概述
本篇分析一下SDWebImage中负责下载图片数据的相关代码，SDWebImageDownloader和SDWebImageDownloaderOperation。
SDWebImageDownloader
SDWebImageDownloader是管理下载图片数据的类，初始化方法代码注释如下：
- (id)init {
if ((self = [super init])) {
_operationClass = [SDWebImageDownloaderOperation class];
_shouldDecompressImages = YES; //是否解压图片
_executionOrder = SDWebImageDownloaderFIFOExecutionOrder;
_downloadQueue = [NSOperationQueue new]; //下载的队列
_downloadQueue.maxConcurrentOperationCount = 6;
_URLCallbacks = [NSMutableDictionary new]; //请求回调dic
#ifdef SD_WEBP
_HTTPHeaders = [@{@"Accept": @"image/webp,image/*;q=0.8"} mutableCopy]; //请求webp图片数据，报文头部accept多包含image/webp字段
#else
_HTTPHeaders = [@{@"Accept": @"image/*;q=0.8"} mutableCopy];
#endif
_barrierQueue = dispatch_queue_create("com.hackemist.SDWebImageDownloaderBarrierQueue", DISPATCH_QUEUE_CONCURRENT); //
_downloadTimeout = 15.0; //请求图片数据超时超时时间
//初始化NSURLSessionConfiguration对象
NSURLSessionConfiguration *sessionConfig = [NSURLSessionConfiguration defaultSessionConfiguration];
sessionConfig.timeoutIntervalForRequest = _downloadTimeout; //设置超时时间
self.session = [NSURLSession sessionWithConfiguration:sessionConfig
delegate:self
delegateQueue:nil]; //创建session
}
return self;
}
该方法初始化了一些参数用于网络请求，SDWebImageDownloader是通过NSURLSession的方式请求数据。同时提供了一些方法用于外部设置。代码注释如下：
//设置请求报文头部
- (void)setValue:(NSString *)value forHTTPHeaderField:(NSString *)field {
if (value) {
self.HTTPHeaders[field] = value;
} else {
[self.HTTPHeaders removeSafeObjectForKey:field];
}
}
//获取设置的报文头部
- (NSString *)valueForHTTPHeaderField:(NSString *)field {
return self.HTTPHeaders[field];
}
//设置下载队列的最大并发个数
- (void)setMaxConcurrentDownloads:(NSInteger)maxConcurrentDownloads {
_downloadQueue.maxConcurrentOperationCount = maxConcurrentDownloads;
}
//获取下载队列的当前operation个数
- (NSUInteger)currentDownloadCount {
return _downloadQueue.operationCount;
}
//获取下载队列的最大并发个数
- (NSInteger)maxConcurrentDownloads {
return _downloadQueue.maxConcurrentOperationCount;
}
SDWebImageDownloader最主要的方法是downloadImageWithURL:options:progress:completed:方法，调用该方法开始下载数据。下面主要分析一下具体执行步骤：
-
首先调用addProgressCallback:completedBlock:forURL:createCallback:方法为当前网络请求关联回调block，包括请求进度的回调和请求结束的回调，主要代码如下：
dispatch_barrier_sync(self.barrierQueue, ^{ BOOL first = NO; if (!self.URLCallbacks[url]) { self.URLCallbacks[url] = [NSMutableArray new]; //url对应block数组 first = YES; } NSMutableArray *callbacksForURL = self.URLCallbacks[url]; NSMutableDictionary *callbacks = [NSMutableDictionary new]; //设置当前请求进度progress回调block和请求完成回调block if (progressBlock) callbacks[kProgressCallbackKey] = [progressBlock copy]; if (completedBlock) callbacks[kCompletedCallbackKey] = [completedBlock copy]; [callbacksForURL addObject:callbacks]; //添加callbacks self.URLCallbacks[url] = callbacksForURL; if (first) { createCallback(); //执行createCallback } });该方法允许设置一组回调block，key是请求的url，数组中每个元素又是一个dictionary，包含请求进度progress回调block和请求完成回调block，最后执行createCallback。 -
downloadImageWithURL方法接着在createCallback中构建请求报文，代码如下：
//创建request对象 NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:url cachePolicy:(options & SDWebImageDownloaderUseNSURLCache ? NSURLRequestUseProtocolCachePolicy : NSURLRequestReloadIgnoringLocalCacheData) timeoutInterval:timeoutInterval]; //设置参数 request.HTTPShouldHandleCookies = (options & SDWebImageDownloaderHandleCookies); request.HTTPShouldUsePipelining = YES;首先创建request对象，根据option值来决定本次网络请求的缓存策略，option是SDWebImageDownloaderOptions类型的枚举值，通过|的方式多选。SDWebImage默认忽略NSURLCache缓存机制，即request的cachePolicy是NSURLRequestReloadIgnoringLocalCacheData，而是用自定义的SDImageCache来缓存数据，上一篇文章分析了SDImageCache，如果option设置了SDWebImageDownloaderUseNSURLCache，则启用NSURLCache缓存机制，并且设置cachePolicy为NSURLRequestUseProtocolCachePolicy，即通过服务端响应报文的缓存策略来决定本地NSURLCache是否缓存数据。另外还支持pipelining和处理cookies。
