Question

I’m trying to track down a freeze problem with an app I am developing. I may be in the deep end of the NSAutoreleasePool and am screwing things up.

The app is playing a midi file. If I comment out the “simRespondToFileNote” code below that uses an NSAutoreleasePool it doesn’t freeze. If I let the code run, it will freeze at seemingly random points The crash log/console output doesn’t seem to indicate where the issue is occurring.

Here is the program flow:

1. I’m using the Bass midi lib (C lib); it plays the midi file in its own thread.

2. When a midi event occurs, a callback is triggered and I wrap the midi event data in an NSDictionary and route it to the main thread so I can do UI updates and some other stuff.

Here is the code that does the routing:

- (void)forwardFileNoteIn:(int) note withVelocity: (int) velocity
{
int position = BASS_ChannelGetPosition(midiFileStream, BASS_POS_MIDI_TICK);
float percent = ((float)position / (float)totalTicks);
int ticksInLoop = outLoopTick - inLoopTick;

QWORD bytes=BASS_ChannelGetPosition(midiFileStream, BASS_POS_BYTE); // get position in bytes
double seconds=BASS_ChannelBytes2Seconds(midiFileStream, bytes); // translate to seconds
int timeStamp =seconds*1000; // translate to milliseconds

NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
NSDictionary *midiData = [NSDictionary dictionaryWithObjectsAndKeys:
                           @"fileNoteIn", @"eventType",
                          [NSNumber numberWithInt:note], @"note",
                          [NSNumber numberWithInt:velocity],@"velocity",
                          [NSNumber numberWithInt:timeStamp],@"timeStamp",
                          [NSNumber numberWithInt:position],@"position",
                          [NSNumber numberWithFloat:percent],@"percentPlayed",
                          [NSNumber numberWithInt:ticksInLoop],@"ticksInLoop",
                          nil];

[delegate performSelectorOnMainThread:@selector(midiFileEvent:)
                            withObject:midiData
                            waitUntilDone:NO];
[pool release];

}

1. From the delegate a message is sent to another object using the NSDictionary instance as a param. That object either sends the NSDictionary instance immediately to another object or queues it to be sent after a short delay (using performSelector: afterDelay: ).

Is it possible that the NSAutoreleasePool is deleting the NSDictionary instance before the queued message is triggered? I am not draining the pool anywhere – should I be doing that?

- (void)simRespondToFileNote:(NSDictionary *)dictionary
{
int velocity = [[dictionary objectForKey:@"velocity"] intValue];

if (velocity == 0){
    // noteOff - send it through
    [delegate routeUserSimMidiEvent:dictionary];
} else {

    float totalPercentCorrect = [dataSource getUserCorrectPercent];

    int note = [[dictionary objectForKey:@"note"] intValue];

    if (totalPercentCorrect < _userAccuracy){

        float lateNoteOnTimeDelay =  (dataSource.postTimeHighAccuracy - (dataSource.postTimeHighAccuracy /4)) / 1000.;
        float lateNoteOffTimeDelay = lateNoteOnTimeDelay + .1; // revise

        NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

        // create noteOff data w/ velocity == 0; timeStamp == 0;
        NSDictionary *midiData = [NSDictionary dictionaryWithObjectsAndKeys:
                                  [NSNumber numberWithInt:note], @"note",
                                  [NSNumber numberWithInt:0],@"velocity",
                                  [NSNumber numberWithUnsignedInt:0],@"timeStamp",
                                  nil];

        [self performSelector:@selector(simLateResponseToFileNote:)  withObject:dictionary afterDelay: lateNoteOnTimeDelay];
        [self performSelector:@selector(simLateResponseToFileNote:)  withObject:midiData afterDelay: lateNoteOffTimeDelay];
        [pool release];

    } else {

        float lateNoteOnTimeDelay =  (dataSource.postTimeLowAccuracy + (dataSource.postTimeLowAccuracy /4)) / 1000.0;
        float lateNoteOffTimeDelay = lateNoteOnTimeDelay + .1; // revise
         NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

        // create noteOff data w/ velocity == 0; timeStamp == 0;
        NSDictionary *midiData =[NSDictionary dictionaryWithObjectsAndKeys:
                                 [NSNumber numberWithInt:note], @"note",
                                 [NSNumber numberWithInt:0],@"velocity",
                                 nil];

        // queue late noteOn
        [self performSelector:@selector(simLateResponseToFileNote:)  withObject:dictionary afterDelay: lateNoteOnTimeDelay];
        // queue late noteOff
        [self performSelector:@selector(simLateResponseToFileNote:)  withObject:midiData afterDelay: lateNoteOffTimeDelay];
        [pool release];

    }
}

}

Answer 1

Your creation of a temporary autorelease pool in simRespondToFileNote: isn’t very useful, but shouldn’t be a problem. Your call to performSelector:withObject:afterDelay: will retain dictionary until its called. If forwardFileNoteIn:withVelocity: is on a thread that doesn’t have an autorelease pool yet, you may have to create it, but generally you do that at the top of the method. If this thread already has a pool, there’s no reason to create on here.

From your description, I would suspect that simLateResponseToFileNote: is blocking the main thread too long. I would look there for a bottleneck.