在开发iOS的客户端应用时，经常需要从服务器下载图片，虽然系统提供了下载工具：NSData、NSURLSession等等方法，但是考虑到图片下载过程中，需要考虑的因素比较多，比如：异步下载、图片缓存、错误处理、编码解码等，以及实际需要中根据不同网络加载不同画质的图片等等需求，因此下载操作不是一个简单的下载动作就可以解决。
针对上述问题，目前常用的开源库就是SDWebImage，它很好的解决了图片的异步下载、图片缓存、错误处理等问题，得到了广泛的应用，使得设置UIImageView、UIButton对象的图片十分方便。本文就从源码的角度，剖析一下这款优秀的开源库的具体实现。
类结构图
SDWebImage的源码的类结构图和下载流程图在官方的说明文档里有介绍，通过UML类结构图详细的介绍了该框架的内部结构，以及通过流程图介绍了具体的下载过程。
下图是我总结的SDWebImage的结构图，简单的把SDWebImage源码文件按照功能进行了划分，方便在阅读源码时，能快速的对源码有一个总体的认识，加快阅读效率。
关键类功能简介：
- 1.下载类
SDWebImageDownloader：提供下载的方法给SDWebImageManager使用，提供了最大并发量的下载控制、超时时间、取消下载、下载挂起、是否解压图片等等功能。同时，还提供了开始下载和停止下载的通知，给使用者监测下载状态，如果使用者不用监测下载状态，就不用监测该通知，这种设计模式很灵活，给使用者提供了更方便的选择。
extern NSString * _Nonnull const SDWebImageDownloadStartNotification;
extern NSString * _Nonnull const SDWebImageDownloadStopNotification;
SDWebImageDownloaderOperation：继承自NSOperation，是图片下载的具体实现类，通过加入到NSOperationQueue中，然后在start方法中来开启下载操作。
- 2.图片缓存
SDImageCacheConfig：主要提供缓存的配置信息，如：是否解压图片、是否缓存到内存、最大缓存时间（默认是一周）和最大缓存的字节数等等。
SDImageCache：缓存实现类，提供最大缓存字节、最大缓存条目的控制，以及缓存到内存及磁盘、从内存或磁盘删除、查询检索和查询缓存信息等功能。
- 3.分类
UIImageView+WebCache：UIImageView的分类，提供了设置UIImageView对象图片的多种方法，下面的方法可以说是SDWebImage框架中最常用的方法。
- (void)sd_setImageWithURL:(nullable NSURL *)url
placeholderImage:(nullable UIImage *)placeholder
options:(SDWebImageOptions)options;
// 带完成block的赋值方法
- (void)sd_setImageWithURL:(nullable NSURL *)url
placeholderImage:(nullable UIImage *)placeholder
options:(SDWebImageOptions)options
completed:(nullable SDExternalCompletionBlock)completedBlock;
UIButton+WebCache：UIButton的分类，提供了设置按钮图片和按钮背景图片的功能
- (void)sd_setImageWithURL:(nullable NSURL *)url
forState:(UIControlState)state
placeholderImage:(nullable UIImage *)placeholder;
- 4.工具类
SDWebImageDecoder：图像解码的工具类，通过imageNames:加载图片会立即进行解码，而通过imageWithContentsOfFile:则不会
SDWebImagePrefetcher：批量图像下载工具，针对UI界面中需要下载多个图片时，又要在滑动中保持流畅体验，则可以使用该工具类批量下载图片，然后在给具体的UI控件设置图片时，就会直接从缓存中取
SDWebImageManager：下载管理类工具，是SDWebImage的核心类，从官方文档的类图中也可以看出，提供了查看图片是否已经缓存、下载图片、缓存图片、取消所有的下载等等功能
- 5.图片格式类
NSData+ImageContentType：根据图片数据的第一个字节来获取图片的格式，可以区分PNG、JPEG、GIF、TIFF和WebP。
以上只是对SDWebImage类结构图的简单分析，如果需要进一步了解各个类的具体实现，请参考文末的资料，已有人详细的介绍了各个类的功能实现原理或方法。
应用
下面介绍一个在应用SDWebImage设置UI图片的源码实现过程
在UIImageView上的应用
设置图片
通过设置URL、占位图片、图片配置、图片下载进度回调和设置完成回调来给UIImageView对象设置图片
// ViewController.m
[self.imageView sd_setImageWithURL:[NSURL URLWithString:@"http://rescdn.qqmail.com/dyimg/20140302/73EB27F4A350.jpg"] placeholderImage:[UIImage imageNamed:@"gift-icon"] options:0 progress:nil completed:nil];
上述代码调用UIImageView+WebCache.m里的方法
- (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];
}
然后调用UIView+WebCache.m中的方法获取图片，然后根据option的类型进行不同的设置
// 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
{
...
if (url) {
...
