Block中直接使用self，YTK中

dispatch_async(dispatch_get_main_queue(), ^{
        [self requestCompletePreprocessor];
        [self requestCompleteFilter];
        YTKRequest *strongSelf = self;
        [strongSelf.delegate requestFinished:strongSelf];
        if (strongSelf.successCompletionBlock) {
            strongSelf.successCompletionBlock(strongSelf);
        }
        [strongSelf clearCompletionBlock];
    });





- (void)clearCompletionBlock {
    // nil out to break the retain cycle.
    self.successCompletionBlock = nil;
    self.failureCompletionBlock = nil;
}