SDWebImage源码解读之分类

第十一篇

前言

我们知道SDWebImageManager是用来管理图片下载的，但我们平时的开发更多的是使用UIImageView和UIButton这两个控件显示图片。

按照正常的想法，我们只需要在他们的分类中，通过SDWebImageManager把图片下载下载之后，再进行赋值就行了。但这样的设计并不是最好的设计，我们在准备提供一项功能的时候，应该要尽可能的弄明白这个功能的使用者是谁？这些使用者的共同点是什么？

UIImageView和UIButton这两个控件都继承自UIView。那么我们是不是可以给UIView赋予加载图片的功能，然后，
UIImageView和UIButton不就都有这个能力了吗？

除了讲解这些之外，在本文的最后，我们给view的layer也添加这个能力。

UIView+WebCache

/**
 * Set the imageView `image` with an `url` and optionally a placeholder image.
 *
 * The download is asynchronous and cached.
 *
 * @param url            The url for the image.
 * @param placeholder    The image to be set initially, until the image request finishes.
 * @param options        The options to use when downloading the image. @see SDWebImageOptions for the possible values.
 * @param operationKey   A string to be used as the operation key. If nil, will use the class name
 * @param setImageBlock  Block used for custom set image code
 * @param progressBlock  A block called while image is downloading
 *                       @note the progress block is executed on a background queue
 * @param completedBlock A block called when operation has been completed. This block has no return value
 *                       and takes the requested UIImage as first parameter. In case of error the image parameter
 *                       is nil and the second parameter may contain an NSError. The third parameter is a Boolean
 *                       indicating if the image was retrieved from the local cache or from the network.
 *                       The fourth parameter is the original image url.
 */
- (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;
                         

实现方法：

- (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 {
    /// 每一个view都给他绑定一个operation，通过一个key来获取这个operation
    /// 如果operationKey为nil，就采用他自身的类的字符串作为key
    NSString *validOperationKey = operationKey ?: NSStringFromClass([self class]);
    /// 这是一个view的分类方法，目的是取消之前绑定的operation
    [self sd_cancelImageLoadOperationWithKey:validOperationKey];
    /// 为自身绑定一个URL
    objc_setAssociatedObject(self, &imageURLKey, url, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    
    /// 如果不是延迟显示placeholder的情况
    if (!(options & SDWebImageDelayPlaceholder)) {
        dispatch_main_async_safe(^{
            /// 在图片下载下来之前，先给自身赋值一个placeholder
            [self sd_setImage:placeholder imageData:nil basedOnClassOrViaCustomSetImageBlock:setImageBlock];
        });
    }
    
    if (url) {
        // check if activityView is enabled or not
        /// 如果设置了显示加载动画，就添加ActivityIndicatorView
        if ([self sd_showActivityIndicatorView]) {
            [self sd_addActivityIndicator];
        }
        
        /// 开始下载图片
        __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;
            /// 下载完成后关闭动画
            [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 {
                    /// 如果是设置了SDWebImageDelayPlaceholder，那么就会在下载完成之后给自身赋值placeholder
                    if ((options & SDWebImageDelayPlaceholder)) {
                        [sself sd_setImage:placeholder imageData:nil basedOnClassOrViaCustomSetImageBlock:setImageBlock];
                        [sself sd_setNeedsLayout];
                    }
                }
                if (completedBlock && finished) {
                    completedBlock(image, error, cacheType, url);
                }
            });
        }];
        /// 绑定operation
        [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);
            }
        });
    }
}

其实这个核心的实现方法也很简单，逻辑也不复杂，需要注意一下下边的方法：

- (void)sd_setImage:(UIImage *)image imageData:(NSData *)imageData basedOnClassOrViaCustomSetImageBlock:(SDSetImageBlock)setImageBlock {
    if (setImageBlock) {
        setImageBlock(image, imageData);
        return;
    }
    
