Es gibt ein merkwürdiges Verhalten, das ich beim Zusammenführen von Videos mit AVFoundation festgestellt habe. Ich bin mir ziemlich sicher, dass ich irgendwo einen Fehler gemacht habe, aber ich bin zu blind, um ihn zu sehen. Mein Ziel ist es nur, 4 Videos zusammenzuführen (später wird es einen Überblendungsübergang zwischen ihnen geben). Jedes Mal, wenn ich versuche, ein Video zu exportieren, erhalte ich folgende Fehlermeldung:

Error Domain=AVFoundationErrorDomain Code=-11821 "Cannot Decode" UserInfo=0x7fd94073cc30 {NSLocalizedDescription=Cannot Decode, NSLocalizedFailureReason=The media data could not be decoded. It may be damaged.}

Das lustigste ist, dass, wenn ich AVAssetExportSession nicht mit AVMutableVideoComposition versorge, alles gut funktioniert! Ich kann nicht verstehen, was ich falsch mache. Die Quellvideos werden von YouTube heruntergeladen und haben die Erweiterung .mp4. Ich kann sie mit MPMoviePlayerController abspielen. Achten Sie beim Überprüfen des Quellcodes sorgfältig auf AVMutableVideoComposition. Ich habe diesen Code in Xcode 6.0.1 auf dem iOS-Simulator getestet.

#import "VideoStitcher.h"
#import <UIKit/UIKit.h>
#import <AVFoundation/AVFoundation.h>
#import <AssetsLibrary/AssetsLibrary.h>

@implementation VideoStitcher
{
    VideoStitcherCompletionBlock _completionBlock;
    AVMutableComposition *_composition;
    AVMutableVideoComposition *_videoComposition;
}

- (instancetype)init
{
    self = [super init];
    if (self)
    {
        _composition = [AVMutableComposition composition];
        _videoComposition = [AVMutableVideoComposition videoComposition];
    }
    return self;
}

- (void)compileVideoWithAssets:(NSArray *)assets completion:(VideoStitcherCompletionBlock)completion
{
    _completionBlock = [completion copy];

    if (assets == nil || assets.count < 2)
    {
        // We need at least two video to make a stitch, right?
        NSAssert(NO, @"VideoStitcher: assets parameter is nil or has not enough items in it");
    }
    else
    {
        [self composeAssets:assets];
        if (_composition != nil) // if stitching went good and no errors were found
            [self exportComposition];
    }
}

- (void)composeAssets:(NSArray *)assets
{
    AVMutableCompositionTrack *compositionVideoTrack = [_composition addMutableTrackWithMediaType:AVMediaTypeVideo
                                                                     preferredTrackID:kCMPersistentTrackID_Invalid];

    NSError *compositionError = nil;
    CMTime currentTime = kCMTimeZero;
    AVAsset *asset = nil;
    for (int i = (int)assets.count - 1; i >= 0; i--) //For some reason videos are compiled in reverse order. Find the bug later. 06.10.14
    {
        asset = assets[i];
        AVAssetTrack *assetVideoTrack = [[asset tracksWithMediaType:AVMediaTypeVideo] firstObject];
        BOOL success = [compositionVideoTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, assetVideoTrack.timeRange.duration)
                                                      ofTrack:assetVideoTrack
                                                       atTime:currentTime
                                                        error:&compositionError];
        if (success)
        {
            CMTimeAdd(currentTime, asset.duration);
        }
        else
        {
            NSLog(@"VideoStitcher: something went wrong during inserting time range in composition");
            if (compositionError != nil)
            {
                NSLog(@"%@", compositionError);
                _completionBlock(nil, compositionError);
                _composition = nil;
                return;
            }
        }
    }

    AVMutableVideoCompositionInstruction *videoCompositionInstruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
    videoCompositionInstruction.timeRange = CMTimeRangeMake(kCMTimeZero, _composition.duration);
    videoCompositionInstruction.backgroundColor = [[UIColor redColor] CGColor];
    _videoComposition.instructions = @[videoCompositionInstruction];
    _videoComposition.renderSize = [self calculateOptimalRenderSizeFromAssets:assets];
    _videoComposition.frameDuration = CMTimeMake(1, 600);
}

- (void)exportComposition
{
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *documentsDirectory = [paths objectAtIndex:0];
    NSString *myPathDocs =  [documentsDirectory stringByAppendingPathComponent:@"testVideo.mov"];
    NSURL *url = [NSURL fileURLWithPath:myPathDocs];


    NSString *filePath = [url path];
    NSFileManager *fileManager = [NSFileManager defaultManager];
    if ([fileManager fileExistsAtPath:filePath]) {
        NSError *error;
        if ([fileManager removeItemAtPath:filePath error:&error] == NO) {
            NSLog(@"removeItemAtPath %@ error:%@", filePath, error);
        }
    }

    AVAssetExportSession *exporter = [[AVAssetExportSession alloc] initWithAsset:_composition
                                                                      presetName:AVAssetExportPreset1280x720];
    exporter.outputURL = url;
    exporter.outputFileType = AVFileTypeQuickTimeMovie;
    exporter.shouldOptimizeForNetworkUse = YES;
    exporter.videoComposition = _videoComposition;
    [exporter exportAsynchronouslyWithCompletionHandler:^{
        [self exportDidFinish:exporter];
    }];
}

- (void)exportDidFinish:(AVAssetExportSession*)session
{
    NSLog(@"%li", session.status);
    if (session.status == AVAssetExportSessionStatusCompleted)
    {
        NSURL *outputURL = session.outputURL;

        // time to call delegate methods, but for testing purposes we save the video in 'photos' app

        ALAssetsLibrary *library = [[ALAssetsLibrary alloc] init];
        if ([library videoAtPathIsCompatibleWithSavedPhotosAlbum:outputURL])
        {
            [library writeVideoAtPathToSavedPhotosAlbum:outputURL completionBlock:^(NSURL *assetURL, NSError *error){
                if (error == nil)
                {
                    NSLog(@"successfully saved video");
                }
                else
                {
                    NSLog(@"saving video failed.\n%@", error);
                }
            }];
        }
    }
    else if (session.status == AVAssetExportSessionStatusFailed)
    {
        NSLog(@"VideoStitcher: exporting failed.\n%@", session.error);
    }
}

- (CGSize)calculateOptimalRenderSizeFromAssets:(NSArray *)assets
{
    AVAsset *firstAsset = assets[0];
    AVAssetTrack *firstAssetVideoTrack = [[firstAsset tracksWithMediaType:AVMediaTypeVideo] firstObject];
    CGFloat maxWidth = firstAssetVideoTrack.naturalSize.height;
    CGFloat maxHeight = firstAssetVideoTrack.naturalSize.width;

    for (AVAsset *asset in assets)
    {
        AVAssetTrack *assetVideoTrack = [[asset tracksWithMediaType:AVMediaTypeVideo] firstObject];
        if (assetVideoTrack.naturalSize.width > maxWidth)
            maxWidth = assetVideoTrack.naturalSize.width;
        if (assetVideoTrack.naturalSize.height > maxHeight)
            maxHeight = assetVideoTrack.naturalSize.height;
    }

    return CGSizeMake(maxWidth, maxHeight);
}

@end

Vielen Dank für Ihre Aufmerksamkeit. Ich bin sehr müde und habe vier Stunden lang versucht, den Bug zu finden. Ich gehe jetzt schlafen.