SDWebImageDownloader 下載圖像資源
當SDWebImage未命中Cache，或用戶明確要求要刷新緩存時，SDWebImage會調用
SDWebImageDownloader對象的
/**
* 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 <SDWebImageOperation>)downloadImageWithURL:(NSURL *)url
options:(SDWebImageDownloaderOptions)options
progress:(SDWebImageDownloaderProgressBlock)progressBlock
completed:(SDWebImageDownloaderCompletedBlock)completedBlock;
方法。
參數很簡單，一目了然。(注意這里會返回一個id<SDWebImageOperation>對象，用于取消異步的下載操作)
這里我們關注一下downloadImageWithURL的回調塊：
typedef void(^SDWebImageDownloaderCompletedBlock)(UIImage *image, NSData *data, NSError *error, BOOL finished);
downloadImageWithURL實現
我們現在來看一下，SDWebDownload下載函數的實現：
- (id <SDWebImageOperation>)downloadImageWithURL:(NSURL *)url options:(SDWebImageDownloaderOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageDownloaderCompletedBlock)completedBlock {
// 這里的SDWebImageDownloaderOperation 是可以擴展的，用戶可以繼承SDWebImageDownloaderOperation 并調用- (void)setOperationClass:(Class)operationClass;
// 來修改operationClass所代表的Class
__block SDWebImageDownloaderOperation *operation;
__weak __typeof(self)wself = self;
// 存儲url對應的progressBlock ，completedBlock。注意這里，只有第一個url才會執行 createCallback^{}，這樣做是防止同一個url同時多次下載
[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回調時，weakSelf可能已經為釋放
if (!sself) return;
__block NSArray *callbacksForURL;
dispatch_sync(sself.barrierQueue, ^{ // 此處用barrierQueue， dispatch_sync， dispatch_barrier_sync配合形成一個對URLCallbacks屬性讀寫鎖的作用
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]; // 將operation 放入opeartion queue中，自動執行start方法 注意
if (wself.executionOrder == SDWebImageDownloaderLIFOExecutionOrder) { // 這里注意executeOrder, LIFO 應該是為了優先響應當前UI的image download的響應
// Emulate LIFO execution order by systematically adding new operations as last operation's dependency
[wself.lastAddedOperation addDependency:operation];
wself.lastAddedOperation = operation;
}
}];
return operation;
}
這里代碼我們將其主體框架先屢清楚。
download的步驟可以分為3步：
第一步，調用addProgressCallback函數
- (void)addProgressCallback:(SDWebImageDownloaderProgressBlock)progressBlock completedBlock:(SDWebImageDownloaderCompletedBlock)completedBlock forURL:(NSURL *)url createCallback:(SDWebImageNoParamsBlock)createCallback
保存當前的下載Progress block與finish block。這里，在addProgressCall方法中，會判斷當前的下載（對應的URL key）是否是第一次被存儲。若是第一次被存儲，則說明該URL還沒有下載，則會繼續調用createCallback，進行下一步的下載操作。若在URLCallbacks中已經存儲過，則僅僅存儲相應回調。而不進行真正下載。這樣，使同一個下載僅同時進行一次，而又可以響應多個回調。
第二步，若當前是第一次下載，則生成對應的下載Operation對象：
- (id)initWithRequest:(NSURLRequest *)request
inSession:(NSURLSession *)session
options:(SDWebImageDownloaderOptions)options
progress:(SDWebImageDownloaderProgressBlock)progressBlock
completed:(SDWebImageDownloaderCompletedBlock)completedBlock
cancelled:(SDWebImageNoParamsBlock)cancelBlock
注意這里的id類型為
SDWebImageDownloaderOperation
其實是繼承自NSOperation的對象。
第三步，將第二步生成的Operation對象放入downloadQueue Operationqueue中，
[wself.downloadQueue addOperation:operation];
這樣，Operation就會自動并發執行了。
SDWebImageDownloaderOperation
既然download是通過operation queue進行的，那么我們就對應的看一下加入到queue里面的
NSOperation 對象SDWebImageDownloaderOperation。
init 函數
- (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的狀態
_finished = NO; // 初始化operation的狀態
_expectedSize = 0;
_unownedSession = session;
responseFromCached = YES; // Initially wrong until `- URLSession:dataTask:willCacheResponse:completionHandler: is called or not called
}
return self;
}
初始化函數沒什么好說的，下面我們來看
NSOperation對象的核心工作函數，start。
- (void)start {
@synchronized (self) {
if (self.isCancelled) { // 老規矩，在使用nsoperation對象的時候，要先檢查是否被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:^{ // 為了確保在App進入后臺時，能夠正確記錄要下載的task，這里mark了后臺任務開始
__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]; // 保持當前線程，讓NSSURLSession 的回調得以返回。 因為operation queue不會保證執行的線程會一直存在。
}
[self.dataTask resume]; // 執行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]; // 這里mark了后臺任務的結束，beginBackgroundTaskWithExpirationHandler必須與endBackgroundTask匹配使用，不然當后臺任務過期時，iOS會kill掉APP
self.backgroundTaskId = UIBackgroundTaskInvalid;
}
#endif
}
在start函數里面，主要做了下面一些事情：
- 判斷當前的operation是否被cancle，若cancel，則直接reset變量，return
- mark后臺工作代碼開始
- 生成request相應的dataTask，并啟動該task。
- 保持住當前的線程，確保當前線程在網絡回調前不會退出。（否則NSURLSession的回調不會返回來）
5.mark后臺工作代碼結束（這里要與begin mark匹配，否則iOS系統會在后臺代碼超時后kill掉我們的APP）
SDWebImageDownloaderOperation的NSURLSessionDataDelegate， NSURLSessionTaskDelegate
對于網絡請求的回調，還是比較容易理解的。這里我們注意以下三個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);
}
}
這里對304 response的處理（文件未更改）。
注釋上說是對于304 response，會直接cancel掉下載的操作，讀取Cache中的內容。
但是我并沒有看到讀取Cache的操作，僅僅將error code置為304來返回。難道是要自己判斷error code為304時，自己再讀取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) { // 這段代碼可以實現 圖片的分段加載
// 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);
}
}
注意這里的
self.expectedSize, SDWebImage實現了圖片的分段加載功能。
#pragma mark NSURLSessionTaskDelegate
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error {
@synchronized(self) {
self.thread = nil; // 將thread解綁，我們已經不需要保活當前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];
}
這個是當dataTask結束時的回調，除了調用completeBlock外，
首先他還做了一件事，
就是將strong引用的thread解除，因為這里我們再也不會獲得來自網絡請求的回調，因此再也不用保持該thread。
