版本记录

前言

我们做APP，文字和图片是绝对不可缺少的元素，特别是图片一般存储在图床里面，一般公司可以委托第三方保存，NB的公司也可以自己存储图片，ios有很多图片加载的第三方框架，其中最优秀的莫过于SDWebImage，它几乎可以满足你所有的需求，用了好几年这个框架，今天想总结一下。感兴趣的可以看其他几篇。
1. SDWebImage探究（一）
2. SDWebImage探究（二）

详析SDWebImage

下面我们就详析SDWebImage的原理。

• 先看一下入口。

UIImageView+WebCache.m

//这个是入口，下载图片的结果方法最终都是走的这个方法

- (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];
}

• 下面进入的是UIView的一个分类

UIView+WebCache.m

- (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 {
    NSString *validOperationKey = operationKey ?: NSStringFromClass([self class]);
    [self sd_cancelImageLoadOperationWithKey:validOperationKey];
    objc_setAssociatedObject(self, &imageURLKey, url, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    
    if (!(options & SDWebImageDelayPlaceholder)) {
        dispatch_main_async_safe(^{
            [self sd_setImage:placeholder imageData:nil basedOnClassOrViaCustomSetImageBlock:setImageBlock];
        });
    }
    
    //如果URL不为空，调用管理器SDWebImageManager
    if (url) {
        //加载指示器
        if ([self sd_showActivityIndicatorView]) {
            [self sd_addActivityIndicator];
        }
        
        __weak __typeof(self)wself = self;
        //调用管理器SDWebImageManager
        id <SDWebImageOperation> operation = [SDWebImageManager.sharedManager loadImageWithURL:url options:options progress:progressBlock completed:^(UIImage *image, NSData *data, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
            //下面的是调用管理器完成的回调
            __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);
                }
            });
        }];
        //取消下载操作
        [self sd_setImageLoadOperation:operation forKey:validOperationKey];
    }
    //如果url为空就抛出异常
    else {
        dispatch_main_async_safe(^{
            //移除指示器
            [self sd_removeActivityIndicator];
            if (completedBlock) {
                //完成以后的回调
                NSError *error = [NSError errorWithDomain:SDWebImageErrorDomain code:-1 userInfo:@{NSLocalizedDescriptionKey : @"Trying to load a nil url"}];
                completedBlock(nil, error, SDImageCacheTypeNone, url);
            }
        });
    }
}

• 调用管理类SDWebImageManager

SDWebImageManager.m

- (id <SDWebImageOperation>)loadImageWithURL:(nullable NSURL *)url
                                     options:(SDWebImageOptions)options
                                    progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                                   completed:(nullable SDInternalCompletionBlock)completedBlock {
    //断言，判断completedBlock是否为空
    NSAssert(completedBlock != nil, @"If you mean to prefetch the image, use -[SDWebImagePrefetcher prefetchURLs] instead");

    //这里是一个容错机制，及时我们给url传递的NSString而不是NSURL，框架也会给我们自动转化
    if ([url isKindOfClass:NSString.class]) {
        url = [NSURL URLWithString:(NSString *)url];
    }

    //防崩溃处理，如果不是NSString和NSURL类型，就赋值为nil，防止app崩溃
    if (![url isKindOfClass:NSURL.class]) {
        url = nil;
    }

    __block SDWebImageCombinedOperation *operation = [SDWebImageCombinedOperation new];
    __weak SDWebImageCombinedOperation *weakOperation = operation;

    BOOL isFailedUrl = NO;
    if (url) {
        //这里的是failedURLs是NSMutableSet集合
        @synchronized (self.failedURLs) {
            //判断集合中是否包含这个url
            isFailedUrl = [self.failedURLs containsObject:url];
        }
    }

    if (url.absoluteString.length == 0 || (!(options & SDWebImageRetryFailed) && isFailedUrl)) {
        //调用操作完成模块
        [self callCompletionBlockForOperation:operation completion:completedBlock error:[NSError errorWithDomain:NSURLErrorDomain code:NSURLErrorFileDoesNotExist userInfo:nil] url:url];
        return operation;
    }

    @synchronized (self.runningOperations) {
        //如果操作operation不为空，就讲该操作放在可变数组runningOperations里面
        [self.runningOperations addObject:operation];
    }
    //获取urlKey作为操作的key
    NSString *key = [self cacheKeyForURL:url];

    //查询该操作是否在缓存操作中
    operation.cacheOperation = [self.imageCache queryCacheOperationForKey:key done:^(UIImage *cachedImage, NSData *cachedData, SDImageCacheType cacheType) {
        //如果缓存中有这个操作，就移除这个operation
        if (operation.isCancelled) {
            [self safelyRemoveOperationFromRunning:operation];
            return;
        }
        
