SDWebImageDownloader 下载图像资源
当SDWebImage未命中Cache，或用户明确要求要刷新缓存时，SDWebImage会调用
SDWebImageDownloader对象的
/**
* Creates a SDWebImageDownloader async downloader instance with a given URL
*
* The delegate will be informed when the image is finish downloaded or an error has happen.
*
* @see SDWebImageDownloaderDelegate
*
* @param url The URL to the image to download
* @param options The options to be used for this download
* @param progressBlock A block called repeatedly while the image is downloading
* @param completedBlock A block called once the download is completed.
* If the download succeeded, the image parameter is set, in case of error,
* error parameter is set with the error. The last parameter is always YES
* if SDWebImageDownloaderProgressiveDownload isn't use. With the
* SDWebImageDownloaderProgressiveDownload option, this block is called
* repeatedly with the partial image object and the finished argument set to NO
* before to be called a last time with the full image and finished argument
* set to YES. In case of error, the finished argument is always YES.
*
* @return A cancellable SDWebImageOperation
*/
- (id )downloadImageWithURL:(NSURL *)url
options:(SDWebImageDownloaderOptions)options
progress:(SDWebImageDownloaderProgressBlock)progressBlock
completed:(SDWebImageDownloaderCompletedBlock)completedBlock;
方法。
参数很简单，一目了然。(注意这里会返回一个id
这里我们关注一下downloadImageWithURL的回调块：
typedef void(^SDWebImageDownloaderCompletedBlock)(UIImage *image, NSData *data, NSError *error, BOOL finished);
downloadImageWithURL实现
我们现在来看一下，SDWebDownload下载函数的实现：
- (id )downloadImageWithURL:(NSURL *)url options:(SDWebImageDownloaderOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageDownloaderCompletedBlock)completedBlock {
// 这里的SDWebImageDownloaderOperation 是可以扩展的，用户可以继承SDWebImageDownloaderOperation 并调用- (void)setOperationClass:(Class)operationClass;
// 来修改operationClass所代表的Class
__block SDWebImageDownloaderOperation *operation;
__weak __typeof(self)wself = self;
// 存储url对应的progressBlock ，completedBlock。注意这里，只有第一个url才会执行 createCallback^{}，这样做是防止同一个url同时多次下载
[self addProgressCallback:progressBlock completedBlock:completedBlock forURL:url createCallback:^{
NSTimeInterval timeoutInterval = wself.downloadTimeout;
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 (wself.headersFilter) {
request.allHTTPHeaderFields = wself.headersFilter(url, [wself.HTTPHeaders copy]);
}
else {
request.allHTTPHeaderFields = wself.HTTPHeaders;
}
operation = [[wself.operationClass alloc] initWithRequest:request
inSession:self.session
options:options
progress:^(NSInteger receivedSize, NSInteger expectedSize) {
SDWebImageDownloader *sself = wself; //在block回调时，weakSelf可能已经为释放
if (!sself) return;
__block NSArray *callbacksForURL;
dispatch_sync(sself.barrierQueue, ^{ // 此处用barrierQueue， dispatch_sync， dispatch_barrier_sync配合形成一个对URLCallbacks属性读写锁的作用
callbacksForURL = [sself.URLCallbacks[url] copy];
});
for (NSDictionary *callbacks in callbacksForURL) {
dispatch_async(dispatch_get_main_queue(), ^{
SDWebImageDownloaderProgressBlock callback = callbacks[kProgressCallbackKey];
if (callback) callback(receivedSize, expectedSize);
});
}
}
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];
}
});
for (NSDictionary *callbacks in callbacksForURL) {
SDWebImageDownloaderCompletedBlock callback = callbacks[kCompletedCallbackKey];
if (callback) callback(image, data, error, finished);
}
}
cancelled:^{
SDWebImageDownloader *sself = wself;
if (!sself) return;
dispatch_barrier_async(sself.barrierQueue, ^{
[sself.URLCallbacks removeObjectForKey:url];
});
}];
operation.shouldDecompressImages = wself.shouldDecompressImages;
if (wself.urlCredential) {
operation.credential = wself.urlCredential;
} else if (wself.username && wself.password) {
