這一篇我們來解決最後的問題，使用 SDWebImage 中下載下傳圖檔的類 SDWebImageDownloader，這個類學習完以後，我們對 SDWebImage 的了解會更加深刻。
1. 下載下傳圖檔
- (id <SDWebImageOperation>)downloadImageWithURL:(NSURL *)url options:(SDWebImageDownloaderOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageDownloaderCompletedBlock)completedBlock {
__block SDWebImageDownloaderOperation *operation;
__weak __typeof(self)wself = self;
// 判斷是否為第一次下載下傳，并且存儲 progressBlock 和 completedBlock，詳情見 1.1
[self addProgressCallback:progressBlock completedBlock:completedBlock forURL:url createCallback:^{
// 第一次下載下傳回調用這個block中的代碼
// 圖檔下載下傳的逾時時間
NSTimeInterval timeoutInterval = wself.downloadTimeout;
if (timeoutInterval == ) {
timeoutInterval = ;
}
// In order to prevent from potential duplicate caching (NSURLCache + SDImageCache) we disable the cache for image requests if told otherwise
// 建立請求
NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:url cachePolicy:(options & SDWebImageDownloaderUseNSURLCache ? NSURLRequestUseProtocolCachePolicy : NSURLRequestReloadIgnoringLocalCacheData) timeoutInterval:timeoutInterval];
// HTTPShouldHandleCookies 為 YES 表示将 cookies 一起發送至伺服器
request.HTTPShouldHandleCookies = (options & SDWebImageDownloaderHandleCookies);
// HTTPShouldUsePipelining 為 YES 可以極大的提高網絡性能，但是 HTTPShouldUsePipelining 也有其局限性，就是伺服器必須按照收到請求的順序傳回對應的資料
request.HTTPShouldUsePipelining = YES;
// 設定頭檔案
if (wself.headersFilter) {
request.allHTTPHeaderFields = wself.headersFilter(url, [wself.HTTPHeaders copy]);
}
else {
request.allHTTPHeaderFields = wself.HTTPHeaders;
}
// 詳情見 2. SDWebImageDownloaderOperation 核心類
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];
});
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];
});
}];
// 是否壓縮圖檔
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];
}
// 優先級
if (options & SDWebImageDownloaderHighPriority) {
operation.queuePriority = NSOperationQueuePriorityHigh;
} else if (options & SDWebImageDownloaderLowPriority) {
operation.queuePriority = NSOperationQueuePriorityLow;
}
// 将 operation 加入隊列中
[wself.downloadQueue addOperation:operation];
if (wself.executionOrder == SDWebImageDownloaderLIFOExecutionOrder) {
// Emulate LIFO execution order by systematically adding new operations as last operation's dependency
// 如果為棧模式，需要添加依賴關系，棧頂完成才能執行下一個 operation
[wself.lastAddedOperation addDependency:operation];
wself.lastAddedOperation = operation;
}
}];
return operation;
}
1.1. 存儲 progressBlock 和 completedBlock
- (void)addProgressCallback:(SDWebImageDownloaderProgressBlock)progressBlock completedBlock:(SDWebImageDownloaderCompletedBlock)completedBlock forURL:(NSURL *)url createCallback:(SDWebImageNoParamsBlock)createCallback {
// The URL will be used as the key to the callbacks dictionary so it cannot be nil. If it is nil immediately call the completed block with no image or data.
if (url == nil) {
if (completedBlock != nil) {
completedBlock(nil, nil, nil, NO);
}
return;
}
// dispatch_barrier_sync : 等待在它前面的多線程執行完再執行這個
dispatch_barrier_sync(self.barrierQueue, ^{
// 判斷是否是第一次下載下傳這個url，如果是第一次下載下傳，将存儲該url
BOOL first = NO;
if (!self.URLCallbacks[url]) {
self.URLCallbacks[url] = [NSMutableArray new];
first = YES;
}
// Handle single download of simultaneous download request for the same URL
// 根據url從self.URLCallbacks字典中取出數組，将傳入的block根據兩個key（kProgressCallbackKey和kCompletedCallbackKey）copy到字典中
NSMutableArray *callbacksForURL = self.URLCallbacks[url];
NSMutableDictionary *callbacks = [NSMutableDictionary new];
if (progressBlock) callbacks[kProgressCallbackKey] = [progressBlock copy];
if (completedBlock) callbacks[kCompletedCallbackKey] = [completedBlock copy];
[callbacksForURL addObject:callbacks];
self.URLCallbacks[url] = callbacksForURL;
if (first) {
createCallback();
}
});
}
2. SDWebImageDownloaderOperation 核心類
在上面我們看到的下載下傳圖檔方法，并沒有涉及到核心代碼，原來是通過以下初始化來進入 SDWebImageDownloaderOperation 核心類
- (id)initWithRequest:(NSURLRequest *)request
inSession:(NSURLSession *)session
options:(SDWebImageDownloaderOptions)options
progress:(SDWebImageDownloaderProgressBlock)progressBlock
completed:(SDWebImageDownloaderCompletedBlock)completedBlock
cancelled:(SDWebImageNoParamsBlock)cancelBlock;
我們進入上面初始化方法的實作中看到，在 init 方法中隻是對傳入參數的指派和擷取，基本上是初始化内容，但是下面有個重寫的 start 方法
- (void)start {
// 線程同步加鎖，重置
@synchronized (self) {
if (self.isCancelled) {
self.finished = YES;
[self reset];
return;
}
#if TARGET_OS_IPHONE && __IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_4_0
// 擷取系統的 application
Class UIApplicationClass = NSClassFromString(@"UIApplication");
BOOL hasApplication = UIApplicationClass && [UIApplicationClass respondsToSelector:@selector(sharedApplication)];
if (hasApplication && [self shouldContinueWhenAppEntersBackground]) {
__weak __typeof__ (self) wself = self;
// 擷取單例
UIApplication * app = [UIApplicationClass performSelector:@selector(sharedApplication)];
// 擷取這個背景線程的表示 UIBackgroundTaskIdentifier
self.backgroundTaskId = [app beginBackgroundTaskWithExpirationHandler:^{
__strong __typeof (wself) sself = wself;
// 背景下載下傳時間到了，就會調用這個 block，如果任務仍在下載下傳就進行取消，調用 endBackgroundTask 方法通知系統該 backgroundTaskId 停止，并把 backgroundTaskId 狀态改為無效
if (sself) {
[sself cancel];
[app endBackgroundTask:sself.backgroundTaskId];
sself.backgroundTaskId = UIBackgroundTaskInvalid;
}
}];
}
#endif
NSURLSession *session = self.unownedSession;
if (!self.unownedSession) {
// 會話配置
NSURLSessionConfiguration *sessionConfig = [NSURLSessionConfiguration defaultSessionConfiguration];
sessionConfig.timeoutIntervalForRequest = ;
/**
* 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.
*/
// 為此任務建立會話
// 我們發送 nil 作為代理隊列，是以建立 session 用于執行所有 delegate 調用和完成處理程式調用的串行操作隊列。
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];
}
// 開始
[self.dataTask resume];
if (self.dataTask) {
if (self.progressBlock) {
self.progressBlock(, 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: 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];
self.backgroundTaskId = UIBackgroundTaskInvalid;
}
#endif
}
SDWebImage 源碼閱讀終于完事了，最後一章有些不細心，等有時間重新整理一下
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