在UIview上弹窗 或者在UIWindow弹窗

- (void)presentAlert:(UIAlertController *)alert  {
    //创建一个新window
    UIWindow *alertWindow = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
    UIViewController *viewController = [[UIViewController alloc] init];
    alertWindow.rootViewController = viewController;
    [alertWindow makeKeyAndVisible];
    [alertWindow.rootViewController presentViewController:alert animated:YES completion:nil];
}
- (void)dismissAlert {
    //将当前页的keyWindow切换回来
    AppDelegate *appDelegate = (id)[UIApplication sharedApplication].delegate;
    [appDelegate.window makeKeyAndVisible];
}