Recorded video sometimes displaying, sometimes not - ios

Question

I have a view controller which records a (front-facing) video of the user. The following code is used:
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *basePath = [paths objectAtIndex:0];
NSString *outputPath = [[NSString alloc] initWithFormat:#"%#", [basePath stringByAppendingPathComponent:#"output.mp4"]];
if ([[NSFileManager defaultManager] isDeletableFileAtPath:outputPath])
[[NSFileManager defaultManager] removeItemAtPath:outputPath error:NULL];
NSURL *outputURL = [[NSURL alloc] initFileURLWithPath:outputPath];
[movieFileOutput startRecordingToOutputFileURL:outputURL recordingDelegate:self];
self.outputURL = outputURL;
Where movieFileOutput is a AVCaptureMovieFileOutput object. I can show you the camera configuration/setup code (this is not an image picker controller or any Apple UI; this films the user in the background while they interact with a completely different screen) but in general, I know it works.
Afterwards, [movieFileOutput stopRecording]; is called and the user is navigated to a new screen where the recording that just occured is played. I pass on the outputURL object to this newly initialized view controller, where the VC plays it using the following code:
self.player = [[MPMoviePlayerController alloc] initWithContentURL:self.outputURL];
self.player.view.frame = CGRectMake(0, 0, self.view.frame.size.width, 320);
self.player.movieSourceType = MPMovieSourceTypeFile;
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(moviePlaybackComplete:) name:MPMoviePlayerPlaybackDidFinishNotification object:self.player];
self.player.controlStyle = MPMovieControlStyleNone;
self.player.shouldAutoplay = YES;
[self.player setFullscreen:YES animated:NO];
[self.player prepareToPlay];
[self.view addSubview:[self.player view]];
[self.player play];
So, the first time, everything works. The app records the video, then the next screen is displayed, and the video is shown. But then, if you click the "Discard" button, which segues all the way back to the start of the app, and do the entire process again, it does not work and instead the player shows a black screen. With that being said, I print out the data length at the path and it is always a large number (hence something exists at that path).
Oddly enough, I decided to just print the NSData object itself like so:
NSLog(#"%#", [NSData dataWithContentsOfURL:self.outputURL]);
And indeed a very large string of characters (perhaps the bytes? not sure) is printed. Yet, for some reason, now everything works fine... when I add this NSLog(), the bug no longer occurs. When I remove it, it occurs again! I have tested this rigorously.
But still there are some odd times with the inclusion of this NSLog() when after three times the black screen is played. I am very confused as to why this bug may occur and how on Earth the inclusion of an NSLog() could change any behavior.
Any ideas?

Answer 1

My guess is that you need to wait for the file to have been saved.
The documentation of AVCaptureFileOutput says:
When recording is stopped either by calling this method, by changing
files using startRecordingToOutputFileURL:recordingDelegate:, or
because of an error, the remaining data that needs to be included to
the file will be written in the background. Therefore, before using
the file, you must wait until the delegate that was specified in
startRecordingToOutputFileURL:recordingDelegate: is notified when all
data has been written to the file using the
captureOutput:didFinishRecordingToOutputFileAtURL:fromConnections:error:
method.