operation.credential = [NSURLCredential credentialWithUser:wself.username password:wself.password persistence:NSURLCredentialPersistenceForSession];
}
if (options & SDWebImageDownloaderHighPriority) {
operation.queuePriority = NSOperationQueuePriorityHigh;
} else if (options & SDWebImageDownloaderLowPriority) {
operation.queuePriority = NSOperationQueuePriorityLow;
}
[wself.downloadQueue addOperation:operation]; // 将operation 放入opeartion queue中，自动执行start方法 注意
if (wself.executionOrder == SDWebImageDownloaderLIFOExecutionOrder) { // 这里注意executeOrder, LIFO 应该是为了优先响应当前UI的image download的响应
// Emulate LIFO execution order by systematically adding new operations as last operation's dependency
[wself.lastAddedOperation addDependency:operation];
wself.lastAddedOperation = operation;
}
}];
return operation;
}
这里代码我们将其主体框架先屡清楚。
download的步骤可以分为3步：
第一步，调用addProgressCallback函数
- (void)addProgressCallback:(SDWebImageDownloaderProgressBlock)progressBlock completedBlock:(SDWebImageDownloaderCompletedBlock)completedBlock forURL:(NSURL *)url createCallback:(SDWebImageNoParamsBlock)createCallback
保存当前的下载Progress block与finish block。这里，在addProgressCall方法中，会判断当前的下载（对应的URL key）是否是第一次被存储。若是第一次被存储，则说明该URL还没有下载，则会继续调用createCallback，进行下一步的下载操作。若在URLCallbacks中已经存储过，则仅仅存储相应回调。而不进行真正下载。这样，使同一个下载仅同时进行一次，而又可以响应多个回调。
第二步，若当前是第一次下载，则生成对应的下载Operation对象：
- (id)initWithRequest:(NSURLRequest *)request
inSession:(NSURLSession *)session
options:(SDWebImageDownloaderOptions)options
progress:(SDWebImageDownloaderProgressBlock)progressBlock
completed:(SDWebImageDownloaderCompletedBlock)completedBlock
cancelled:(SDWebImageNoParamsBlock)cancelBlock
注意这里的id类型为
SDWebImageDownloaderOperation
其实是继承自NSOperation的对象。
第三步，将第二步生成的Operation对象放入downloadQueue Operationqueue中，
[wself.downloadQueue addOperation:operation];
这样，Operation就会自动并发执行了。
SDWebImageDownloaderOperation
既然download是通过operation queue进行的，那么我们就对应的看一下加入到queue里面的
NSOperation 对象SDWebImageDownloaderOperation。
init 函数
- (id)initWithRequest:(NSURLRequest *)request
inSession:(NSURLSession *)session
options:(SDWebImageDownloaderOptions)options
progress:(SDWebImageDownloaderProgressBlock)progressBlock
completed:(SDWebImageDownloaderCompletedBlock)completedBlock
cancelled:(SDWebImageNoParamsBlock)cancelBlock {
if ((self = [super init])) {
_request = request;
_shouldDecompressImages = YES;
_options = options;
_progressBlock = [progressBlock copy];
_completedBlock = [completedBlock copy];
_cancelBlock = [cancelBlock copy];
_executing = NO; // 初始化operation的状态
_finished = NO; // 初始化operation的状态
_expectedSize = 0;
_unownedSession = session;
responseFromCached = YES; // Initially wrong until `- URLSession:dataTask:willCacheResponse:completionHandler: is called or not called
}
return self;
}
初始化函数没什么好说的，下面我们来看
NSOperation对象的核心工作函数，start。
- (void)start {
@synchronized (self) {
if (self.isCancelled) { // 老规矩，在使用nsoperation对象的时候，要先检查是否被cancel掉了
self.finished = YES;
[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)];
if (hasApplication && [self shouldContinueWhenAppEntersBackground]) {
__weak __typeof__ (self) wself = self;
UIApplication * app = [UIApplicationClass performSelector:@selector(sharedApplication)];
self.backgroundTaskId = [app beginBackgroundTaskWithExpirationHandler:^{ // 为了确保在App进入后台时，能够正确记录要下载的task，这里mark了后台任务开始
__strong __typeof (wself) sself = wself;
if (sself) {
[sself cancel];
[app endBackgroundTask:sself.backgroundTaskId];
sself.backgroundTaskId = UIBackgroundTaskInvalid;
}
}];
}
#endif
NSURLSession *session = self.unownedSession;
if (!self.unownedSession) {
NSURLSessionConfiguration *sessionConfig = [NSURLSessionConfiguration defaultSessionConfiguration];
sessionConfig.timeoutIntervalForRequest = 15;
/**
* Create the session for this task
* We send nil as delegate queue so that the session creates a serial operation queue for performing all delegate
* method calls and completion handler calls.
