AFURLSessionManager里监听事件为什么要判断object是否是NSURLSessionDownloadTask

• (void)observeValueForKeyPath:(NSString )keyPath ofObject:(id)object change:(NSDictionary<NSString ,id> )change context:(void )context {

     if ([object isKindOfClass:[NSURLSessionTask class]] || [object isKindOfClass:[NSURLSessionDownloadTask class]]) {
         if ([keyPath isEqualToString:NSStringFromSelector(@selector(countOfBytesReceived))]) {
             self.downloadProgress.completedUnitCount = [change[@"new"] longLongValue];
         } else if ([keyPath isEqualToString:NSStringFromSelector(@selector(countOfBytesExpectedToReceive))]) {
             self.downloadProgress.totalUnitCount = [change[@"new"] longLongValue];
         } else if ([keyPath isEqualToString:NSStringFromSelector(@selector(countOfBytesSent))]) {
             self.uploadProgress.completedUnitCount = [change[@"new"] longLongValue];
         } else if ([keyPath isEqualToString:NSStringFromSelector(@selector(countOfBytesExpectedToSend))]) {
             self.uploadProgress.totalUnitCount = [change[@"new"] longLongValue];
         }
     }
     else if ([object isEqual:self.downloadProgress]) {
         if (self.downloadProgressBlock) {
             self.downloadProgressBlock(object);
         }
     }
     else if ([object isEqual:self.uploadProgress]) {
         if (self.uploadProgressBlock) {
             self.uploadProgressBlock(object);
         }
     }

  }

在第一行判断object时，已经判断了是否是NSURLSessionTask，为什么还要判断或是否是NSURLSessionDownloadTask，NSURLSessionDownloadTask不是已经是NSURLSessionTask的子类了么？

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NSURLSessionTask实际上是Class cluster，通过NSURLSession生成的task返回的并不一定是指定的task类型。因此kindOfClass并不总会生效，具体可以参见AFURLSessionManager.m在load方法中的说明。
特定于当前问题，是由于iOS 7上__NSCFURLSessionDownloadTask的基类并不是__NSCFURLSessionTask，因此isKindOfClass会出错。查看对应的commit就可以知道了。