本次學習從SDWebImage 常用的圖片載入開始 ，由淺入深的剖析實現過程

    UIImageView *imgV = [[UIImageView alloc] initWithFrame:CGRectMake(100, 100, 100, 100)];
    [imgV sd_setImageWithURL:[NSURL URLWithString:@""] placeholderImage:nil];

首先進入方法 ，該方法是UIImageView 的分類中實現 UIImageView+WebCache.h

- (void)sd_setImageWithURL:(nullable NSURL
 
 *)url placeholderImage:(nullable UIImage *)placeholder {
    [self sd_setImageWithURL:url placeholderImage:placeholder options:0 progress:nil completed:nil];
}

繼續深入

- (void)sd_setImageWithURL:(nullable NSURL *)url
          placeholderImage:(nullable UIImage *)placeholder
                   options:(SDWebImageOptions)options
                  progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                 completed:(nullable SDExternalCompletionBlock)completedBlock {

 // 此處為UIImageView 的父類UIView 的分類方法 UIView+WebCache
 

    [self sd_internalSetImageWithURL:url
                    placeholderImage:placeholder
                             options:options
                        operationKey:nil
                       setImageBlock:nil
                            progress:progressBlock
                           completed:completedBlock];
}
 

檢視該方法詳細實現。分析見註釋

- (void)sd_internalSetImageWithURL:(nullable NSURL *)url
                  placeholderImage:(nullable UIImage *)placeholder
                           options:(SDWebImageOptions)options
                      operationKey:(nullable NSString *)operationKey
                     setImageBlock:(nullable SDSetImageBlock)setImageBlock
                          progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                         completed:(nullable SDExternalCompletionBlock)completedBlock {

   // 首先會根據operationKey 傳入的值來設定validOperationKey，當為nil時，設定為當前類名的字串
   // 我們在呼叫 sd_setImageWithURL: placeholderImage: 時，這裡傳入的是nil 所以此處validOperationKey = @"UIImageView"

    NSString *validOperationKey = operationKey ?: NSStringFromClass([self class]);

  // 該方法將根據validOperationKey的值去取消對應的圖片載入任務
  // 該方法的詳細實現在下面
    [self sd_cancelImageLoadOperationWithKey:validOperationKey];

// 將當前url 通過imageURLKey 與self 繫結,由於self不同，這樣就實現了 url 與對應的UIImageView繫結。
    objc_setAssociatedObject(self, &imageURLKey, url, OBJC_ASSOCIATION_RETAIN_NONATOMIC);

// 在本例的簡單呼叫中options = 0,所以 判斷為真，進入分支

// dispatch_main_async_safe() 這個巨集是為了判斷是否在主執行緒，如果不在則回到主執行緒呼叫。其實現見下

 if (!(options & SDWebImageDelayPlaceholder)) {
        dispatch_main_async_safe(^{
// 確保在主執行緒下呼叫
// 此處由引數可見 是為了先設定placeholder 即佔位圖，實現沒有圖片時顯示佔位圖，本例中 placeholder,setImageBlock  引數均為nil
// 該方法實現見下
          [self sd_setImage:placeholder imageData:nil basedOnClassOrViaCustomSetImageBlock:setImageBlock];
        });
    }
    if (url) {
        // check if activityView is enabled or not
        if ([self sd_showActivityIndicatorView]) {
            [self sd_addActivityIndicator];
        }

        __weak __typeof(self)wself = self;
        id <SDWebImageOperation> operation = [SDWebImageManager.sharedManager loadImageWithURL:url options:options progress:progressBlock completed:^(UIImage *image, NSData *data, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
            __strong __typeof (wself) sself = wself;
            [sself sd_removeActivityIndicator];
            if (!sself) {
                return;
            }
            dispatch_main_async_safe(^{
                if (!sself) {
                    return;
                }
                if (image && (options & SDWebImageAvoidAutoSetImage) && completedBlock) {
                    completedBlock(image, error, cacheType, url);
                    return;
                } else if (image) {
                    [sself sd_setImage:image imageData:data basedOnClassOrViaCustomSetImageBlock:setImageBlock];
                    [sself sd_setNeedsLayout];
                } else {
                    if ((options & SDWebImageDelayPlaceholder)) {
                        [sself sd_setImage:placeholder imageData:nil basedOnClassOrViaCustomSetImageBlock:setImageBlock];
                        [sself sd_setNeedsLayout];
                    }
                }
                if (completedBlock && finished) {
                    completedBlock(image, error, cacheType, url);
                }
            });
        }];
        [self sd_setImageLoadOperation:operation forKey:validOperationKey];
    } else {
        dispatch_main_async_safe(^{
            [self sd_removeActivityIndicator];
            if (completedBlock) {
                NSError *error = [NSError errorWithDomain:SDWebImageErrorDomain code:-1 userInfo:@{NSLocalizedDescriptionKey : @"Trying to load a nil url"}];
                completedBlock(nil, error, SDImageCacheTypeNone, url);
            }
        });
    }
}

dispatch_main_async_safe() 的實現，
dispatch_queue_get_label獲取當前佇列標籤，使用strcmp()函式和主執行緒佇列標籤比較

#ifndef dispatch_main_async_safe
#define dispatch_main_async_safe(block)\
    if (strcmp(dispatch_queue_get_label(DISPATCH_CURRENT_QUEUE_LABEL), dispatch_queue_get_label(dispatch_get_main_queue())) == 0) {\
        block();\
    } else {\
        dispatch_async(dispatch_get_main_queue(), block);\
    }
#endif

該方法實現在UIView的分類UIView+WebCacheOperation

typedef NSMutableDictionary<NSString *, id> SDOperationsDictionary;
- (void)sd_cancelImageLoadOperationWithKey:(nullable NSString *)key {
    // Cancel in progress downloader from queue
    SDOperationsDictionary *operationDictionary = [self operationDictionary];
    id operations = operationDictionary[key];
    if (operations) {
        if ([operations isKindOfClass:[NSArray class]]) {
            for (id <SDWebImageOperation> operation in operations) {
                if (operation) {
                    [operation cancel];
                }
            }
        } else if ([operations conformsToProtocol:@protocol(SDWebImageOperation)]){
            [(id<SDWebImageOperation>) operations cancel];
        }
        [operationDictionary removeObjectForKey:key];
    }
}

- (SDOperationsDictionary *)operationDictionary {
// 通過變數繫結的方式，獲取到operations
 SDOperationsDictionary *operations = objc_getAssociatedObject(self, &loadOperationKey);
    if (operations) {
        return operations;
    }
// 第一次進入時，operations需要新建立，並進行變數繫結給loadOperationKey，然後返回
   operations = [NSMutableDictionary dictionary];
    objc_setAssociatedObject(self, &loadOperationKey, operations, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    return operations;
}

- (void)sd_setImage:(UIImage *)image imageData:(NSData *)imageData basedOnClassOrViaCustomSetImageBlock:(SDSetImageBlock)setImageBlock {
    if (setImageBlock) { // 本例中為nil 
        setImageBlock(image, imageData);
        return;
    }
// 下面程式碼就比較簡單了，根據self的類執行不同方法設定佔位圖。
 #if SD_UIKIT || SD_MAC // 平臺相容適配巨集
    if ([self isKindOfClass:[UIImageView class]]) {
        UIImageView *imageView = (UIImageView *)self;
        imageView.image = image;
    }
#endif
#if SD_UIKIT
    if ([self isKindOfClass:[UIButton class]]) {
        UIButton *button = (UIButton *)self;
        [button setImage:image forState:UIControlStateNormal];
    }
#endif
}