在我的应用程序中，我正在录制小型视频，并将它们作为AVAsset添加到NSMutableArray中，以便保留所捕获内容的记录.当用户按下按钮将其合并时，最终结果仅是拍摄的第一个视频(例如，如果拍摄了三个短视频，则合并后的最终结果仅是第一个视频，而其他视频则不显示).我在NSMutableArray中迭代并将视频拼接在一起的代码在这里:

In my app, I'm recording small videos and adding them into an NSMutableArray as AVAsset so that i keep record of what has been captured. when the user press a button to merge them, the final result is only the first video taken (example, if three short videos where taken, the final result after merging is only the first video and the others do not appear). my code on iterating in the NSMutableArray and stitching the videos together is here:

if (self.capturedVideos.count != 0) {        
    //Create AVMutableComposition Object.This object will hold our multiple AVMutableCompositionTrack.
    AVMutableComposition* mixComposition = [[AVMutableComposition alloc] init];

    for (AVAsset *asset in self.capturedVideos) {
        //check if the video is the first one captures so that it  is placed at time 0.
        if ([self.capturedVideos indexOfObject:asset] == 0) {
            AVMutableCompositionTrack *firstTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
            [firstTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, asset.duration) ofTrack:[[asset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0] atTime:kCMTimeZero error:nil];
            previousAsset = asset;
        } else{
            AVMutableCompositionTrack *track = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
            [track insertTimeRange:CMTimeRangeMake(kCMTimeZero, asset.duration) ofTrack:[[asset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0] atTime:previousAsset.duration error:nil];
            previousAsset = asset;
        }
    }

    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *documentsDirectory = [paths objectAtIndex:0];
    NSString *myPathDocs =  [documentsDirectory stringByAppendingPathComponent:
    [NSString stringWithFormat:@"mergeVideo-%d.mov",arc4random() % 1000]];
    NSURL *url = [NSURL fileURLWithPath:myPathDocs];
    // 5 - Create exporter
    AVAssetExportSession *exporter = [[AVAssetExportSession alloc] initWithAsset:mixComposition presetName:AVAssetExportPresetHighestQuality];
    exporter.outputURL=url;
    exporter.outputFileType = AVFileTypeQuickTimeMovie;
    exporter.shouldOptimizeForNetworkUse = YES;
    [exporter exportAsynchronouslyWithCompletionHandler:^{
        dispatch_async(dispatch_get_main_queue(), ^{
            [self exportDidFinish:exporter];
        });
    }];
}

for循环之后是用于导出要保存在相机胶卷中的视频.那我的错误在哪里?持续时间是正确的(因此不会出现重叠现象).但是，我对此有所怀疑.我在@implementation之后在花括号中添加了一个实例变量，它是previousAsset，它跟踪先前添加的资产，从而知道将下一个放置在何处.它属于AVAsset类，因此我没有初始化它，因为当我尝试显示给我一个错误时.

what's after the for loop is for exporting the video to be saved in camera roll. so where is my mistake? the durations are right (so there is no over lapping). however, i'm doubting in something. There is an instance variable i added after @implementation in braces which is previousAsset which tracks the previous asset added thus knowing where to place the next one. it's of class AVAsset so i didn't initialize it because when i try to it's showing me an error.

previousAsset = [[AVAsset分配] init];

previousAsset = [[AVAsset alloc] init];