Consecutive calls to startRecordingToOutputFileURL:

问题

The Apple docs seem to indicate that while recording video to a file, the app can change the URL on the fly with no problem. But I'm seeing a problem. When I try this, the recording delegate gets called with an error...

The operation couldn’t be completed. (OSStatus error -12780.) Info dictionary is: { AVErrorRecordingSuccessfullyFinishedKey = 0; }

(funky single quote in "couldn't" comes from logging [error localizedDescription])

Here's the code, which is basically tweaks to WWDC10 AVCam sample:

1) Start recording. Start timer to change the output URL every few seconds

- (void) startRecording
{
    // start the chunk timer
    self.chunkTimer = [NSTimer scheduledTimerWithTimeInterval:5
                                                       target:self
                                                     selector:@selector(chunkTimerFired:)
                                                     userInfo:nil
                                                      repeats:YES];

    AVCaptureConnection *videoConnection = [AVCamCaptureManager connectionWithMediaType:AVMediaTypeVideo fromConnections:[[self movieFileOutput] connections]];
    if ([videoConnection isVideoOrientationSupported]) {
        [videoConnection setVideoOrientation:[self orientation]];
    }

    if ([[UIDevice currentDevice] isMultitaskingSupported]) {
        [self setBackgroundRecordingID:[[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:^{}]];
    }

    NSURL *fileUrl = [[ChunkManager sharedInstance] nextURL];
    NSLog(@"now recording to %@", [fileUrl absoluteString]);
    [[self movieFileOutput] startRecordingToOutputFileURL:fileUrl recordingDelegate:self];
}

2) When the timer fires, change the output file name without stopping recording

- (void)chunkTimerFired:(NSTimer *)aTimer {

    if ([[UIDevice currentDevice] isMultitaskingSupported]) {
        [self setBackgroundRecordingID:[[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:^{}]];
    }

    NSURL *nextUrl = [self nextURL];
    NSLog(@"changing capture output to %@", [[nextUrl absoluteString] lastPathComponent]);

    [[self movieFileOutput] startRecordingToOutputFileURL:nextUrl recordingDelegate:self];
}

Note: [self nextURL] generates file urls like file-0.mov, file-5.mov, file-10.mov and so on.

3) This gets called each time the file changes, and every other invocation is an error...

- (void)              captureOutput:(AVCaptureFileOutput *)captureOutput
didFinishRecordingToOutputFileAtURL:(NSURL *)outputFileURL
                    fromConnections:(NSArray *)connections
                              error:(NSError *)error
{
    id delegate = [self delegate];
    if (error && [delegate respondsToSelector:@selector(someOtherError:)]) {
        NSLog(@"got an error, tell delegate");
        [delegate someOtherError:error];
    }

    if ([self backgroundRecordingID]) {
        if ([[UIDevice currentDevice] isMultitaskingSupported]) {
            [[UIApplication sharedApplication] endBackgroundTask:[self backgroundRecordingID]];
        }
        [self setBackgroundRecordingID:0];
    }

    if ([delegate respondsToSelector:@selector(recordingFinished)]) {
        [delegate recordingFinished];
    }
}

When this runs, file-0 gets written, then we see error -12780 right after changing the url to file-5, file-10 gets written, then an error, then okay, and so on.

It's appears that changing the URL on the fly doesn't work, but it stops the writing which allows the next URL change to work.

回答1:

Thanks all, for the review and good thoughts on this. Here's the word from Apple DTS...

I spoke with our AV Foundation engineers, and it is definitely a bug in that this method is not doing what the documentation says it should ("You do not need to call stopRecording before calling this method while another recording is in progress."). Please file a bug report using the Apple Bug Reporter (http://developer.apple.com/bugreporter/) so the team can investigate. Make sure and include your minimal project in the report.

I've filed this with Apple as bug 11632087

回答2:

In the docs it's stated this:

If a file at the given URL already exists when capturing starts, recording to the new file will fail.

Are you sure you check that nextUrl is a non-existing file name?

回答3:

According to the documentation, calling to 2 consecutive startRecordingToOutputFileURL is not supported.

you can read about it here

In iOS, this frame accurate file switching is not supported. You must call stopRecording before calling this method again to avoid any errors.

来源：https://stackoverflow.com/questions/10858793/consecutive-calls-to-startrecordingtooutputfileurl