关于sdwebimage的疑问

刚在看sdwebimage的源码，在SDWebImageDownloaderOperation这个类的start方法里，

- (void)start {
    @synchronized (self) {
        if (self.isCancelled) {
            self.finished = YES;
            [self reset];
            return;
        }

#if TARGET_OS_IPHONE && __IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_4_0
        if ([self shouldContinueWhenAppEntersBackground]) {
            __weak __typeof__ (self) wself = self;
            self.backgroundTaskId = [[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:^{
                __strong __typeof (wself) sself = wself;

                if (sself) {
                    [sself cancel];

                    [[UIApplication sharedApplication] endBackgroundTask:sself.backgroundTaskId];
                    sself.backgroundTaskId = UIBackgroundTaskInvalid;
                }
            }];
        }
#endif

        self.executing = YES;
        self.connection = [[NSURLConnection alloc] initWithRequest:self.request delegate:self startImmediately:NO];
        self.thread = [NSThread currentThread];
    }

    [self.connection start];

    if (self.connection) {
        if (self.progressBlock) {
            self.progressBlock(0, NSURLResponseUnknownLength);
        }
        dispatch_async(dispatch_get_main_queue(), ^{
            [[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStartNotification object:self];
        });

        if (floor(NSFoundationVersionNumber) <= NSFoundationVersionNumber_iOS_5_1) {
            // Make sure to run the runloop in our background thread so it can process downloaded data
            // Note: we use a timeout to work around an issue with NSURLConnection cancel under iOS 5
            //       not waking up the runloop, leading to dead threads (see https://github.com/rs/SDWebImage/issues/466)
            CFRunLoopRunInMode(kCFRunLoopDefaultMode, 10, false);
        }
        else {
            CFRunLoopRun();
        }

        if (!self.isFinished) {
            [self.connection cancel];
            [self connection:self.connection didFailWithError:[NSError errorWithDomain:NSURLErrorDomain code:NSURLErrorTimedOut userInfo:@{NSURLErrorFailingURLErrorKey : self.request.URL}]];
        }
    }
    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
    if (self.backgroundTaskId != UIBackgroundTaskInvalid) {
        [[UIApplication sharedApplication] endBackgroundTask:self.backgroundTaskId];
        self.backgroundTaskId = UIBackgroundTaskInvalid;
    }
#endif
}

 [self.connection start];

开启了connection， 然后判断connection是否存在，如果存在的话，里面有一段

 if (!self.isFinished) {
            [self.connection cancel];
            [self connection:self.connection didFailWithError:[NSError errorWithDomain:NSURLErrorDomain code:NSURLErrorTimedOut userInfo:@{NSURLErrorFailingURLErrorKey : self.request.URL}]];
        }

这一段我就不理解了，如果它没有结束，就取消connection。刚刚开启connection，现在就取消connection？而且是不结束的时候取消，这是什么意思？

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可以这么理解，在下载有结果（正常完成，或者发生错误）之前，代码一直会阻塞在 CFRunLoopRun() 或者是 CFRunLoopRunInMode(kCFRunLoopDefaultMode, 10, false) 这里，就是说调用完 start 之后，下载一直就在进行中，一直到下载完成了或者出错了（这两种情况下都会调用 CFRunLoopStop )，这个阻塞才会解除。

这时候，如果发现 isFinished 还是 NO， 那就说明是超时了，取消当前的连接，抛出超时错误。

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beginBackgroundTaskWithExpirationHandler:是在你进入后台超时的时候才会走到，当它超时的时候你就该取消连接以免占用资源

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核心代码在于这里

CFRunLoopRun();

它会无限期阻塞当前线程 直到调用CFRunLoopStop未知