Editing a video saved in camera roll and re-save it - Objective c

Question

I have a video on my camera roll and I must resize it and save it back in, but my app crashes in the following line:

[firstTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, firstAsset.duration) ofTrack:[[firstAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0] atTime:kCMTimeZero error:nil];

This is the error in the console:

CameraCaptureTutorial[2006:811774] *** Terminating app due to uncaught exception 'NSRangeException', reason: '*** -[__NSArrayM objectAtIndex:]: index 0 beyond bounds for empty array'

What would be the proper way to complete this task? here is my complete code:

 //Here where load our movie Assets using AVURLAsset
NSURL *url = [NSTemporaryDirectory() stringByAppendingPathComponent:[@"movie" stringByAppendingPathExtension:@"mp4"]];
AVURLAsset* firstAsset = [AVURLAsset URLAssetWithURL: url options:nil];

AVURLAsset * secondAsset = [AVURLAsset URLAssetWithURL: url options:nil];

//Create AVMutableComposition Object.This object will hold our multiple AVMutableCompositionTrack.
AVMutableComposition* mixComposition = [[AVMutableComposition alloc] init];

//Here we are creating the first AVMutableCompositionTrack.See how we are adding a new track to our AVMutableComposition.
AVMutableCompositionTrack *firstTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
//Now we set the length of the firstTrack equal to the length of the firstAsset and add the firstAsset to out newly created track at kCMTimeZero so video plays from the start of the track.

//This is where the issue starts...    
[firstTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, firstAsset.duration) ofTrack:[[firstAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0] atTime:kCMTimeZero error:nil];

//Now we repeat the same process for the 2nd track as we did above for the first track.Note that the new track also starts at kCMTimeZero meaning both tracks will play simultaneously.
AVMutableCompositionTrack *secondTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
[secondTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, secondAsset.duration) ofTrack:[[secondAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0] atTime:kCMTimeZero error:nil];

//See how we are creating AVMutableVideoCompositionInstruction object.This object will contain the array of our AVMutableVideoCompositionLayerInstruction objects.You set the duration of the layer.You should add the lenght equal to the lingth of the longer asset in terms of duration.
AVMutableVideoCompositionInstruction * MainInstruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
MainInstruction.timeRange = CMTimeRangeMake(kCMTimeZero, firstAsset.duration);
//We will be creating 2 AVMutableVideoCompositionLayerInstruction objects.Each for our 2 AVMutableCompositionTrack.here we are creating AVMutableVideoCompositionLayerInstruction for out first track.see how we make use of Affinetransform to move and scale our First Track.so it is displayed at the bottom of the screen in smaller size.(First track in the one that remains on top).
AVMutableVideoCompositionLayerInstruction *FirstlayerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:firstTrack];
CGAffineTransform Scale = CGAffineTransformMakeScale(0.7f,0.7f);
CGAffineTransform Move = CGAffineTransformMakeTranslation(230,230);
[FirstlayerInstruction setTransform:CGAffineTransformConcat(Scale,Move) atTime:kCMTimeZero];

//Here we are creating AVMutableVideoCompositionLayerInstruction for out second track.see how we make use of Affinetransform to move and scale our second Track.
AVMutableVideoCompositionLayerInstruction *SecondlayerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:secondTrack];
CGAffineTransform SecondScale = CGAffineTransformMakeScale(1.2f,1.5f);
CGAffineTransform SecondMove = CGAffineTransformMakeTranslation(0,0);
[SecondlayerInstruction setTransform:CGAffineTransformConcat(SecondScale,SecondMove) atTime:kCMTimeZero];

//Now we add our 2 created AVMutableVideoCompositionLayerInstruction objects to our AVMutableVideoCompositionInstruction in form of an array.
MainInstruction.layerInstructions = [NSArray arrayWithObjects:FirstlayerInstruction,SecondlayerInstruction,nil];;

//Now we create AVMutableVideoComposition object.We can add mutiple AVMutableVideoCompositionInstruction to this object.We have only one AVMutableVideoCompositionInstruction object in our example.You can use multiple AVMutableVideoCompositionInstruction objects to add multiple layers of effects such as fade and transition but make sure that time ranges of the AVMutableVideoCompositionInstruction objects dont overlap.
AVMutableVideoComposition *MainCompositionInst = [AVMutableVideoComposition videoComposition];
MainCompositionInst.instructions = [NSArray arrayWithObject:MainInstruction];
MainCompositionInst.frameDuration = CMTimeMake(1, 30);
MainCompositionInst.renderSize = CGSizeMake(640, 360);

AVAssetExportSession *exporter = [[AVAssetExportSession alloc] initWithAsset:mixComposition presetName:nil];

exporter.outputURL = [NSURL URLWithString:[NSTemporaryDirectory() stringByAppendingPathComponent:[@"video" stringByAppendingPathExtension:@"mp4"]]];
// Set the output file type to be a QuickTime movie.
exporter.outputFileType = AVFileTypeQuickTimeMovie;
exporter.shouldOptimizeForNetworkUse = YES;
exporter.videoComposition = MainCompositionInst;

[exporter exportAsynchronouslyWithCompletionHandler:^{
    if (AVAssetExportSessionStatusCompleted == exporter.status)
    {
        NSLog(@"done processing video!");
        [[[ALAssetsLibrary alloc] init] writeVideoAtPathToSavedPhotosAlbum:exporter.outputURL completionBlock:^(NSURL *assetURL, NSError *error) {
            if (error)
                NSLog(@"%@", error);

            [[NSFileManager defaultManager] removeItemAtURL:exporter.outputURL error:nil];
        }];
    }
    else
    {
        NSLog(@"%ld", exporter.status);
    }
}];

Answer 1

It looks like firstAsset does not have any tracks. Unless you saved a video at this url "NSURL *url = [NSTemporaryDirectory() stringByAppendingPathComponent:[@"movie" stringByAppendingPathExtension:@"mp4"]];" earlier in your code then you can guarantee that anything is there.

You would need to do one of two things: 1. Create an AVAsset from a url that you get from your photo library probably using UIImagePickercontroller or 2. Embed the media in your app and use [NSBundle mainBundle] URLForResource:withExtension:]