ios 中请求主线程刷新UI

 1 if ([NSThread isMainThread])
 2 {
 3     [self.downloadMapBtn setImage:[UIImage imageNamed:@"download_map.png"] forState:UIControlStateNormal];
 4     [self.downloadMapBtn setNeedsDisplay];
 5 }
 6 else
 7 {
 8     dispatch_sync(dispatch_get_main_queue(), ^{
 9         //Update UI in UI thread here
10         [self.downloadMapBtn setImage:[UIImage imageNamed:@"download_map.png"] forState:UIControlStateNormal];
11         [self.downloadMapBtn setNeedsDisplay];
14     });
15 }