValue = self.player.duration;
[self.player prepareToPlay];
[[self player] play];
self.updateTimer = [NSTimer scheduledTimerWithTimeInterval:0.1 target:self selector:#selector(updateSeekBar) userInfo:nil repeats:YES];
});
});
This works pretty well - the segue completes quickly and the data is loaded off of the main thread. However, if the user presses the Back button while the audio is playing, it continues.
My questions:
Am I on the right track with dispatch_async for this? If so, how should I go about killing the player when the user presses the Back button?
Thanks!

Answer 1

You should probably move away from using -[NSData dataWithContentsOfURL:] and start managing the network operation yourself using NSURLSession. -[NSData dataWithContentsOfURL:] is always going to load 100% of the data before playing anything, and it's synchronous, so calling it on the main thread is a good way to get the OS to kill your app for stalling the main thread. Doing the loading with dispatch_async keeps you from stalling the main thread, but doesn't solve the problem where it's going to load 100% of the data before you can do anything with it.