        //如果缓存中没有图片，就调用先判断是否可以下载
        if ((!cachedImage || options & SDWebImageRefreshCached) && (![self.delegate respondsToSelector:@selector(imageManager:shouldDownloadImageForURL:)] || [self.delegate imageManager:self shouldDownloadImageForURL:url])) {
            if (cachedImage && options & SDWebImageRefreshCached) {
                //如果缓存中有图片，设置了SDWebImageRefreshCached，就会重新下载，NSURLCache从服务器重新下载
                [self callCompletionBlockForOperation:weakOperation completion:completedBlock image:cachedImage data:cachedData error:nil cacheType:cacheType finished:YES url:url];
            }

            //如果没有图片，利用代理进行下载，下面是下载选项的枚举
            SDWebImageDownloaderOptions downloaderOptions = 0;
            if (options & SDWebImageLowPriority) downloaderOptions |= SDWebImageDownloaderLowPriority;
            if (options & SDWebImageProgressiveDownload) downloaderOptions |= SDWebImageDownloaderProgressiveDownload;
            if (options & SDWebImageRefreshCached) downloaderOptions |= SDWebImageDownloaderUseNSURLCache;
            if (options & SDWebImageContinueInBackground) downloaderOptions |= SDWebImageDownloaderContinueInBackground;
            if (options & SDWebImageHandleCookies) downloaderOptions |= SDWebImageDownloaderHandleCookies;
            if (options & SDWebImageAllowInvalidSSLCertificates) downloaderOptions |= SDWebImageDownloaderAllowInvalidSSLCertificates;
            if (options & SDWebImageHighPriority) downloaderOptions |= SDWebImageDownloaderHighPriority;
            if (options & SDWebImageScaleDownLargeImages) downloaderOptions |= SDWebImageDownloaderScaleDownLargeImages;
            
            if (cachedImage && options & SDWebImageRefreshCached) {
                downloaderOptions &= ~SDWebImageDownloaderProgressiveDownload;
                downloaderOptions |= SDWebImageDownloaderIgnoreCachedResponse;
            }
            
            //下载图片并给出回调结果
            SDWebImageDownloadToken *subOperationToken = [self.imageDownloader downloadImageWithURL:url options:downloaderOptions progress:progressBlock completed:^(UIImage *downloadedImage, NSData *downloadedData, NSError *error, BOOL finished) {
                __strong __typeof(weakOperation) strongOperation = weakOperation;
                if (!strongOperation || strongOperation.isCancelled) {
                    //如果操作不存在或者操作被取消，就什么都不做，如果我们调用完成block，对于同一个对象，在这个block和其他的完成block之间就会产生竞争状态，因此如果这个被二次调用，我们就会覆盖新数据。
                }
                else if (error) {
                    [self callCompletionBlockForOperation:strongOperation completion:completedBlock error:error url:url];
                    //错误码处理
                    if (   error.code != NSURLErrorNotConnectedToInternet
                        && error.code != NSURLErrorCancelled
                        && error.code != NSURLErrorTimedOut
                        && error.code != NSURLErrorInternationalRoamingOff
                        && error.code != NSURLErrorDataNotAllowed
                        && error.code != NSURLErrorCannotFindHost
                        && error.code != NSURLErrorCannotConnectToHost) {
                        @synchronized (self.failedURLs) {
                            [self.failedURLs addObject:url];
                        }
                    }
                }
                else {
                    //如果下载失败的话，就将这个url从集合set中移除
                    if ((options & SDWebImageRetryFailed)) {
                        @synchronized (self.failedURLs) {
                            [self.failedURLs removeObject:url];
                        }
                    }
                    
                    BOOL cacheOnDisk = !(options & SDWebImageCacheMemoryOnly);

                    if (options & SDWebImageRefreshCached && cachedImage && !downloadedImage) {
                        // Image refresh hit the NSURLCache cache, do not call the completion block
                    }
                    else if (downloadedImage && (!downloadedImage.images || (options & SDWebImageTransformAnimatedImage)) && [self.delegate respondsToSelector:@selector(imageManager:transformDownloadedImage:withURL:)]) {
                        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
                            UIImage *transformedImage = [self.delegate imageManager:self transformDownloadedImage:downloadedImage withURL:url];

                            if (transformedImage && finished) {
                                BOOL imageWasTransformed = ![transformedImage isEqual:downloadedImage];
                                // pass nil if the image was transformed, so we can recalculate the data from the image
                                [self.imageCache storeImage:transformedImage imageData:(imageWasTransformed ? nil : downloadedData) forKey:key toDisk:cacheOnDisk completion:nil];
                            }
                            
