主线程 更新 UI

// 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!";
});