仅测试了本机拍摄的mp4文件

- (void)setFileName:(NSString *)fileName{
    _fileName = [fileName copy];
    
    // 这里是获取本地沙盒的视频，远程url原理一样
    // 记住如果是http地址记得用[NSURL URLWithString:urlStr]
    // 若是本地视频地址要用[NSURL fileURLWithPath:path]
    _videoUrl = [NSURL fileURLWithPath:[[[NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) firstObject] stringByAppendingPathComponent:VideoRecordsDirectory] stringByAppendingPathComponent:_fileName]];
    
    AVURLAsset *asset = [[AVURLAsset alloc] initWithURL:_videoUrl options:nil];
    
    // 获取某一帧图片，这里获取第一帧
    _videoImage = [self getthumImageForAsset:asset atTime:0];
}

// 获取视频某一帧的图片
- (UIImage *)getthumImageForAsset:(AVURLAsset *)asset atTime:(NSTimeInterval)time{
    
    NSParameterAssert(asset);
    
    AVAssetImageGenerator *assetImageGenerator = [[AVAssetImageGenerator alloc] initWithAsset:asset];
    assetImageGenerator.appliesPreferredTrackTransform = YES;
    assetImageGenerator.apertureMode = AVAssetImageGeneratorApertureModeEncodedPixels;
    
    // 获取任意帧要设定这个，然后可根据传过来的参数time获取任意帧
    //    assetImageGenerator.requestedTimeToleranceAfter = kCMTimeZero;
    //    assetImageGenerator.requestedTimeToleranceBefore = kCMTimeZero;
    
    CGImageRef thumImageRef = NULL;
    
    // 获取哪一时间的帧图片
    CFTimeInterval thumImageTime = time;
    
    NSError *thumImageGenerationError = nil;
    
    // 获取每秒多少帧
    //    float fps = [[[asset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0] nominalFrameRate];
    // 重写CMTimeMake(a,b) a当前第几帧, b每秒钟多少帧.当前播放时间a/b CMTimeMakeWithSeconds(a,b) a当前时间,b每秒钟多少帧
    // 这个时候即可获取第N帧图片
    //    thumImageRef = [assetImageGenerator copyCGImageAtTime:CMTimeMake(time, fps) actualTime:NULL error:&thumImageGenerationError];
    
    thumImageRef = [assetImageGenerator copyCGImageAtTime:CMTimeMake(thumImageTime, 60) actualTime:NULL error:&thumImageGenerationError];
    
    if (!thumImageRef)
        NSLog(@"thumImageGenerationError %@", thumImageGenerationError);
    
    UIImage *thumImage = thumImageRef ? [[UIImage alloc] initWithCGImage:thumImageRef] : nil;
    
    return thumImage;
}

/** 获取视频的尺寸 */
+ (void)getVideoSizeWithURL:(NSURL *)URL complete:(void(^)(CGSize videoSize))complete{
    
    AVURLAsset *asset = [[AVURLAsset alloc] initWithURL:URL options:nil];
    
    // 获取
    // loadValuesAsynchronouslyForKeys是官方提供异步加载track的方法，防止线程阻塞
    // 加载track是耗时操作
    [asset loadValuesAsynchronouslyForKeys:@[@"tracks"] completionHandler:^{
        
        // 一般视频都有至少两个track(轨道)，根据track.mediaType判断track类型
        // AVMediaTypeVideo表示视频轨道，AVMediaTypeAudio代表音频轨道，其他类型可以查看文档。
        // 根据track的naturalSize属性即可获得视频尺寸
        NSArray *array = asset.tracks;
        CGSize videoSize = CGSizeZero;
        
        for (AVAssetTrack *track in array) {
            
            if ([track.mediaType isEqualToString:AVMediaTypeVideo]) {
                
                // 注意修正naturalSize的宽高
                videoSize = CGSizeApplyAffineTransform(track.naturalSize, track.preferredTransform);//CGSizeMake(track.naturalSize.height, track.naturalSize.width);
                
                break;
            }
        }
        
        if (asset.playable) {
            
            dispatch_async(dispatch_get_main_queue(), ^{
                
                !complete ? : complete(videoSize);
            });
        }
    }];
}