Cocoa: AVAsset loaded from file has 0 tracks - objective-c

Question

I'm attempting to concatenate some audio files using the technique shown here. My audio files are .m4a and I can verify that they play fine in Quicktime. Here's the code I'm trying to use to concatenate them:
[currFile.audioContent writeToFile:tempOldFilePath atomically:NO];
AVURLAsset *oldAudioAsset = [AVURLAsset URLAssetWithURL:[NSURL URLWithString:tempOldFilePath] options:nil];
AVURLAsset *newAudioAsset = [AVURLAsset URLAssetWithURL:[NSURL URLWithString:tempInputFilePath] options:nil];
NSLog(#"oldAsset num tracks = %lu",(unsigned long)oldAudioAsset.tracks.count);
NSLog(#"newAsset num tracks = %lu",(unsigned long)newAudioAsset.tracks.count);
AVAssetTrack *oldTrack = [[oldAudioAsset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0];
AVAssetTrack *newTrack = [[newAudioAsset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0];
AVMutableComposition *mutableComposition = [AVMutableComposition composition];
AVMutableCompositionTrack *compTrack = [mutableComposition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
NSError *error=nil;
[compTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, oldTrack.timeRange.duration) ofTrack:oldTrack atTime:kCMTimeZero error:&error];
if (error) {
NSLog(#"%#",error.localizedDescription);
error=nil;
}
[compTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, newTrack.timeRange.duration) ofTrack:newTrack
atTime:oldTrack.timeRange.duration error:&error];
if (error) {
NSLog(#"%#",error.localizedDescription);
error=nil;
}
exporter = [[AVAssetExportSession alloc] initWithAsset:mutableComposition presetName:AVAssetExportPresetAppleM4A];
exporter.outputURL = [NSURL URLWithString:tempCompFilePath];
exporter.outputFileType = AVFileTypeAppleM4A;
[exporter exportAsynchronouslyWithCompletionHandler:^{
NSLog(#"handller");
NSError *error=nil;
NSData *newData = [NSData dataWithContentsOfFile:tempCompFilePath options:0 error:&error];
NSLog(#"%lu",(unsigned long)newData.length);
if (error) {
NSLog(#"%#",error.localizedDescription);
}
currFile.audioContent = newData;
[[AppDelegate sharedDelegate] saveAction:nil];
}];
The first problem I noticed is that the exporter's handler method is never called. I'm guessing the reason for this is the other problem I noticed: After created my AVAssets from URL, log statements show that they contain 0 tracks. Apple's example doesn't exactly show how the AVAssets are loaded.
Any advice on how to get this working?

Answer 1

As you've already found, you need to use fileURLWithPath:, not URLWithString:, to create your URLs.
URLWithString: expects a string that describes a URL, such as #"file:///path/to/file" or #"http://example.com/". When your string describes a path alone, such as #"/path/to/file", you must use fileURLWithPath:, which will fill in the missing pieces correctly.
More technically, URLWithString: will interpret a path as simply a URL with only a path but no particular scheme, which you could go on to use relative to a base URL in any file-oriented scheme, such as HTTP (GET /path/to/file). fileURLWithPath: will interpret a path as a local file path, and return a file: URL accordingly.

Answer 2

I found the reason why this error was occurred and eventually solve it.
Originally I had set the source file path as ".mp4".
But the type of recorded video file was MOV so I changed as ".mov".
NSString *source_file_path = #"temp_video.mov"
instead of
NSString *source_file_path = #"temp_video.mp4"
The problem was fixed and it is working well now.
Hope to be helpful for all.

Answer 3

Apparently I was using the wrong NSURL method. I changed it to this:
AVURLAsset *oldAudioAsset = [AVURLAsset URLAssetWithURL:[NSURL fileURLWithPath:tempOldFilePath] options:nil];
Now my AVAssets each have 1 track in them. If anyone can provide a good explanation as to why this is necessary, I will accept that answer.