__weak __typeof(self)wself = self;
// 开始加载图片
id <SDWebImageOperation> operation = [SDWebImageManager.sharedManager loadImageWithURL:url options:options progress:progressBlock completed:^(UIImage *image, NSData *data, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
...
dispatch_main_async_safe(^{
if (!sself) {
return;
}
if (image && (options & SDWebImageAvoidAutoSetImage) && completedBlock) {
// 把图片放到completedBlock里处理，一般是手动设置图片，因为这样可以对图片做进一步处理
completedBlock(image, error, cacheType, url);
return;
} else if (image) {
// 设置图片
[sself sd_setImage:image imageData:data basedOnClassOrViaCustomSetImageBlock:setImageBlock];
[sself sd_setNeedsLayout];
} else {
// 延迟加载占位图（获取图片之后）
if ((options & SDWebImageDelayPlaceholder)) {
[sself sd_setImage:placeholder imageData:nil basedOnClassOrViaCustomSetImageBlock:setImageBlock];
[sself sd_setNeedsLayout];
}
}
// 回调完成block，如果是nil，则不调用
if (completedBlock && finished) {
completedBlock(image, error, cacheType, url);
}
});
}];
} else {
// 处理url为nil的情况
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);
}
});
}
}
加载图片的具体实现代码在SDWebImageManager里面，先从缓存中取图片，如果缓存中没有图片，就从网络下载，然后设置图片，最后再缓存该图片
// SDWebImageManager.m
- (id <SDWebImageOperation>)loadImageWithURL:(nullable NSURL *)url
options:(SDWebImageOptions)options
progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
completed:(nullable SDInternalCompletionBlock)completedBlock
{
...
// 从缓存中取图片
operation.cacheOperation = [self.imageCache queryCacheOperationForKey:key done:^(UIImage *cachedImage, NSData *cachedData, SDImageCacheType cacheType) {
if (operation.isCancelled) {
// 如果操作被取消，就从runningOperations操作数组从把该操作删除
[self safelyRemoveOperationFromRunning:operation];
return;
}
if ((!cachedImage || options & SDWebImageRefreshCached) && (![self.delegate respondsToSelector:@selector(imageManager:shouldDownloadImageForURL:)] || [self.delegate imageManager:self shouldDownloadImageForURL:url])) {
if (cachedImage && options & SDWebImageRefreshCached) {
// 如果options设置为更新缓存，那么就需要从服务器从新下载图片，然后更新本地缓存
[self callCompletionBlockForOperation:weakOperation completion:completedBlock image:cachedImage data:cachedData error:nil cacheType:cacheType finished:YES url:url];
}
...
// 创建下载器，从服务器下载图片
SDWebImageDownloadToken *subOperationToken = [self.imageDownloader downloadImageWithURL:url options:downloaderOptions progress:progressBlock completed:^(UIImage *downloadedImage, NSData *downloadedData, NSError *error, BOOL finished) {
__strong __typeof(weakOperation) strongOperation = weakOperation;
...
else {
// 设置了options为失败了重试，则会把失败的url加入failedURLs数组
if ((options & SDWebImageRetryFailed)) {
@synchronized (self.failedURLs) {
[self.failedURLs removeObject:url];
}
}
...
} else if (downloadedImage && (!downloadedImage.images || (options & SDWebImageTransformAnimatedImage)) && [self.delegate respondsToSelector:@selector(imageManager:transformDownloadedImage:withURL:)]) {
// 对图片进行transform操作
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 {
// 缓存图片，有缓存到内存和磁盘两种方式
if (downloadedImage && finished) {
[self.imageCache storeImage:downloadedImage imageData:downloadedData forKey:key toDisk:cacheOnDisk completion:nil];
}
// 回调完成的block
[self callCompletionBlockForOperation:strongOperation completion:completedBlock image:downloadedImage data:downloadedData error:nil cacheType:SDImageCacheTypeNone finished:finished url:url];
}
}
if (finished) {
// 下载完成，就从runningOperations数组中删除操作
[self safelyRemoveOperationFromRunning:strongOperation];
}
}];
// 设置取消下载的回调
operation.cancelBlock = ^{
[self.imageDownloader cancel:subOperationToken];
__strong __typeof(weakOperation) strongOperation = weakOperation;
[self safelyRemoveOperationFromRunning:strongOperation];
};
} else if (cachedImage) {
// 从缓存在取到图片，回调完成block
__strong __typeof(weakOperation) strongOperation = weakOperation;
[self callCompletionBlockForOperation:strongOperation completion:completedBlock image:cachedImage data:cachedData error:nil cacheType:cacheType finished:YES url:url];
[self safelyRemoveOperationFromRunning:operation];
}
...
