SDWebImage是iOS开发常用的一个开源库，主要作用是异步下载图片并显示在UIImageView上。具体功能有：
1.异步加载图片；
2.异步缓存图片在内存和磁盘；
3.支持GIF、WebP图片
4.支持后台图片解压缩；
5.同一个URL不会被反复下载；
6.同一个非法的URL不会被反复加载；
7.线程安全， 不会阻塞主线程。

主要流程：

Paste_Image.png

UIImageView+WebCache

我们最常用到的接口应该是这几个：

- (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder;
- (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options;
- (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options completed:(SDWebImageCompletionBlock)completedBlock;

不过最后都是调用到下面这个接口，

- (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageCompletionBlock)completedBlock {
    // 取消之前的下载操作
    [self sd_cancelCurrentImageLoad];
    // 动态关联url，唯一key
    objc_setAssociatedObject(self, &imageURLKey, url, OBJC_ASSOCIATION_RETAIN_NONATOMIC);

    if (!(options & SDWebImageDelayPlaceholder)) {
        // 将任务放在主线程
        dispatch_main_async_safe(^{
            // 展位图
            self.image = placeholder;
        });
    }
    
    if (url) {
         // 是否显示进度
        // check if activityView is enabled or not
        if ([self showActivityIndicatorView]) {
            [self addActivityIndicator];
        }

        __weak __typeof(self)wself = self;
        id <SDWebImageOperation> operation = [SDWebImageManager.sharedManager downloadImageWithURL:url options:options progress:progressBlock completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
            [wself removeActivityIndicator];
            if (!wself) return;
            dispatch_main_sync_safe(^{
                if (!wself) return;
                if (image && (options & SDWebImageAvoidAutoSetImage) && completedBlock)
                {
                    completedBlock(image, error, cacheType, url);
                    return;
                }
                else if (image) {
                    wself.image = image;
                    // 重新布局
                    [wself setNeedsLayout];
                } else {
                    if ((options & SDWebImageDelayPlaceholder)) {
                        wself.image = placeholder;
                        [wself setNeedsLayout];
                    }
                }
                if (completedBlock && finished) {
                    completedBlock(image, error, cacheType, url);
                }
            });
        }];
        // 前面把UIImageView的下载操作取消了，重新为UIImageView绑定新的操作
        [self sd_setImageLoadOperation:operation forKey:@"UIImageViewImageLoad"];
    } else {
        dispatch_main_async_safe(^{
            [self removeActivityIndicator];
            // 出错处理
            if (completedBlock) {
                NSError *error = [NSError errorWithDomain:SDWebImageErrorDomain code:-1 userInfo:@{NSLocalizedDescriptionKey : @"Trying to load a nil url"}];
                completedBlock(nil, error, SDImageCacheTypeNone, url);
            }
        });
    }
}

SDWebImageOptions

typedef NS_OPTIONS(NSUInteger, SDWebImageOptions) {
     /**
     下载失败会重新尝试下载
     */
    SDWebImageRetryFailed = 1 << 0,
    /**
     UIScrollView滚动时，延迟下载
     */
    SDWebImageLowPriority = 1 << 1,
    /**
     只缓存到内存中
     */
    SDWebImageCacheMemoryOnly = 1 << 2,
    /**
     图片变下载变显示，默认的是下载完成再显示
     */
    SDWebImageProgressiveDownload = 1 << 3,
    /**
     刷新缓存
     */
    SDWebImageRefreshCached = 1 << 4,
    /**
     后台下载
     */
    SDWebImageContinueInBackground = 1 << 5,
    SDWebImageHandleCookies = 1 << 6,
    SDWebImageAllowInvalidSSLCertificates = 1 << 7,
    SDWebImageHighPriority = 1 << 8,
    SDWebImageDelayPlaceholder = 1 << 9,
    SDWebImageTransformAnimatedImage = 1 << 10,
    SDWebImageAvoidAutoSetImage = 1 << 11
}

SDWebImageDownloaderProgressBlock 进度block。
SDWebImageCompletionBlock 结束block。