还是读sdwebimage的源码,在sdwebimagedownloader里的下面这个方法里
- (id)downloadImageWithURL:(NSURL *)url options:(SDWebImageDownloaderOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageDownloaderCompletedBlock)completedBlock { __block SDWebImageDownloaderOperation *operation; __weak __typeof(self)wself = self; [self addProgressCallback:progressBlock andCompletedBlock: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 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) { 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.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]; if (wself.executionOrder == SDWebImageDownloaderLIFOExecutionOrder) { // Emulate LIFO execution order by systematically adding new operations as last operation's dependency [wself.lastAddedOperation addDependency:operation]; wself.lastAddedOperation = operation; } }]; return operation; }
为什么 还要SDWebImageDownloader *sself = wself;执行这一句?
直接操作wself不行吗?
直接把wself赋给sself有什么区别吗?
Block 外面使用 weak self 是为了防止 Block 引起循环引用;而当 Block 回调是异步的情况下,为了防止回调执行时 weak self 被清理了,所以加一个 strong 化的过程。这样即避免了循环引用,又保证了对象的正常生命周期。
wself 是个 weak 引用,weak 引用可能随时会被释放(当所有它的 strong 引用都释放了,ref count 达到 0 ),你疑惑的那一句代码, 就是给 wself 弄了一个 strong 引用(sself),其实从名字也能看出来,这样就可以保证在那个 block 内, wself 不会被释放掉。