                            [self callCompletionBlockForOperation:strongOperation completion:completedBlock image:transformedImage data:downloadedData error:nil cacheType:SDImageCacheTypeNone finished:finished url:url];
                        });
                    }
                    else {
                        if (downloadedImage && finished) {
                            [self.imageCache storeImage:downloadedImage imageData:downloadedData forKey:key toDisk:cacheOnDisk completion:nil];
                        }
                        [self callCompletionBlockForOperation:strongOperation completion:completedBlock image:downloadedImage data:downloadedData error:nil cacheType:SDImageCacheTypeNone finished:finished url:url];
                    }
                }

                if (finished) {
                    //完成后才操作集合中移除该操作
                    [self safelyRemoveOperationFromRunning:strongOperation];
                }
            }];
            //如果操作取消了，同样从操作中移除该操作
            operation.cancelBlock = ^{
                [self.imageDownloader cancel:subOperationToken];
                __strong __typeof(weakOperation) strongOperation = weakOperation;
                [self safelyRemoveOperationFromRunning:strongOperation];
            };
        }
        //如果有缓存图片
        else if (cachedImage) {
            __strong __typeof(weakOperation) strongOperation = weakOperation;
            //调用完成回调，并且将该操作从数组中移除
            [self callCompletionBlockForOperation:strongOperation completion:completedBlock image:cachedImage data:cachedData error:nil cacheType:cacheType finished:YES url:url];
            [self safelyRemoveOperationFromRunning:operation];
        }
        else {
            //缓存中没有图片，也不用代理下载，并移除操作
            __strong __typeof(weakOperation) strongOperation = weakOperation;
            [self callCompletionBlockForOperation:strongOperation completion:completedBlock image:nil data:nil error:nil cacheType:SDImageCacheTypeNone finished:YES url:url];
            [self safelyRemoveOperationFromRunning:operation];
        }
    }];

    return operation;
}

• 在上面的那个步骤下载前，需要先检查缓存SDImageCache。

SDImageCache.m

//查询是否缓存过该操作
- (nullable NSOperation *)queryCacheOperationForKey:(nullable NSString *)key done:(nullable SDCacheQueryCompletedBlock)doneBlock
{
    //不存在这个key，就调用完成block
    if (!key) {
        if (doneBlock) {
            doneBlock(nil, nil, SDImageCacheTypeNone);
        }
        return nil;
    }

    // 先检查内存是否存在该图片
    UIImage *image = [self imageFromMemoryCacheForKey:key];
    if (image) {
        NSData *diskData = nil;
        if ([image isGIF]) {//gif
            diskData = [self diskImageDataBySearchingAllPathsForKey:key];
        }
        if (doneBlock) {
            doneBlock(image, diskData, SDImageCacheTypeMemory);
        }
        return nil;
    }

    //实例化一个新操作
    NSOperation *operation = [NSOperation new];
    dispatch_async(self.ioQueue, ^{
        if (operation.isCancelled) {
            //取消操作就返回
            return;
        }

        @autoreleasepool {
            NSData *diskData = [self diskImageDataBySearchingAllPathsForKey:key];
            UIImage *diskImage = [self diskImageForKey:key];
            //如果盘中存在图片，并且如果可以使用缓存
            if (diskImage && self.config.shouldCacheImagesInMemory) {
                //图片的尺寸
                NSUInteger cost = SDCacheCostForImage(diskImage);
                //将硬盘中的图片在缓存中存一份
                [self.memCache setObject:diskImage forKey:key cost:cost];
            }

            if (doneBlock) {
                dispatch_async(dispatch_get_main_queue(), ^{
                    //完成的回调，返回图片，数据以及图片的获取途径
//                    typedef NS_ENUM(NSInteger, SDImageCacheType) {
//                       //缓存中不存在图片，要从网上下载
//                        SDImageCacheTypeNone,
//                       //图片缓存在磁盘
//                        SDImageCacheTypeDisk,
//                         图片缓存在内存
//                        SDImageCacheTypeMemory
//                    };

                    doneBlock(diskImage, diskData, SDImageCacheTypeDisk);
                });
            }
        }
    });

    return operation;
}

• 如果上面的缓存中没有，下面就是下载图片了SDWebImageDownloader。

SDWebImageDownloader.m

//下面就是下载图片的方法了

- (nullable SDWebImageDownloadToken *)downloadImageWithURL:(nullable NSURL *)url
                                                   options:(SDWebImageDownloaderOptions)options
                                                  progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                                                 completed:(nullable SDWebImageDownloaderCompletedBlock)completedBlock {
    __weak SDWebImageDownloader *wself = self;

    //进度回调
    return [self addProgressCallback:progressBlock completedBlock:completedBlock forURL:url createCallback:^SDWebImageDownloaderOperation *{
        __strong __typeof (wself) sself = wself;
        //设置超时15s
        NSTimeInterval timeoutInterval = sself.downloadTimeout;
        if (timeoutInterval == 0.0) {
            timeoutInterval = 15.0;
        }

