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];
});
}