#if SD_UIKIT || SD_MAC
    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
}

这个方法说明如果我们设置了setImageBlock,那么view的显示图片的逻辑应该写到这个Block中，如果setImageBlock为nil，就判断是不是UIImageView和UIButton。

UIImageView的实现

- (void)sd_setImageWithURL:(nullable NSURL *)url
          placeholderImage:(nullable UIImage *)placeholder
                   options:(SDWebImageOptions)options
                  progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                 completed:(nullable SDExternalCompletionBlock)completedBlock {
    [self sd_internalSetImageWithURL:url
                    placeholderImage:placeholder
                             options:options
                        operationKey:nil
                       setImageBlock:nil
                            progress:progressBlock
                           completed:completedBlock];
}

CALay的实现

- (void)sd_setImageWithURL:(nullable NSURL *)url
          placeholderImage:(nullable UIImage *)placeholder
                   options:(SDWebImageOptions)options
                  progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                 completed:(nullable SDExternalCompletionBlock)completedBlock {
    NSString *validOperationKey = @"CALayerImages" ?: NSStringFromClass([self class]);
    [self sd_cancelImageLoadOperationWithKey:validOperationKey];
    objc_setAssociatedObject(self, &imageURLKey, url, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    
    if (!(options & SDWebImageDelayPlaceholder)) {
        dispatch_main_async_safe(^{
            self.contents = (__bridge id _Nullable)placeholder.CGImage;
        });
    }
    
    if (url) {
        __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;

            if (!sself) {
                return;
            }
            dispatch_main_async_safe(^{
                if (!sself) {
                    return;
                }
                if (image && (options & SDWebImageAvoidAutoSetImage) && completedBlock) {
                    completedBlock(image, error, cacheType, url);
                    return;
                } else if (image) {
                    self.contents = (__bridge id _Nullable)image.CGImage;
                   
                } else {
                    if ((options & SDWebImageDelayPlaceholder)) {
                        self.contents = (__bridge id _Nullable)placeholder.CGImage;
                    }
                }
                if (completedBlock && finished) {
                    completedBlock(image, error, cacheType, url);
                }
            });
        }];
        [self sd_setImageLoadOperation:operation forKey:validOperationKey];
    } else {
        dispatch_main_async_safe(^{
   
            if (completedBlock) {
                NSError *error = [NSError errorWithDomain:SDWebImageErrorDomain code:-1 userInfo:@{NSLocalizedDescriptionKey : @"Trying to load a nil url"}];
                completedBlock(nil, error, SDImageCacheTypeNone, url);
            }
        });
    }
}




- (SDOperationsDictionary *)operationDictionary {
    SDOperationsDictionary *operations = objc_getAssociatedObject(self, &loadOperationKey);
    if (operations) {
        return operations;
    }
    operations = [NSMutableDictionary dictionary];
    objc_setAssociatedObject(self, &loadOperationKey, operations, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    return operations;
}

- (void)sd_setImageLoadOperation:(nullable id)operation forKey:(nullable NSString *)key {
    if (key) {
        [self sd_cancelImageLoadOperationWithKey:key];
        if (operation) {
            SDOperationsDictionary *operationDictionary = [self operationDictionary];
            operationDictionary[key] = operation;
        }
    }
}

- (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];
    }
}

- (void)sd_removeImageLoadOperationWithKey:(nullable NSString *)key {
    if (key) {
        SDOperationsDictionary *operationDictionary = [self operationDictionary];
        [operationDictionary removeObjectForKey:key];
    }
}

实现思路跟给view添加该能力完全相同。

 [self.imageView.layer sd_setImageWithURL:url placeholderImage:[UIImage imageNamed:@"4-3397163ecdb3855a0a4139c34a695885.jpg"] options:0 progress:nil completed:nil];

总结

SDWebImage的源码就看到这里了，下一篇我会把这十一篇中用到的知识点进行一个总结。