UIImageView+WebCache

主要功能是給UIImageView添加的一些便利操作，也就是下載、緩存、顯示3大部分的顯示部分
1、獲取獲取設置的下載url，
知識點；使用runtime、地址作為唯一key

static char imageURLKey;
- (NSURL *)sd_imageURL {
    return objc_getAssociatedObject(self, &imageURLKey);
}

2、設置下載圖片的URL，各種組合的便利方法
知識點：
2.1、開始新下載時取消上一次的操作，這是設計一個庫時應該考慮的避免意外的錯誤已經節省資源
2.2、使用runtime保存操作的url
2.3、options不包含SDWebImageDelayPlaceholder才設置placeHolder
2.4、將UI操作放在主線程，并抽成一個宏

#define dispatch_main_async_safe(block)\
  if ([NSThread isMainThread]) {\
      block();\
  } else {\
      dispatch_async(dispatch_get_main_queue(), block);\
  }

2.5、調用SDWebImageManager的download方法
2.6、持有下載的操作operation以及刪除時取消操作

- (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageCompletionBlock)completedBlock {
// 取消當前下載，從新設置內容 
   [self sd_cancelCurrentImageLoad];
    objc_setAssociatedObject(self, &imageURLKey, url, OBJC_ASSOCIATION_RETAIN_NONATOMIC);

    if (!(options & SDWebImageDelayPlaceholder)) {
        dispatch_main_async_safe(^{
            self.image = placeholder;
        });
    }
    
    if (url) {

        // 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;
                    [wself setNeedsLayout];
                } else {
                    if ((options & SDWebImageDelayPlaceholder)) {
                        wself.image = placeholder;
                        [wself setNeedsLayout];
                    }
                }
                if (completedBlock && finished) {
                    completedBlock(image, error, cacheType, url);
                }
            });
        }];
        [self sd_setImageLoadOperation:operation forKey:@"UIImageViewImageLoad"];
    } else {
        dispatch_main_async_safe(^{
            [self removeActivityIndicator];
            if (completedBlock) {
                NSError *error = [NSError errorWithDomain:SDWebImageErrorDomain code:-1 userInfo:@{NSLocalizedDescriptionKey : @"Trying to load a nil url"}];
                completedBlock(nil, error, SDImageCacheTypeNone, url);
            }
        });
    }
}

3、設置下載動畫組的圖片下載地址
如果加載的是一個動畫組的圖片，則遍歷所有url進行下載，把所有一個組的操作放入一個數組進行持有，下載好的圖片則每下載一個就放入組里開始動畫

- (void)sd_setAnimationImagesWithURLs:(NSArray *)arrayOfURLs;

4、設置圖片加載中的Indicator

/**
 *  Show activity UIActivityIndicatorView
 */
- (void)setShowActivityIndicatorView:(BOOL)show;

/**
 *  set desired UIActivityIndicatorViewStyle
 *
 *  @param style The style of the UIActivityIndicatorView
 */
- (void)setIndicatorStyle:(UIActivityIndicatorViewStyle)style;

5、取消當前下載中的圖片，主要場景為離開當前頁面，不用再繼續下載

/**
 * Cancel the current download
 */
- (void)sd_cancelCurrentImageLoad;

- (void)sd_cancelCurrentAnimationImagesLoad;