*/
self.ownedSession = [NSURLSession sessionWithConfiguration:sessionConfig
delegate:self
delegateQueue:nil];
session = self.ownedSession;
}
self.dataTask = [session dataTaskWithRequest:self.request];
self.executing = YES;
self.thread = [NSThread currentThread]; // 保持当前线程，让NSSURLSession 的回调得以返回。 因为operation queue不会保证执行的线程会一直存在。
}
[self.dataTask resume]; // 执行dataTask
if (self.dataTask) {
if (self.progressBlock) {
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);
}
}
#if TARGET_OS_IPHONE && __IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_4_0
Class UIApplicationClass = NSClassFromString(@"UIApplication");
if(!UIApplicationClass || ![UIApplicationClass respondsToSelector:@selector(sharedApplication)]) {
return;
}
if (self.backgroundTaskId != UIBackgroundTaskInvalid) {
UIApplication * app = [UIApplication performSelector:@selector(sharedApplication)];
[app endBackgroundTask:self.backgroundTaskId]; // 这里mark了后台任务的结束，beginBackgroundTaskWithExpirationHandler必须与endBackgroundTask匹配使用，不然当后台任务过期时，iOS会kill掉APP
self.backgroundTaskId = UIBackgroundTaskInvalid;
}
#endif
}
在start函数里面，主要做了下面一些事情：
- 判断当前的operation是否被cancle，若cancel，则直接reset变量，return
- mark后台工作代码开始
- 生成request相应的dataTask，并启动该task。
- 保持住当前的线程，确保当前线程在网络回调前不会退出。（否则NSURLSession的回调不会返回来）
5.mark后台工作代码结束（这里要与begin mark匹配，否则iOS系统会在后台代码超时后kill掉我们的APP）
SDWebImageDownloaderOperation的NSURLSessionDataDelegate， NSURLSessionTaskDelegate
对于网络请求的回调，还是比较容易理解的。这里我们注意以下三个delegate方法：
#pragma mark NSURLSessionDataDelegate
- (void)URLSession:(NSURLSession *)session
dataTask:(NSURLSessionDataTask *)dataTask
didReceiveResponse:(NSURLResponse *)response
completionHandler:(void (^)(NSURLSessionResponseDisposition disposition))completionHandler {
//'304 Not Modified' is an exceptional one
if (![response respondsToSelector:@selector(statusCode)] || ([((NSHTTPURLResponse *)response) statusCode] < 400 && [((NSHTTPURLResponse *)response) statusCode] != 304)) {
NSInteger expected = response.expectedContentLength > 0 ? (NSInteger)response.expectedContentLength : 0;
self.expectedSize = expected;
if (self.progressBlock) {
self.progressBlock(0, expected);
}
self.imageData = [[NSMutableData alloc] initWithCapacity:expected];
self.response = response;
dispatch_async(dispatch_get_main_queue(), ^{
[[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadReceiveResponseNotification object:self];
});
}
else {
NSUInteger code = [((NSHTTPURLResponse *)response) statusCode];
//This is the case when server returns '304 Not Modified'. It means that remote image is not changed.
