AVAudioEngine: no sound

I'm trying to test AVAudioEngine, watched the WWDC conferences about it, and did everything by the book to try to play a simple file.

Despite everything being exactly the same as in the samples that I found (WWDC and a few other places), despite everything seeming fine (no error, seems to be running), I have no sound output.

Here's the code:

NSError *error = Nil;

[AVAudioSession.sharedInstance setCategory:AVAudioSessionCategoryPlayback withOptions:AVAudioSessionCategoryOptionMixWithOthers error: &error];
[AVAudioSession.sharedInstance setActive:YES error:&error];

AVAudioEngine *engine = [[AVAudioEngine alloc] init];

AVAudioPlayerNode *player = [[AVAudioPlayerNode alloc] init];
[engine attachNode:player];

NSURL *fileURL = [[NSBundle mainBundle] URLForResource:@"piano" withExtension:@"wav"];
AVAudioFile *file = [[AVAudioFile alloc] initForReading:fileURL error:&error];

AVAudioMixerNode *mainMixer = [engine mainMixerNode];
AVAudioOutputNode *outputNode = engine.outputNode;

[engine connect:player to:mainMixer format:file.processingFormat];
[engine connect:engine.mainMixerNode to:outputNode format:nil];
[engine prepare];

if (!engine.isRunning) {
    if (![engine startAndReturnError:&error]) {
        //TODO
    }
}

[player scheduleFile:file atTime:nil completionHandler:nil];
[player play];

NSLog(@"Engine description: %@", [engine description]);

Here's the output of the log call:

Engine description: 
________ GraphDescription ________
AVAudioEngineGraph 0x10100d820: initialized = 1, running = 1, number of nodes = 3

     ******** output chain ********

     node 0x28388d200 {'auou' 'rioc' 'appl'}, 'I'
         inputs = 1
             (bus0, en1) <- (bus0) 0x2838a0880, {'aumx' 'mcmx' 'appl'}, [ 2 ch,  44100 Hz, 'lpcm' (0x00000029) 32-bit little-endian float, deinterleaved]

     node 0x2838a0880 {'aumx' 'mcmx' 'appl'}, 'I'
         inputs = 1
             (bus0, en1) <- (bus0) 0x282a8df00, {'augn' 'sspl' 'appl'}, [ 2 ch,  44100 Hz, 'lpcm' (0x00000029) 32-bit little-endian float, deinterleaved]
         outputs = 1
             (bus0, en1) -> (bus0) 0x28388d200, {'auou' 'rioc' 'appl'}, [ 2 ch,  44100 Hz, 'lpcm' (0x00000029) 32-bit little-endian float, deinterleaved]

     node 0x282a8df00 {'augn' 'sspl' 'appl'}, 'I'
         outputs = 1
             (bus0, en1) -> (bus0) 0x2838a0880, {'aumx' 'mcmx' 'appl'}, [ 2 ch,  44100 Hz, 'lpcm' (0x00000029) 32-bit little-endian float, deinterleaved]
______________________________________

I have no idea how to diagnose this and can't find any mistake or reason for which this would not work.

Any idea?

Keeping the question in case anyone has this problem: as I was testing, all my variables were local variables in a test method, and as soon as the method was finished it was probably GC by ARC or something.

Having the AVAudioEngine variable outside (keeping a reference to it) fixed it.