-
downloadImageWithURL方法接着创建一个SDWebImageDownloaderOperation类型的NSOperation对象，将本次网络请求作为一个operation执行。代码注释如下：
//创建一个operation对象 operation = [[wself.operationClass alloc] initWithRequest:request inSession:self.session options:options progress:^(NSInteger receivedSize, NSInteger expectedSize) { SDWebImageDownloader *sself = wself; if (!sself) return; __block NSArray *callbacksForURL; dispatch_sync(sself.barrierQueue, ^{ callbacksForURL = [sself.URLCallbacks[url] copy]; //取出callback数组 }); for (NSDictionary *callbacks in callbacksForURL) { //遍历callback数组 dispatch_async(dispatch_get_main_queue(), ^{ SDWebImageDownloaderProgressBlock callback = callbacks[kProgressCallbackKey]; //取出progress的callback if (callback) callback(receivedSize, expectedSize); //执行block }); } } completed:^(UIImage *image, NSData *data, NSError *error, BOOL finished) { SDWebImageDownloader *sself = wself; if (!sself) return; __block NSArray *callbacksForURL; dispatch_barrier_sync(sself.barrierQueue, ^{ callbacksForURL = [sself.URLCallbacks[url] copy]; if (finished) { [sself.URLCallbacks removeObjectForKey:url]; //删除回调 } }); for (NSDictionary *callbacks in callbacksForURL) { //遍历callback数组 SDWebImageDownloaderCompletedBlock callback = callbacks[kCompletedCallbackKey]; if (callback) callback(image, data, error, finished); //执行block } } cancelled:^{ SDWebImageDownloader *sself = wself; if (!sself) return; dispatch_barrier_async(sself.barrierQueue, ^{ [sself.URLCallbacks removeObjectForKey:url]; //删除url }); }]; operation.shouldDecompressImages = wself.shouldDecompressImages; //图片数据下载完成后是否要解压请求图片数据过程中触发progress回调，receivedSize是当前接收数据的大小，expectedSize是数据总大小，请求完成触发completed回调，删除url对应的相关callback，并且执行SDWebImageDownloaderCompletedBlock。当operation被cancel，触发cancelled回调，删除url对应的相关callback。
-
最后将operation加入downloadQueue队列中，开始执行当前网络请求任务。同时设置了operation的优先级和依赖关系。
由于SDWebImageDownloader将self设置为urlsession的delegate，当发起网络请求时，触发urlsession的回调方法。SDWebImageDownloader实现了urlsession的相关代理方法，但是逻辑交给当前dataTask对应的SDWebImageDownloaderOperation对象来处理，通过operationWithTask找到dataTask对应的operation。
- (SDWebImageDownloaderOperation *)operationWithTask:(NSURLSessionTask *)task {
SDWebImageDownloaderOperation *returnOperation = nil;
for (SDWebImageDownloaderOperation *operation in self.downloadQueue.operations) {
//遍历downloadQueue中当前所有operation，匹配task
if (operation.dataTask.taskIdentifier == task.taskIdentifier) {
returnOperation = operation;
break;
}
}
return returnOperation; //返回
}
SDWebImageDownloaderOperation
SDWebImageDownloaderOperation是继承NSOperation的类，对应一次网络请求任务，首先通过初始化方法初始化一些参数设置，参数值有SDWebImageDownloader传入。
start方法
当operation加入downloadQueue中，触发start方法，相关代码注释如下：
- (void)start {
@synchronized (self) { //加锁，多线程数据同步
if (self.isCancelled) { //如果operation被取消
self.finished = YES; //则operation结束
[self reset];
return;
}
#if TARGET_OS_IPHONE && __IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_4_0
Class UIApplicationClass = NSClassFromString(@"UIApplication");
BOOL hasApplication = UIApplicationClass && [UIApplicationClass respondsToSelector:@selector(sharedApplication)];
//如果当前operation标记为在app进入后台时继续下载图片数据
if (hasApplication && [self shouldContinueWhenAppEntersBackground]) {
__weak __typeof__ (self) wself = self;
UIApplication * app = [UIApplicationClass performSelector:@selector(sharedApplication)];
//app进入后允许执行一段时间，超过期限执行ExpirationHandler
self.backgroundTaskId = [app beginBackgroundTaskWithExpirationHandler:^{
__strong __typeof (wself) sself = wself;
if (sself) {