//In case of 304 we need just cancel the operation and return cached image from the cache.
if (code == 304) {
[self cancelInternal];
} else {
[self.dataTask cancel];
}
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];
}
if (completionHandler) {
completionHandler(NSURLSessionResponseAllow);
}
}
这里对304 response的处理（文件未更改）。
注释上说是对于304 response，会直接cancel掉下载的操作，读取Cache中的内容。
但是我并没有看到读取Cache的操作，仅仅将error code置为304来返回。难道是要自己判断error code为304时，自己再读取Cache？
- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask didReceiveData:(NSData *)data {
[self.imageData appendData:data];
if ((self.options & SDWebImageDownloaderProgressiveDownload) && self.expectedSize > 0 && self.completedBlock) {
// The following code is from http://www.cocoaintheshell.com/2011/05/progressive-images-download-imageio/
// Thanks to the author @Nyx0uf
// Get the total bytes downloaded
const NSInteger totalSize = self.imageData.length;
// Update the data source, we must pass ALL the data, not just the new bytes
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);
if (val) CFNumberGetValue(val, kCFNumberLongType, &height);
val = CFDictionaryGetValue(properties, kCGImagePropertyPixelWidth);
if (val) CFNumberGetValue(val, kCFNumberLongType, &width);
val = CFDictionaryGetValue(properties, kCGImagePropertyOrientation);
if (val) CFNumberGetValue(val, kCFNumberNSIntegerType, &orientationValue);
CFRelease(properties);
// When we draw to Core Graphics, we lose orientation information,
// which means the image below born of initWithCGIImage will be
// oriented incorrectly sometimes. (Unlike the image born of initWithData
// in didCompleteWithError.) So save it here and pass it on later.
orientation = [[self class] orientationFromPropertyValue:(orientationValue == -1 ? 1 : orientationValue)];
}
}
if (width + height > 0 && totalSize < self.expectedSize) { // 这段代码可以实现 图片的分段加载
// Create the image
CGImageRef partialImageRef = CGImageSourceCreateImageAtIndex(imageSource, 0, NULL);
#ifdef TARGET_OS_IPHONE
// Workaround for iOS anamorphic image
if (partialImageRef) {
const size_t partialHeight = CGImageGetHeight(partialImageRef);
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
CGContextRef bmContext = CGBitmapContextCreate(NULL, width, height, 8, width * 4, colorSpace, kCGBitmapByteOrderDefault | kCGImageAlphaPremultipliedFirst);
CGColorSpaceRelease(colorSpace);
if (bmContext) {
CGContextDrawImage(bmContext, (CGRect){.origin.x = 0.0f, .origin.y = 0.0f, .size.width = width, .size.height = partialHeight}, partialImageRef);
CGImageRelease(partialImageRef);
partialImageRef = CGBitmapContextCreateImage(bmContext);
CGContextRelease(bmContext);
}
else {
CGImageRelease(partialImageRef);
partialImageRef = nil;
}
}
#endif
if (partialImageRef) {
UIImage *image = [UIImage imageWithCGImage:partialImageRef scale:1 orientation:orientation];
NSString *key = [[SDWebImageManager sharedManager] cacheKeyForURL:self.request.URL];
UIImage *scaledImage = [self scaledImageForKey:key image:image];
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);
}
}
注意这里的
self.expectedSize, SDWebImage实现了图片的分段加载功能。
#pragma mark NSURLSessionTaskDelegate
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error {
@synchronized(self) {
self.thread = nil; // 将thread解绑，我们已经不需要保活当前thread
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 (![[NSURLCache sharedURLCache] cachedResponseForRequest:_request]) {
responseFromCached = NO;
}
if (completionBlock) {
if (self.options & SDWebImageDownloaderIgnoreCachedResponse && responseFromCached) {
completionBlock(nil, nil, nil, YES);
} else if (self.imageData) {
UIImage *image = [UIImage sd_imageWithData:self.imageData];
NSString *key = [[SDWebImageManager sharedManager] cacheKeyForURL:self.request.URL];
image = [self scaledImageForKey:key image:image];
// Do not force decoding animated GIFs
if (!image.images) {
if (self.shouldDecompressImages) {
image = [UIImage decodedImageWithImage:image];
}
}
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);
}
} else {
completionBlock(nil, nil, [NSError errorWithDomain:SDWebImageErrorDomain code:0 userInfo:@{NSLocalizedDescriptionKey : @"Image data is nil"}], YES);
}
}
}
self.completionBlock = nil;
[self done];
}
这个是当dataTask结束时的回调，除了调用completeBlock外，
首先他还做了一件事，
就是将strong引用的thread解除，因为这里我们再也不会获得来自网络请求的回调，因此再也不用保持该thread。