ios设置视频封面，获取视频任意帧，并进行三级缓存

背景：

项目中有很多视频展示，点击播放的功能，视频的展示封面需要获取视频第一帧画面显示。又因为在列表中用的多，滑动时每次去获取比较耗时。

思路：

创建UIImageView的分类，参考SDWebImage实现，获取到视频第一帧画面时，以视频链接哈希串作为key,image作为值，利用SDImageCache进行图片的三级缓存(磁盘、内存和网络层)。
流程：

• 给UIImageView设置图片；
• 先去缓存查找，有则返回；
• 再去磁盘查找，有则返回；
• 都没找到，截取视频某一帧画面转为image，缓存至磁盘。

实现:

#import "UIButton+VideoImage.h"
#import 

@implementation UIButton (VideoImage)

- (void)videoImageWithvideoURL:(NSURL *)videoURL atTime:(NSTimeInterval)time {
    
    //先从缓存中查找是否有图片
    SDImageCache *cache =  [SDImageCache sharedImageCache];
    UIImage *memoryImage =  [cache imageFromMemoryCacheForKey:videoURL.absoluteString];
    if (memoryImage) {
        [self setBackgroundImage:memoryImage forState:UIControlStateNormal];
        return;
    }else{
        //再从磁盘中查找是否有图片
        UIImage *diskImage =  [cache imageFromDiskCacheForKey:videoURL.absoluteString];
        if (diskImage) {
            [self setBackgroundImage:diskImage forState:UIControlStateNormal];
            return;
        }
    }
    if (!time) {
        time = 1;
    }
    //如果都不存在，开启异步线程截取对应时间点的画面，转成图片缓存至磁盘
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        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;
        
        dispatch_async(dispatch_get_main_queue(), ^{
            SDImageCache * cache =  [SDImageCache sharedImageCache];
            [cache storeImage:thumbnailImage forKey:videoURL.absoluteString toDisk:YES completion:nil];
            [self setBackgroundImage:thumbnailImage forState:UIControlStateNormal];
        });
        
    });
    
}

@end