iOS实现多个弹框按顺序依次弹出效果

- (void)viewDidAppear:(BOOL)animated {
 [super viewDidAppear:animated];
 UIAlertController *alert = [UIAlertController alertControllerWithTitle:@"弹框1" message:@"第一个弹框" preferredStyle:UIAlertControllerStyleAlert];
 [alert addAction:[UIAlertAction actionWithTitle:@"取消" style:UIAlertActionStyleCancel handler:nil]];
 [self presentViewController:alert animated:YES completion:nil];

 UIAlertController *alert2 = [UIAlertController alertControllerWithTitle:@"弹框2" message:@"第二个弹框" preferredStyle:UIAlertControllerStyleAlert];
 [alert2 addAction:[UIAlertAction actionWithTitle:@"取消" style:UIAlertActionStyleCancel handler:nil]];
 [self presentViewController:alert2 animated:YES completion:nil];
}

- (void)viewDidAppear:(BOOL)animated {
 [super viewDidAppear:animated];
 //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);
 //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);
 //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);
 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];
 });
 });
}