iOS macOS 回到主线程的三种方式

简单说将代码同步到主线程执行的三种方法如下：
// 1.NSThread
[self performSelectorOnMainThread:@selector(updateUI) withObject:nil waitUntilDone:NO];

- (void)updateUI {
    // UI更新代码
    self.alert.text = @"Thanks!";
}

// 2.NSOperationQueue
[[NSOperationQueue mainQueue] addOperationWithBlock:^{
    // UI更新代码
    self.alert.text = @"Thanks!";
    }];

// 3.GCD
dispatch_async(dispatch_get_main_queue(), ^{
   // UI更新代码
   self.alert.text = @"Thanks!";
});

感谢：奋拓达 https://www.jianshu.com/p/c1f96d734616