        // NSURLCache + SDImageCache)防止重复缓存，开始请求图片
        NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:url cachePolicy:(options & SDWebImageDownloaderUseNSURLCache ? NSURLRequestUseProtocolCachePolicy : NSURLRequestReloadIgnoringLocalCacheData) timeoutInterval:timeoutInterval];
        request.HTTPShouldHandleCookies = (options & SDWebImageDownloaderHandleCookies);
        request.HTTPShouldUsePipelining = YES;
        //过滤HTTP请求，该字典用来HTTP请求的header
        if (sself.headersFilter) {
            request.allHTTPHeaderFields = sself.headersFilter(url, [sself.HTTPHeaders copy]);
        }
        else {
            request.allHTTPHeaderFields = sself.HTTPHeaders;
        }
        SDWebImageDownloaderOperation *operation = [[sself.operationClass alloc] initWithRequest:request inSession:sself.session options:options];
        operation.shouldDecompressImages = sself.shouldDecompressImages;
        
        //设置默认的 URL credential
        if (sself.urlCredential) {
            operation.credential = sself.urlCredential;
        } else if (sself.username && sself.password) {
            operation.credential = [NSURLCredential credentialWithUser:sself.username password:sself.password persistence:NSURLCredentialPersistenceForSession];
        }
        
        //设置队列的优先级级别
        if (options & SDWebImageDownloaderHighPriority) {
            operation.queuePriority = NSOperationQueuePriorityHigh;
        } else if (options & SDWebImageDownloaderLowPriority) {
            operation.queuePriority = NSOperationQueuePriorityLow;
        }
        
        //将操作放在NSOperationQueue中
        [sself.downloadQueue addOperation:operation];
        //设置操作的执行顺序
        //typedef NS_ENUM(NSInteger, SDWebImageDownloaderExecutionOrder) {
            //队列模式  先进先出，这个是默认值
            //SDWebImageDownloaderFIFOExecutionOrder,
            //堆栈模式，后进先出
            //SDWebImageDownloaderLIFOExecutionOrder
        //};

        if (sself.executionOrder == SDWebImageDownloaderLIFOExecutionOrder) {
            //添加操作的依赖关系，这里利用lastAddedOperation记录操作
            [sself.lastAddedOperation addDependency:operation];
            sself.lastAddedOperation = operation;
        }

        return operation;
    }];
}

• 上面的SDWebImageDownloadToken是从下面这个方法中获取的。

SDWebImageDownloader.m

//加入进度回调，返回SDWebImageDownloadToken对象

- (nullable SDWebImageDownloadToken *)addProgressCallback:(SDWebImageDownloaderProgressBlock)progressBlock
                                           completedBlock:(SDWebImageDownloaderCompletedBlock)completedBlock
                                                   forURL:(nullable NSURL *)url
                                           createCallback:(SDWebImageDownloaderOperation *(^)())createCallback {
    //如果URL用于回调字典的key，不能为空，如果为空，就会立即调用完成block，不带有图片和数据
    if (url == nil) {
        if (completedBlock != nil) {
            completedBlock(nil, nil, nil, NO);
        }
        return nil;
    }

    __block SDWebImageDownloadToken *token = nil;

    //加入GCD阻塞
    dispatch_barrier_sync(self.barrierQueue, ^{
        //根据url作为key返回操作
        SDWebImageDownloaderOperation *operation = self.URLOperations[url];
        if (!operation) {
            operation = createCallback();
            self.URLOperations[url] = operation;

            __weak SDWebImageDownloaderOperation *woperation = operation;
            operation.completionBlock = ^{
              SDWebImageDownloaderOperation *soperation = woperation;
              if (!soperation)
                  return;
             //如果这次加入的操作已经存在，就移除该操作
              if (self.URLOperations[url] == soperation) {
                  [self.URLOperations removeObjectForKey:url];
              };
            };
        }
        id downloadOperationCancelToken = [operation addHandlersForProgress:progressBlock completed:completedBlock];
        //重新初始化SDWebImageDownloadToken对象
        token = [SDWebImageDownloadToken new];
        token.url = url;
        token.downloadOperationCancelToken = downloadOperationCancelToken;
    });

    return token;
}

• 下载完图片就对图片进行解码。

#import <Foundation/Foundation.h>
#import "SDWebImageCompat.h"

@interface UIImage (ForceDecode)

+ (nullable UIImage *)decodedImageWithImage:(nullable UIImage *)image;

+ (nullable UIImage *)decodedAndScaledDownImageWithImage:(nullable UIImage *)image;

@end

  主要的这些功能了，剩下的就是下载完成以后，对图片进行进一步的缓存处理。

后记

  这个框架写的确实很好，从整个架构的考虑，到细节处理，从编程思想到具体实现，都是我们可以学习的地方，未完，待续~~~

ocean && sky