}];
return operation;
}
从缓存中取图片，是先从内存中取，如果在内存中取到，就在当前线程中直接回调doneBlock；如果内存中没有，就开子线程从磁盘中取，如果取到图片，就回调doneBlock
// SDImageCache.m
- (nullable NSOperation *)queryCacheOperationForKey:(nullable NSString *)key done:(nullable SDCacheQueryCompletedBlock)doneBlock {
...
// First check the in-memory cache...
UIImage *image = [self imageFromMemoryCacheForKey:key];
if (image) {
NSData *diskData = nil;
if ([image isGIF]) {
diskData = [self diskImageDataBySearchingAllPathsForKey:key];
}
if (doneBlock) {
doneBlock(image, diskData, SDImageCacheTypeMemory);
}
return nil;
}
NSOperation *operation = [NSOperation new];
dispatch_async(self.ioQueue, ^{
if (operation.isCancelled) {
// do not call the completion if cancelled
return;
}
@autoreleasepool {
// 从磁盘中取图片的data
NSData *diskData = [self diskImageDataBySearchingAllPathsForKey:key];
// 从磁盘中直接取图片
UIImage *diskImage = [self diskImageForKey:key];
if (diskImage && self.config.shouldCacheImagesInMemory) {
NSUInteger cost = SDCacheCostForImage(diskImage);
// 缓存到内存中
[self.memCache setObject:diskImage forKey:key cost:cost];
}
if (doneBlock) {
dispatch_async(dispatch_get_main_queue(), ^{
doneBlock(diskImage, diskData, SDImageCacheTypeDisk);
});
}
}
});
return operation;
}
图片的下载过程是在SDWebImageDownloader.m中进行的，实质是通过SDWebImageDownloaderOperation(继承自NSOperation)对象，把该对象加入到downloadQueue里，然后在start方法里通过NSURLSession来下载图片。（其中，NSOperation有两个方法：main和start，如果想使用同步，那么最简单方法的就是把逻辑写在main()中，使用异步，需要把逻辑写到start()中，然后加入到队列之中）
// SDWebImageDownloader.m
- (nullable SDWebImageDownloadToken *)downloadImageWithURL:(nullable NSURL *)url
options:(SDWebImageDownloaderOptions)options
progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
completed:(nullable SDWebImageDownloaderCompletedBlock)completedBlock {
__weak SDWebImageDownloader *wself = self;
return [self addProgressCallback:progressBlock completedBlock:completedBlock forURL:url createCallback:^SDWebImageDownloaderOperation *{
__strong __typeof (wself) sself = wself;
NSTimeInterval timeoutInterval = sself.downloadTimeout;
// 设置超时时间为15s
if (timeoutInterval == 0.0) {
timeoutInterval = 15.0;
}
// In order to prevent from potential duplicate caching (NSURLCache + SDImageCache) we disable the cache for image requests if told otherwise
NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:url cachePolicy:(options & SDWebImageDownloaderUseNSURLCache ? NSURLRequestUseProtocolCachePolicy : NSURLRequestReloadIgnoringLocalCacheData) timeoutInterval:timeoutInterval];
request.HTTPShouldHandleCookies = (options & SDWebImageDownloaderHandleCookies);
request.HTTPShouldUsePipelining = YES;
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;
...
// 加入操作队列，开始下载
[sself.downloadQueue addOperation:operation];
...
return operation;
}];
}
把SDWebImageDownloaderOperation对象加入到操作队列，就开始调用该对象的start方法。
// SDWebImageDownloaderOperation.m
- (void)start {
// 如果操作被取消，就reset设置
@synchronized (self) {
if (self.isCancelled) {
self.finished = YES;
[self reset];
return;
}
...
