在尝试将视频与AVFoundation合并时，我发现了一种奇怪的行为。我很确定我在某个地方犯了一个错误，但是我太盲目了。我的目标只是合并4个视频（之后它们之间会有交叉淡入淡出过渡）。 我每次尝试导出视频时都会收到此错误：
Error Domain=AVFoundationErrorDomain Code=-11821 "Cannot Decode" UserInfo=0x7fd94073cc30 {NSLocalizedDescription=Cannot Decode, NSLocalizedFailureReason=The media data could not be decoded. It may be damaged.}
最有趣的是，如果我不使用AVMutableVideoComposition提供AVAssetExportSession，那么一切正常！我无法理解我做错了什么。源视频从youtube下载并具有.mp4扩展名。我可以用MPMoviePlayerController播放它们。在检查源代码时，请仔细查看AVMutableVideoComposition。 我在iOS模拟器上的Xcode 6.0.1中测试了这段代码。
#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
感谢您的关注。我真的很累，我一直试图找到这个虫子连续四个小时。我现在要去睡觉了。
答案 0 :(得分：4)
我终于找到了解决方案。错误描述导致我的方向错误：＆＃34;无法解码。媒体数据无法解码。它可能会被损坏。＆＃34;。根据此说明，您可能会认为您的视频文件存在问题。我花了5个小时试验格式，调试等。
嗯，答案完全不同！
我的错误是我忘记了CMTimeADD（）返回值。我认为它改变了第一个参数的值，在代码中你可以看到：
CMTime currentTime = kCMTimeZero;
for (int i = (int)assets.count - 1; i >= 0; i--)
{
CMTimeAdd(currentTime, asset.duration); //HERE!! I don't actually increment the value! currentTime is always kCMTimeZero
}
videoCompositionInstruction.timeRange = CMTimeRangeMake(kCMTimeZero, _composition.duration); // And that's where everything breaks!
我所学到的教训：使用AVFoundation时，请务必检查您的时间值！这非常重要，否则你会遇到很多错误。