ios 中NSData 拼接成功，但写入本地种只有前面的data

NSData *videoData = [NSData dataWithContentsOfURL:outputFileURL];
    NSLog(@"%ld",videoData.length);
    if (!self.data) {
        self.data = [[NSMutableData alloc] initWithData:videoData];
    }else {
        //NSMutableData *newData = [NSMutableData dataWithData:videoData];
        
        [self.data appendBytes:videoData.bytes length:videoData.length];
        //[self.data appendData:videoData];
        NSLog(@"self.newdata= %ld",self.data.length);
       NSString *path = [NSTemporaryDirectory() stringByAppendingPathComponent:
         [NSString stringWithFormat:@"%@.%@",
          @"video",@"mp4"]];
       
          [self.data writeToFile:path atomically:NO];

         NSURL *viedeoUrl = [NSURL fileURLWithPath:path];
       // [self.data writeToURL:viedeoUrl atomically:NO];
        NSLog(@"%ld",[NSData dataWithContentsOfURL:viedeoUrl].length);
        NSLog(@"path = %@",viedeoUrl);
        
        ALAssetsLibrary * library = [[ALAssetsLibrary alloc]init];
        [library writeVideoAtPathToSavedPhotosAlbum:viedeoUrl completionBlock:^(NSURL *assetURL, NSError *error) {
                    NSLog(@"%@",assetURL);
             NSLog(@"%ld",[NSData dataWithContentsOfURL:assetURL].length);
            NSLog(@"error:%@",error);
        }];