SDWebImage源码之DownLoader

SDWebImageDownloader下载器

源码+注释：

- (nullable SDWebImageDownloadToken *)downloadImageWithURL:(nullable NSURL *)url
                                                   options:(SDWebImageDownloaderOptions)options
                                                   context:(nullable SDWebImageContext *)context
                                                  progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                                                 completed:(nullable SDWebImageDownloaderCompletedBlock)completedBlock {
    // The URL will be used as the key to the callbacks dictionary so it cannot be nil. If it is nil immediately call the completed block with no image or data.
    if (url == nil) {//没有url直接 执行 complateblock
        if (completedBlock) {
            NSError *error = [NSError errorWithDomain:SDWebImageErrorDomain code:SDWebImageErrorInvalidURL userInfo:@{NSLocalizedDescriptionKey : @"Image url is nil"}];
            completedBlock(nil, nil, error, YES);
        }
        return nil;
    }
    
    SD_LOCK(self.operationsLock);//信号量为1实现加锁 operation
    NSOperation<SDWebImageDownloaderOperation> *operation = [self.URLOperations objectForKey:url];
    //已经完成 或者取消的opertion 都被删除 则下边重新新建opertion
    if (!operation || operation.isFinished || operation.isCancelled) {
//新建op
        operation = [self createDownloaderOperationWithUrl:url options:options context:context];
        if (!operation) {
//如果新建op失败 则解锁 返回并执行complateblock
            SD_UNLOCK(self.operationsLock);
            if (completedBlock) {
                NSError *error = [NSError errorWithDomain:SDWebImageErrorDomain code:SDWebImageErrorInvalidDownloadOperation userInfo:@{NSLocalizedDescriptionKey : @"Downloader operation is nil"}];
                completedBlock(nil, nil, error, YES);
            }
            return nil;
        }
        @weakify(self);//weakself
        operation.completionBlock = ^{
            @strongify(self);//延迟释放self'
            if (!self) {//如果释放了 则返回
                return;
            }
            SD_LOCK(self.operationsLock);//加锁 并删除url
            [self.URLOperations removeObjectForKey:url];
            SD_UNLOCK(self.operationsLock);
        };
        self.URLOperations[url] = operation;
        //加入到队列和字典中
        [self.downloadQueue addOperation:operation];
    }
    else if (!operation.isExecuting) {
//重置op的优先级
        if (options & SDWebImageDownloaderHighPriority) {
            operation.queuePriority = NSOperationQueuePriorityHigh;
        } else if (options & SDWebImageDownloaderLowPriority) {
            operation.queuePriority = NSOperationQueuePriorityLow;
        } else {
            operation.queuePriority = NSOperationQueuePriorityNormal;
        }
    }
//解锁
    SD_UNLOCK(self.operationsLock);
    
    id downloadOperationCancelToken = [operation addHandlersForProgress:progressBlock completed:completedBlock];
    //信息记录都token中 可以执行cancel操作
    SDWebImageDownloadToken *token = [[SDWebImageDownloadToken alloc] initWithDownloadOperation:operation];
    token.url = url;
    token.request = operation.request;
    token.downloadOperationCancelToken = downloadOperationCancelToken;
    token.downloader = self;
    
    return token;
}
//取消token
- (void)cancel:(nullable SDWebImageDownloadToken *)token {
    NSURL *url = token.url;
    if (!url) {
        return;
    }//加锁
    SD_LOCK(self.operationsLock);
    NSOperation<SDWebImageDownloaderOperation> *operation = [self.URLOperations objectForKey:url];
    if (operation) {
//取消下载 
        BOOL canceled = [operation cancel:token.downloadOperationCancelToken];
        if (canceled) {
            [self.URLOperations removeObjectForKey:url];
        }
    }
    SD_UNLOCK(self.operationsLock);
}
//取消NSOperationQueue中的所有的下载op
- (void)cancelAllDownloads {
    [self.downloadQueue cancelAllOperations];
}

统一下载入口

- (void)sd_internalSetImageWithURL:(nullable NSURL *)url
                  placeholderImage:(nullable UIImage *)placeholder
                           options:(SDWebImageOptions)options
                           context:(nullable SDWebImageContext *)context
                     setImageBlock:(nullable SDSetImageBlock)setImageBlock
                          progress:(nullable SDImageLoaderProgressBlock)progressBlock
                         completed:(nullable SDInternalCompletionBlock)completedBlock {
    context = [context copy]; // copy to avoid mutable object
    NSString *validOperationKey = context[SDWebImageContextSetImageOperationKey];
    if (!validOperationKey) {
        validOperationKey = NSStringFromClass([self class]);
    }
    self.sd_latestOperationKey = validOperationKey;
//取消view old的下载任务
    [self sd_cancelImageLoadOperationWithKey:validOperationKey];
    self.sd_imageURL = url;
    
    if (!(options & SDWebImageDelayPlaceholder)) {
        dispatch_main_async_safe(^{
            [self sd_setImage:placeholder imageData:nil basedOnClassOrViaCustomSetImageBlock:setImageBlock cacheType:SDImageCacheTypeNone imageURL:url];
        });
    }
    
    if (url) {
        // reset the progress
        self.sd_imageProgress.totalUnitCount = 0;
        self.sd_imageProgress.completedUnitCount = 0;
        