[sself cancel];
[app endBackgroundTask:sself.backgroundTaskId];
sself.backgroundTaskId = UIBackgroundTaskInvalid;
}
}];
}
#endif
NSURLSession *session = self.unownedSession;
if (!self.unownedSession) { //如果没有在初始化方法设置unownedSession，则创建一个urlsession对象
NSURLSessionConfiguration *sessionConfig = [NSURLSessionConfiguration defaultSessionConfiguration];
sessionConfig.timeoutIntervalForRequest = 15;
self.ownedSession = [NSURLSession sessionWithConfiguration:sessionConfig delegate:self delegateQueue:nil];
session = self.ownedSession;
}
//通过session创建一个dataTask
self.dataTask = [session dataTaskWithRequest:self.request];
self.executing = YES; //当前operation标记为执行状态
self.thread = [NSThread currentThread]; //获取当前的线程
}
[self.dataTask resume]; //开始执行dataTask
if (self.dataTask) {
if (self.progressBlock) { //当前进度为0
self.progressBlock(0, NSURLResponseUnknownLength);
}
dispatch_async(dispatch_get_main_queue(), ^{
[[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStartNotification object:self];
});
}
else {
if (self.completedBlock) {
self.completedBlock(nil, nil, [NSError errorWithDomain:NSURLErrorDomain code:0 userInfo:@{NSLocalizedDescriptionKey : @"Connection can't be initialized"}], YES);
}
}
...
}
该方法主要开启一个NSURLSessionTask，并且调用resume方法执行task，开始进行网络请求。
控制operation生命周期
提供了setFinished:方法，setExecuting:方法控制operation的状态，实现是修改operation的状态值，然后手动抛通知给外部。同时提供了cancel和cancelInternal方法取消当前operation。代码注释如下：
- (void)cancelInternal {
if (self.isFinished) return;
[super cancel]; //取消operation
if (self.cancelBlock) self.cancelBlock();
//取消当前网络请求
if (self.dataTask) {
[self.dataTask cancel];
dispatch_async(dispatch_get_main_queue(), ^{
[[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStopNotification object:self];
});
//operation状态为结束
if (self.isExecuting) self.executing = NO;
if (!self.isFinished) self.finished = YES;
}
[self reset]; //相关属性置为nil
}
NSURLSessionDataDelegate
网络请求执行的过程中，触发NSURLSession的相关代理方法，SDWebImageDownloader是delegate，实现了相关代理方法，会调用operation来执行。operation实现了以下4个方法：
-
-(void)URLSession:dataTask:didReceiveResponse:completionHandler:方法
当请求建立连接时，服务器发送响应给客户端，触发该方法，代码注释如下：- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask didReceiveResponse:(NSURLResponse *)response completionHandler:(void (^)(NSURLSessionResponseDisposition disposition))completionHandler { //根据服务端响应的statusCode执行不同逻辑 if (![response respondsToSelector:@selector(statusCode)] || ([((NSHTTPURLResponse *)response) statusCode] < 400 && [((NSHTTPURLResponse *)response) statusCode] != 304)) { //连接成功 NSInteger expected = response.expectedContentLength > 0 ? (NSInteger)response.expectedContentLength : 0; self.expectedSize = expected; if (self.progressBlock) { self.progressBlock(0, expected); } //创建imageData接收数据 self.imageData = [[NSMutableData alloc] initWithCapacity:expected]; self.response = response; dispatch_async(dispatch_get_main_queue(), ^{ //抛通知 [[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadReceiveResponseNotification object:self]; }); } else { NSUInteger code = [((NSHTTPURLResponse *)response) statusCode]; if (code == 304) { [self cancelInternal]; //304表示服务端图片没有更新，结束operation，用缓存中的图片数据 } else { [self.dataTask cancel];//请求发生错误，取消本次请求， } dispatch_async(dispatch_get_main_queue(), ^{ [[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStopNotification object:self]; }); if (self.completedBlock) { self.completedBlock(nil, nil, [NSError errorWithDomain:NSURLErrorDomain code:[((NSHTTPURLResponse *)response) statusCode] userInfo:nil], YES); } [self done]; //结束operation } if (completionHandler) { completionHandler(NSURLSessionResponseAllow); } }该方法发生在客户端收到服务端的响应，但是还没开始传输数据的时候，如果相应报文的statusCode小于400且不是304，则说明连接正常，则创建imageData结束数据。如果statusCode是304，说明网络请求开启缓存功能，且客户端的请求报文中带有上次缓存到本地的lastModified和ETag信息，服务端在对比本地资源和报文中的字段，发现资源没有修改后，返回304，且不返回响应的报文body数据。具体可以参考这篇文章。这种情况下不创建imageData接收数据，直接取消dataTask，结束operation，即使用缓存中的图片数据。如果statusCode不是304，则说明请求失败，取消dataTask，最后在done方法中结束operation。
