ios 中请求主线程刷新UI

IOS中只有主线程 才能立即刷新UI。如果是通过侦听异步消息，触发回调函数，或者调用异步方法，请求刷新UI，都会产生线程阻塞和延迟的问题。

可以通过 

        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];


            });
        }

这样的方法来进行 消息派送给主线程，进行刷新。

相关阅读

http://www.ios-developer.net/iphone-ipad-programmer/development/threads/updating-ui-controls-on-background-threads

http://www.raywenderlich.com/4295/multithreading-and-grand-central-dispatch-on-ios-for-beginners-tutorial