GCD flow how to write - iphone

Question

I am trying to make screen shot of avplayer when video start playing so i need to run this code in fast in background so it will not block main thread and other controls run fast simultaneous,trying to run that code GCD format i am not able to run please help me to do that it stops at where i add into my array(in array i am adding UIImage Object)...
if (isCaptureScreenStart)
{
if (CMTimeGetSeconds([avPlayer currentTime])>0)
{
if (avFramesArray!=nil)
{
queue = dispatch_queue_create("array", NULL);
dispatch_sync(queue, ^{
[avFramesArray addObject:[self screenshotFromPlayer:avPlayer maximumSize:avPlayerLayer.frame.size :CMTimeGetSeconds([avPlayer currentTime])]];//stop at this line
NSLog(#"count:%d",[avFramesArray count]);
dispatch_sync(dispatch_get_main_queue(), ^{
NSLog(#"Frame are created:%d",[avFramesArray count]);
if ([avFramesArray count]==0)
{
NSLog(#"Frame are over");
}
});
});
}
}
}
dispatch_release(queue);
Edit:
I think i need to use dispatch_group_async this block now..please give some guideline that how to use:
if (isCaptureScreenStart)
{
if (CMTimeGetSeconds([avPlayer currentTime])>0)
{
if (avFramesArray!=nil) {
dispatch_group_async(serial_group1, serial_dispatch_queue1, ^{
[avFramesArray addObject:[self screenshotFromPlayer:avPlayer maximumSize:avPlayerLayer.frame.size :CMTimeGetSeconds([avPlayer currentTime])]];
});
}
}
dispatch_group_notify(serial_group1, serial_dispatch_queue1, ^{
NSLog(#"task competed");
});
}
Now I am using this block but above execution is contentious running and if i use dispatch_suspend(serial_dispatch_queue1); its stop but again i need to start block execution then what i need to use i have also try with dispatch_resume(serial_dispatch_queue1); again load but system show me crash

Answer 1

dispatch_release(queue); don't do it there, the dispatch queue that you are calling its going to a backThread, so wat is happening is :-
your queue is getting released before the block of code executes.
since your queue looks like an ivar, release it in dealloc. Rest, your code looks fine ..put a breakpoint inside and check if the block is executing.
EDIT
I dont understand, what u are trying to achieve by suspending the queue, there is no need to do it. You dont need to check whether the block has finished executing. The block will finish and then call the dispatch_async , get the main queue and update the UI from there.
Now, when you are creating the queue, create it lazily in your method. take the queue as an ivar in header file:
#interface YourFileController : UIViewController {
dispatch_queue_t queue;
}
Then in your method modify it as such:
if (isCaptureScreenStart)
{
if (CMTimeGetSeconds([avPlayer currentTime])>0)
{
if (avFramesArray!=nil)
{
if (!queue)
queue = dispatch_queue_create("array", DISPATCH_QUEUE_SERIAL);
dispatch_sync(queue, ^{
[avFramesArray addObject:[self screenshotFromPlayer:avPlayer maximumSize:avPlayerLayer.frame.size :CMTimeGetSeconds([avPlayer currentTime])]];//stop at this line
NSLog(#"count:%d",[avFramesArray count]);
dispatch_sync(dispatch_get_main_queue(), ^{
NSLog(#"Frame are created:%d",[avFramesArray count]);
if ([avFramesArray count]==0)
{
NSLog(#"Frame are over");
}
});
});
}
}
}
NOTE : DISPATCH_QUEUE_SERIAL creates a serial queue, meaning all the blocks submitted to it will execute serially in First in First Out order. Once all the blocks submitted get executed, the queue stays ;) ..submit another block to it and it executes the block :D
this represents one whole block:-
[avFramesArray addObject:[self screenshotFromPlayer:avPlayer maximumSize:avPlayerLayer.frame.size :CMTimeGetSeconds([avPlayer currentTime])]];//stop at this line
NSLog(#"count:%d",[avFramesArray count]);
dispatch_sync(dispatch_get_main_queue(), ^{
NSLog(#"Frame are created:%d",[avFramesArray count]);
if ([avFramesArray count]==0)
{
NSLog(#"Frame are over");
}
});