NSURLSession *session = self.unownedSession;
if (!self.unownedSession) {
// 创建session的配置
NSURLSessionConfiguration *sessionConfig = [NSURLSessionConfiguration defaultSessionConfiguration];
sessionConfig.timeoutIntervalForRequest = 15;
// 创建session对象
self.ownedSession = [NSURLSession sessionWithConfiguration:sessionConfig
delegate:self
delegateQueue:nil];
session = self.ownedSession;
}
self.dataTask = [session dataTaskWithRequest:self.request];
self.executing = YES;
}
// 开始下载任务
[self.dataTask resume];
if (self.dataTask) {
for (SDWebImageDownloaderProgressBlock progressBlock in [self callbacksForKey:kProgressCallbackKey]) {
progressBlock(0, NSURLResponseUnknownLength, self.request.URL);
}
dispatch_async(dispatch_get_main_queue(), ^{
[[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStartNotification object:self];
});
} else {
// 创建任务失败
[self callCompletionBlocksWithError:[NSError errorWithDomain:NSURLErrorDomain code:0 userInfo:@{NSLocalizedDescriptionKey : @"Connection can't be initialized"}]];
}
...
}
在下载过程中，会涉及鉴权、响应的statusCode判断（404、304等等），以及收到数据后的进度回调等等，在最后的didCompleteWithError里做最后的处理，然后回调完成的block，下面仅分析一下didCompleteWithError的方法
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error {
...
if (error) {
[self callCompletionBlocksWithError:error];
} else {
if ([self callbacksForKey:kCompletedCallbackKey].count > 0) {
/**
* See #1608 and #1623 - apparently, there is a race condition on `NSURLCache` that causes a crash
* Limited the calls to `cachedResponseForRequest:` only for cases where we should ignore the cached response
* and images for which responseFromCached is YES (only the ones that cannot be cached).
* Note: responseFromCached is set to NO inside `willCacheResponse:`. This method doesn't get called for large images or images behind authentication
*/
if (self.options & SDWebImageDownloaderIgnoreCachedResponse && responseFromCached && [[NSURLCache sharedURLCache] cachedResponseForRequest:self.request]) {
// 如果options是忽略缓存，而图片又是从缓存中取的，就给回调传入nil
[self callCompletionBlocksWithImage:nil imageData:nil error:nil finished:YES];
} else if (self.imageData) {
UIImage *image = [UIImage sd_imageWithData:self.imageData];
// 缓存图片
NSString *key = [[SDWebImageManager sharedManager] cacheKeyForURL:self.request.URL];
// 跳转图片的大小
image = [self scaledImageForKey:key image:image];
// Do not force decoding animated GIFs
if (!image.images) {
// 不是Gif图像
if (self.shouldDecompressImages) {
if (self.options & SDWebImageDownloaderScaleDownLargeImages) {
#if SD_UIKIT || SD_WATCH
image = [UIImage decodedAndScaledDownImageWithImage:image];
[self.imageData setData:UIImagePNGRepresentation(image)];
#endif
} else {
image = [UIImage decodedImageWithImage:image];
}
}
}
if (CGSizeEqualToSize(image.size, CGSizeZero)) {
// 下载是图片大小的0
[self callCompletionBlocksWithError:[NSError errorWithDomain:SDWebImageErrorDomain code:0 userInfo:@{NSLocalizedDescriptionKey : @"Downloaded image has 0 pixels"}]];
} else {
// 把下载的图片作为参数回调
[self callCompletionBlocksWithImage:image imageData:self.imageData error:nil finished:YES];
}
} else {
[self callCompletionBlocksWithError:[NSError errorWithDomain:SDWebImageErrorDomain code:0 userInfo:@{NSLocalizedDescriptionKey : @"Image data is nil"}]];
}
}
}
...
}
以上就是给UIImageView对象设置图片的过程，可以看出还是比较复杂的，考虑的情况也比较多，不得不佩服作者的编码能力。至于UIButton的图片设置过程，分析情况类似，在此不做分析。
SDWebImage的源码中在设置图片的过程中，还应用了多种技术：GCD的线程组、锁机制、并发控制、队列、图像解码、缓存控制等等，是一个综合性十分强的项目了，通过阅读源码，对这些技术的使用也有了进一步的认知，对作者的编程功力的深厚深深折服。
SDWebImage的解析到此结束，本文只是简单的从源码结构、UIImageView的使用角度进行了简单的分析，希望对阅读源码的朋友有一些帮助，如果文中有不足之处，还望不吝指出，互相学习。
参考资料