#if SD_UIKIT || SD_MAC
        // check and start image indicator
        [self sd_startImageIndicator];
        id<SDWebImageIndicator> imageIndicator = self.sd_imageIndicator;
#endif
        
        SDWebImageManager *manager = context[SDWebImageContextCustomManager];
        if (!manager) {
            manager = [SDWebImageManager sharedManager];
        }
        
        @weakify(self);
        SDImageLoaderProgressBlock combinedProgressBlock = ^(NSInteger receivedSize, NSInteger expectedSize, NSURL * _Nullable targetURL) {
            @strongify(self);
            NSProgress *imageProgress = self.sd_imageProgress;
            imageProgress.totalUnitCount = expectedSize;
            imageProgress.completedUnitCount = receivedSize;
#if SD_UIKIT || SD_MAC
            if ([imageIndicator respondsToSelector:@selector(updateIndicatorProgress:)]) {
                double progress = imageProgress.fractionCompleted;
                dispatch_async(dispatch_get_main_queue(), ^{
                    [imageIndicator updateIndicatorProgress:progress];
                });
            }
#endif
            if (progressBlock) {
                progressBlock(receivedSize, expectedSize, targetURL);
            }
        };
        id <SDWebImageOperation> operation = [manager loadImageWithURL:url options:options context:context progress:combinedProgressBlock completed:^(UIImage *image, NSData *data, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
            @strongify(self);
            if (!self) { return; }
            // if the progress not been updated, mark it to complete state
            if (finished && !error && self.sd_imageProgress.totalUnitCount == 0 && self.sd_imageProgress.completedUnitCount == 0) {
                self.sd_imageProgress.totalUnitCount = SDWebImageProgressUnitCountUnknown;
                self.sd_imageProgress.completedUnitCount = SDWebImageProgressUnitCountUnknown;
            }
            
#if SD_UIKIT || SD_MAC
            // check and stop image indicator
            if (finished) {
                [self sd_stopImageIndicator];
            }
#endif
            
            BOOL shouldCallCompletedBlock = finished || (options & SDWebImageAvoidAutoSetImage);
            BOOL shouldNotSetImage = ((image && (options & SDWebImageAvoidAutoSetImage)) ||
                                      (!image && !(options & SDWebImageDelayPlaceholder)));
            SDWebImageNoParamsBlock callCompletedBlockClojure = ^{
                if (!self) { return; }
                if (!shouldNotSetImage) {
                    [self sd_setNeedsLayout];
                }
                if (completedBlock && shouldCallCompletedBlock) {
                    completedBlock(image, data, error, cacheType, finished, url);
                }
            };
            
            // case 1a: we got an image, but the SDWebImageAvoidAutoSetImage flag is set
            // OR
            // case 1b: we got no image and the SDWebImageDelayPlaceholder is not set
            if (shouldNotSetImage) {
                dispatch_main_async_safe(callCompletedBlockClojure);
                return;
            }
            
            UIImage *targetImage = nil;
            NSData *targetData = nil;
            if (image) {
                // case 2a: we got an image and the SDWebImageAvoidAutoSetImage is not set
                targetImage = image;
                targetData = data;
            } else if (options & SDWebImageDelayPlaceholder) {
                // case 2b: we got no image and the SDWebImageDelayPlaceholder flag is set
                targetImage = placeholder;
                targetData = nil;
            }
            
#if SD_UIKIT || SD_MAC
            // check whether we should use the image transition
            SDWebImageTransition *transition = nil;
            if (finished && (options & SDWebImageForceTransition || cacheType == SDImageCacheTypeNone)) {
                transition = self.sd_imageTransition;
            }
#endif
            dispatch_main_async_safe(^{
#if SD_UIKIT || SD_MAC
                [self sd_setImage:targetImage imageData:targetData basedOnClassOrViaCustomSetImageBlock:setImageBlock transition:transition cacheType:cacheType imageURL:imageURL];
#else
                [self sd_setImage:targetImage imageData:targetData basedOnClassOrViaCustomSetImageBlock:setImageBlock cacheType:cacheType imageURL:imageURL];
#endif
                callCompletedBlockClojure();
            });
        }];
        [self sd_setImageLoadOperation:operation forKey:validOperationKey];
    } else {
#if SD_UIKIT || SD_MAC
        [self sd_stopImageIndicator];
#endif
        dispatch_main_async_safe(^{
            if (completedBlock) {
                NSError *error = [NSError errorWithDomain:SDWebImageErrorDomain code:SDWebImageErrorInvalidURL userInfo:@{NSLocalizedDescriptionKey : @"Image url is nil"}];
                completedBlock(nil, nil, error, SDImageCacheTypeNone, YES, url);
            }
        });
    }
}

最后放上镇楼图：

SDWebimage的框架概览