SDWebImage是iOS開發常用的一個開源庫，主要作用是異步下載圖片并顯示在UIImageView上。具體功能有：
1.異步加載圖片；
2.異步緩存圖片在內存和磁盤；
3.支持GIF、WebP圖片
4.支持后臺圖片解壓縮；
5.同一個URL不會被反復下載；
6.同一個非法的URL不會被反復加載；
7.線程安全， 不會阻塞主線程。

主要流程：

Paste_Image.png

UIImageView+WebCache

我們最常用到的接口應該是這幾個：

- (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;

不過最后都是調用到下面這個接口，

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

    if (!(options & SDWebImageDelayPlaceholder)) {
        // 將任務放在主線程
        dispatch_main_async_safe(^{
            // 展位圖
            self.image = placeholder;
        });
    }
    
    if (url) {
         // 是否顯示進度
        // 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的下載操作取消了，重新為UIImageView綁定新的操作
        [self sd_setImageLoadOperation:operation forKey:@"UIImageViewImageLoad"];
    } else {
        dispatch_main_async_safe(^{
            [self removeActivityIndicator];
            // 出錯處理
            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) {
     /**
     下載失敗會重新嘗試下載
     */
    SDWebImageRetryFailed = 1 << 0,
    /**
     UIScrollView滾動時，延遲下載
     */
    SDWebImageLowPriority = 1 << 1,
    /**
     只緩存到內存中
     */
    SDWebImageCacheMemoryOnly = 1 << 2,
    /**
     圖片變下載變顯示，默認的是下載完成再顯示
     */
    SDWebImageProgressiveDownload = 1 << 3,
    /**
     刷新緩存
     */
    SDWebImageRefreshCached = 1 << 4,
    /**
     后臺下載
     */
    SDWebImageContinueInBackground = 1 << 5,
    SDWebImageHandleCookies = 1 << 6,
    SDWebImageAllowInvalidSSLCertificates = 1 << 7,
    SDWebImageHighPriority = 1 << 8,
    SDWebImageDelayPlaceholder = 1 << 9,
    SDWebImageTransformAnimatedImage = 1 << 10,
    SDWebImageAvoidAutoSetImage = 1 << 11
}

SDWebImageDownloaderProgressBlock 進度block。
SDWebImageCompletionBlock 結束block。