ios 中请求主线程刷新UI

        if ([NSThread isMainThread])
        {
            [self.downloadMapBtn setImage:[UIImage imageNamed:@"download_map.png"] forState:UIControlStateNormal];
            [self.downloadMapBtn setNeedsDisplay];
        }
        else
        {
            dispatch_sync(dispatch_get_main_queue(), ^{
                //Update UI in UI thread here
                [self.downloadMapBtn setImage:[UIImage imageNamed:@"download_map.png"] forState:UIControlStateNormal];
                [self.downloadMapBtn setNeedsDisplay];




            });
        }