多弹窗按顺序依次弹出

通过dispatch_queue_t及dispatch_semaphore_t实现

 //创建一个队列，串行并行都可以，主要为了操作信号量
    dispatch_queue_t queue = dispatch_queue_create("com.se7en.alert", DISPATCH_QUEUE_SERIAL);
    dispatch_async(queue, ^{
    //创建一个初始为0的信号量
    dispatch_semaphore_t sema = dispatch_semaphore_create(0);
        NSLog(@"弹框1");
        [NSThread sleepForTimeInterval:1];
    //第一个弹框，UI的创建和显示，要在主线程
    dispatch_async(dispatch_get_main_queue(), ^{
     UIAlertController *alert = [UIAlertController alertControllerWithTitle:@"弹框1" message:@"第一个弹框" preferredStyle:UIAlertControllerStyleAlert];
     [alert addAction:[UIAlertAction actionWithTitle:@"取消" style:UIAlertActionStyleCancel handler:^(UIAlertAction * _Nonnull action) {
     //点击Alert上的按钮，我们发送一次信号。
     dispatch_semaphore_signal(sema);
     }]];
     [self presentViewController:alert animated:YES completion:nil];
    });
    
    //等待信号触发，注意，这里是在我们创建的队列中等待
    dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
        NSLog(@"弹框2");
        [NSThread sleepForTimeInterval:1];
    //上面的等待到信号触发之后，再创建第二个Alert
    dispatch_async(dispatch_get_main_queue(), ^{
     UIAlertController *alert = [UIAlertController alertControllerWithTitle:@"弹框2" message:@"第二个弹框" preferredStyle:UIAlertControllerStyleAlert];
     [alert addAction:[UIAlertAction actionWithTitle:@"取消" style:UIAlertActionStyleCancel handler:^(UIAlertAction * _Nonnull action) {
     dispatch_semaphore_signal(sema);
     }]];
     [self presentViewController:alert animated:YES completion:nil];
    });
    
    //同理，创建第三个Alert
    dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
        NSLog(@"弹框3");
        [NSThread sleepForTimeInterval:1];
    dispatch_async(dispatch_get_main_queue(), ^{
     UIAlertController *alert = [UIAlertController alertControllerWithTitle:@"弹框3" message:@"第三个弹框" preferredStyle:UIAlertControllerStyleAlert];
     [alert addAction:[UIAlertAction actionWithTitle:@"取消" style:UIAlertActionStyleCancel handler:^(UIAlertAction * _Nonnull action) {
     dispatch_semaphore_signal(sema);
     }]];
     [self presentViewController:alert animated:YES completion:nil];
    });
    });

结果：

2020-08-03 14:10:17.143774+0800 GCDDemo[4731:161332] 弹框1
2020-08-03 14:10:20.254767+0800 GCDDemo[4731:161332] 弹框2
2020-08-03 14:10:22.363951+0800 GCDDemo[4731:161332] 弹框3