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];  
    });  
}