-
-(void)URLSession: dataTask: didReceiveData:方法
当开始下载数据时，触发该方法，如果数据比较多，会不断触发，代码注释如下：
- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask didReceiveData:(NSData *)data { [self.imageData appendData:data]; //1.拼接数据 if ((self.options & SDWebImageDownloaderProgressiveDownload) && self.expectedSize > 0 && self.completedBlock) { //当前已经下载的数据大小 const NSInteger totalSize = self.imageData.length; CGImageSourceRef imageSource = CGImageSourceCreateWithData((__bridge CFDataRef)self.imageData, NULL); if (width + height == 0) { CFDictionaryRef properties = CGImageSourceCopyPropertiesAtIndex(imageSource, 0, NULL); if (properties) { NSInteger orientationValue = -1; CFTypeRef val = CFDictionaryGetValue(properties, kCGImagePropertyPixelHeight); //图像高度 if (val) CFNumberGetValue(val, kCFNumberLongType, &height); val = CFDictionaryGetValue(properties, kCGImagePropertyPixelWidth); //图像宽度 if (val) CFNumberGetValue(val, kCFNumberLongType, &width); val = CFDictionaryGetValue(properties, kCGImagePropertyOrientation); //图像方向 if (val) CFNumberGetValue(val, kCFNumberNSIntegerType, &orientationValue); CFRelease(properties); orientation = [[self class] orientationFromPropertyValue:(orientationValue == -1 ? 1 : orientationValue)]; //图片方向 } } //图像宽度和高度不为0，还没接受完数据 if (width + height > 0 && totalSize < self.expectedSize) { CGImageRef partialImageRef = CGImageSourceCreateImageAtIndex(imageSource, 0, NULL); #ifdef TARGET_OS_IPHONE if (partialImageRef) { const size_t partialHeight = CGImageGetHeight(partialImageRef); CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB(); //创建context CGContextRef bmContext = CGBitmapContextCreate(NULL, width, height, 8, width * 4, colorSpace, kCGBitmapByteOrderDefault | kCGImageAlphaPremultipliedFirst); CGColorSpaceRelease(colorSpace); if (bmContext) { //画位图 CGContextDrawImage(bmContext, (CGRect){.origin.x = 0.0f, .origin.y = 0.0f, .size.width = width, .size.height = partialHeight}, partialImageRef); CGImageRelease(partialImageRef); //取出位图数据 partialImageRef = CGBitmapContextCreateImage(bmContext); CGContextRelease(bmContext); } else { CGImageRelease(partialImageRef); partialImageRef = nil; } } #endif if (partialImageRef) { //创建image对象 UIImage *image = [UIImage imageWithCGImage:partialImageRef scale:1 orientation:orientation]; NSString *key = [[SDWebImageManager sharedManager] cacheKeyForURL:self.request.URL]; //缩放图片 UIImage *scaledImage = [self scaledImageForKey:key image:image]; //需要解码 if (self.shouldDecompressImages) { image = [UIImage decodedImageWithImage:scaledImage]; } else { image = scaledImage; } CGImageRelease(partialImageRef); dispatch_main_sync_safe(^{ if (self.completedBlock) { self.completedBlock(image, nil, nil, NO); } }); } } CFRelease(imageSource); } if (self.progressBlock) { self.progressBlock(self.imageData.length, self.expectedSize); } }首先将拼接数据，如果option包含SDWebImageDownloaderProgressiveDownload，即支持边下载边展示图片，首先获取图片的宽、高、方向等信息，然后创建上下文对象bmContext，并调用CGContextDrawImage方法绘制图像，最后生成UIImage对象，通过completedBlock返回。
-
-(void)URLSession: dataTask: willCacheResponse: completionHandler:方法
当服务器缓存数据到本地时，触发该方法，代码注释如下：
- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask willCacheResponse:(NSCachedURLResponse *)proposedResponse completionHandler:(void (^)(NSCachedURLResponse *cachedResponse))completionHandler { responseFromCached = NO; //不是从缓存中取数据 NSCachedURLResponse *cachedResponse = proposedResponse; if (self.request.cachePolicy == NSURLRequestReloadIgnoringLocalCacheData) { cachedResponse = nil; //不缓存数据 } if (completionHandler) { completionHandler(cachedResponse); } }首先将responseFromCached置为NO，说明本次数据不是200 from cache，而是从服务器下载的，然后判断cachePolicy如果是NSURLRequestReloadIgnoringLocalCacheData，说明采用的缓存策略是忽略本地NSURLCache缓存，这个时候不将数据存入本地。上文分析到SDWebImage默认忽略NSURLCache缓存，即request的cachePolicy是NSURLRequestReloadIgnoringLocalCacheData，所以该方法不将cachedResponse存入本地。（勘误：经过实验，无论是200 from cache还是200，都会触发NSURLSession的该方法，而NSURLConnection不会触发）。
-
-(void)URLSession: task: didCompleteWithError:方法
当数据下载完成时，触发该方法，代码注释如下：
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error { @synchronized(self) { self.thread = nil; self.dataTask = nil; dispatch_async(dispatch_get_main_queue(), ^{ [[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStopNotification object:self]; if (!error) { [[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadFinishNotification object:self]; } }); } if (error) { if (self.completedBlock) { self.completedBlock(nil, nil, error, YES); } } else { SDWebImageDownloaderCompletedBlock completionBlock = self.completedBlock; if (completionBlock) { if (self.options & SDWebImageDownloaderIgnoreCachedResponse && responseFromCached && [[NSURLCache sharedURLCache] cachedResponseForRequest:self.request]) { //如果缓存中存在数据，且没有更新缓存，则返回nil，即图片数据不更新 completionBlock(nil, nil, nil, YES); } else if (self.imageData) { //存在数据，处理数据 UIImage *image = [UIImage sd_imageWithData:self.imageData]; NSString *key = [[SDWebImageManager sharedManager] cacheKeyForURL:self.request.URL]; image = [self scaledImageForKey:key image:image]; // GIF图不解压 if (!image.images) { if (self.shouldDecompressImages) { image = [UIImage decodedImageWithImage:image];//解压图片 } } if (CGSizeEqualToSize(image.size, CGSizeZero)) { completionBlock(nil, nil, [NSError errorWithDomain:SDWebImageErrorDomain code:0 userInfo:@{NSLocalizedDescriptionKey : @"Downloaded image has 0 pixels"}], YES); } else { completionBlock(image, self.imageData, nil, YES); //返回图片 } } else { completionBlock(nil, nil, [NSError errorWithDomain:SDWebImageErrorDomain code:0 userInfo:@{NSLocalizedDescriptionKey : @"Image data is nil"}], YES); } } } self.completionBlock = nil; [self done]; //完成本次operation }该方法首先判断缓存策略，如果设置了option是SDWebImageRefreshCached（参考SDWebImageManager中的代码），则downloadOption是SDWebImageDownloaderIgnoreCachedResponse和SDWebImageDownloaderUseNSURLCache，说明使用NSURLCache的缓存机制来决定图片的更新，通过服务器的cache-control字段来控制，具体情况分析如下：
一、第一次下载图片：服务器cache-control指定缓存时间，会将response存入NSURLCache，同时会进入else if(self.imageData)中，将数据转成图片，并调用completionBlock()回调，将图片显示出来，并存入SDWebImageCache中。
二、第二次下载相同url的图片，如果本地缓存未过期，即NSURLCache中存在缓存数据，responseFromCached=YES，同时由于设置了SDWebImageDownloaderIgnoreCachedResponse选项，则completionBlock回调nil给外层。如果本地缓存过期则从服务端重新下载数据，responseFromCached=NO，进入else if(self.imageData)中，和（1）一样处理。
这种机制可以通过实现相同url图片的更新，而不是SDWebImage默认的机制，一个url对应一张图片，如果下载到本地（SDWebImageCache中存储），则从缓存中取，不再下载。
但是让我疑惑的是SD实现NSURLSession的-(void)URLSession: dataTask: willCacheResponse: completionHandler:方法，每次都会触发，无论数据是否是从服务器下载还是缓存中取，导致responseFromCached=NO，每次都会进入else if (self.imageData)这个逻辑分支。之前老的SD版本基于NSURLConnection，如果是200 from cache，不会触发类似的willCacheResponse方法。这里不太清楚作者的用意。
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-(void)URLSession: task:didReceiveChallenge: completionHandler:方法
当发送HTTPS的url时，触发该方法用于校验服务端下发的证书。具体不进行分析。
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