Question

I need to selectively (and reliably) turn off processing of sampleBuffers in my captureOutput:didOutputSampleBuffer:fromConnection: method. As you know it’s called from a GCD queue and not on the main thread… but I’m taking user input from a UIButton (on the main thread of course) and telling my camera object to halt all processing by setting a BOOL flag.

However, sometimes I’m seeing 1 extra frame slip through the cracks after processing was supposedly stopped. Is there any way I can be absolutely sure that nothing will be processed after the button is pressed? Right now I’m doing a trivial test:

// in ViewController:
- (IBAction)tappedStop:(id)sender {
    NSLog("stop processing!");
    _camera.capturing = NO;
}

// in my camera obj:
- (void)captureOutput:(AVCaptureOutput *)captureOutput did... {
    if (!capturing) {
        return;
    }
    NSLog(@"processing!");
}

I’ve tried using @synchronized, a static BOOL, and using a semaphore, but to no avail… sometimes that extra frame still sneaks in. Anyone have and ideas? There’s probably some GCD method that does what I want but I’m not sure how to go about it.

Here’s how the results (sometimes) look in my debug console (shortened to make it more readable):

2012-09-29 23:29:01.869 __33-_block_invoke_0 [Line 322] processing!
2012-09-29 23:29:01.910 __33-_block_invoke_0 [Line 322] processing!
2012-09-29 23:29:01.953 __33-_block_invoke_0 [Line 322] processing!
2012-09-29 23:29:01.994 __33-_block_invoke_0 [Line 322] processing!
2012-09-29 23:29:02.047 __33-_block_invoke_0 [Line 322] processing!
2012-09-29 23:29:02.078 __33-_block_invoke_0 [Line 322] processing!
2012-09-29 23:29:02.121 __33-_block_invoke_0 [Line 322] processing!
2012-09-29 23:29:02.166 -[ViewController tappedButton:] [Line 913] stop processing!
2012-09-29 23:29:02.161 __33-_block_invoke_0 [Line 322] processing!
...

But usually (about 4 out of 5 times) my console looks like:

2012-09-29 23:29:01.869 __33-_block_invoke_0 [Line 322] processing!
2012-09-29 23:29:01.910 __33-_block_invoke_0 [Line 322] processing!
2012-09-29 23:29:01.953 __33-_block_invoke_0 [Line 322] processing!
2012-09-29 23:29:01.994 __33-_block_invoke_0 [Line 322] processing!
2012-09-29 23:29:02.047 __33-_block_invoke_0 [Line 322] processing!
2012-09-29 23:29:02.078 __33-_block_invoke_0 [Line 322] processing!
2012-09-29 23:29:02.121 __33-_block_invoke_0 [Line 322] processing!
2012-09-29 23:29:02.166 -[ViewController tappedButton:] [Line 913] stop processing!

I should probably also mention that I don’t have access to the original queue that’s calling captureOutput:didOutput... since it’s in a superClass of a framework that I can’t control.

Answer 1

I dunno why I didn’t think of this sooner, but if I wrap my entire captureOutput:didOutput… method in an async call to the main thread, even though it seems kind of non-ideal, turning off sampleBuffer processing works as expected. I guess it makes sense; my touch event always comes from the main thread, and since I don’t have a handle on which thread was used in my buffer output queue, the only way (that I can see) to reliably check isRecording is to do so from the main thread too.

- (void)captureOutput:(AVCaptureOutput *)captureOutput did... {
    dispatch_async(dispatch_get_main_queue(), ^{
        if (!isRecording) {
            return;
        }

        NSLog(@"processing!");

        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
            // do actual processing
        });
    });
}

If anyone has any other suggestions though I’d love to hear ’em. Thanks!