iOS拍摄视频，获取视频的某一秒图片

UIImagePickerController *picker = [[UIImagePickerController alloc] init];
        picker.sourceType = UIImagePickerControllerSourceTypeCamera;//sourcetype有三种分别是camera，photoLibrary和photoAlbum
        NSArray *availableMedia = [UIImagePickerController availableMediaTypesForSourceType:UIImagePickerControllerSourceTypeCamera];//Camera所支持的Media格式都有哪些,共有两个分别是@"public.image",@"public.movie"
        picker.mediaTypes = [NSArray arrayWithObject:availableMedia[1]];//设置媒体类型为public.movie
        picker.videoQuality = UIImagePickerControllerQualityTypeHigh;
        picker.videoMaximumDuration = 10.0f;//30秒
        picker.delegate = self;//设置委托
        [self presentViewController:picker animated:YES completion:nil];

- (UIImage *)thumbnailImageForVideo:(NSURL *)videoURL atTime:(NSTimeInterval)time{
    AVURLAsset *asset = [[AVURLAsset alloc] initWithURL:videoURL options:nil];
    NSParameterAssert(asset);
    AVAssetImageGenerator *assetImageGenerator =[[AVAssetImageGenerator alloc] initWithAsset:asset];
    assetImageGenerator.appliesPreferredTrackTransform = YES;
    assetImageGenerator.apertureMode =AVAssetImageGeneratorApertureModeEncodedPixels;
    CGImageRef thumbnailImageRef = NULL;
    CFTimeInterval thumbnailImageTime = time;
    NSError *thumbnailImageGenerationError = nil;
    thumbnailImageRef = [assetImageGenerator copyCGImageAtTime:CMTimeMake(thumbnailImageTime, 60)actualTime:NULL error:&thumbnailImageGenerationError];
if(!thumbnailImageRef){
    NSLog(@"thumbnailImageGenerationError %@",thumbnailImageGenerationError);
    }
    UIImage *thumbnailImage = thumbnailImageRef ? [[UIImage alloc]initWithCGImage:thumbnailImageRef] : nil;
    return thumbnailImage;
}