can not show alertcontroller when page is displayed using PushModalAsync - xamarin.forms

Question

I am trying to show toast message in android and iOS from xamarin.forms project using Dependency Service. In iOS project message is shown on MainPage or NavigationPage. but when I navigate a second page on button click using PushModalAsync, message is not displayed.
How I navigate the page
public LoginPage()
{
Device.BeginInvokeOnMainThread(() =>
{
CustomToast.LongMessage("Hiiiiii"); // Message shown
});
Navigation.PushModalAsync(new RegisterPage()); //Doesn't show
//var reg = new RegisterPage();
//Application.Current.MainPage = reg; // toast shown here
}
Code for alertController in iOS :
const double SHORT_DELAY = 2.0;
NSTimer alertDelay;
UIAlertController alert;
public void LongAlert(string message)
{
ShowAlert(message, LONG_DELAY);
}
public void ShortAlert(string message)
{
ShowAlert(message, SHORT_DELAY);
}
void ShowAlert(string message, double seconds)
{
try
{
if (alert == null && alertDelay == null)
{
alertDelay = NSTimer.CreateScheduledTimer(seconds, (obj) =>
{
Device.BeginInvokeOnMainThread(() =>
{
DismissMessage();
});
});
Device.BeginInvokeOnMainThread(() =>
{
try
{
alert = UIAlertController.Create("", message, UIAlertControllerStyle.ActionSheet);
UIApplication.SharedApplication.KeyWindow.RootViewController.PresentViewController(alert, true, null);
}
catch (Exception ex)
{
var Error = ex.Message;
}
});
}
}
catch (Exception ex)
{
TraceLog("Message iOS ShowAlert : " + ex.Message);
}
}
void DismissMessage()
{
if (alert != null)
{
alert.DismissViewController(true, null);
alert = null;
}
if (alertDelay != null)
{
alertDelay.Dispose();
alertDelay = null;
}
}
And I call this from my register page constructor
Device.BeginInvokeOnMainThread(() =>
{
CustomToast.LongMessage("Hiiiiii");
});
It doesn't go in catch anywhere but its not displayed also. can anyone please suggest some advice ?

Answer 1

This is because RegisterPage is a presented page on your LoginPage, UIApplication.SharedApplication.KeyWindow.RootViewController this code can't retrieve a correct view controller for RegisterPage. It just presented an action sheet on the previous page, but your app has reached a new page then this Toast can be shown on the screen.
Firstly, you have to find out the top page on the window:
UIViewController topViewControllerWithRootViewController(UIViewController rootViewController)
{
if (rootViewController is UITabBarController)
{
UITabBarController tabBarController = (UITabBarController)rootViewController;
return topViewControllerWithRootViewController(tabBarController.SelectedViewController);
}
else if (rootViewController is UINavigationController)
{
UINavigationController navigationController = (UINavigationController)rootViewController;
return topViewControllerWithRootViewController(navigationController.VisibleViewController);
}
else if (rootViewController.PresentedViewController != null)
{
UIViewController presentedViewController = rootViewController.PresentedViewController;
return topViewControllerWithRootViewController(presentedViewController);
}
return rootViewController;
}
Secondly, adjust your presenting code like:
Device.BeginInvokeOnMainThread(() =>
{
try
{
alert = UIAlertController.Create("", messages, UIAlertControllerStyle.ActionSheet);
topViewControllerWithRootViewController(UIApplication.SharedApplication.KeyWindow.RootViewController).PresentViewController(alert, true, null);
}
catch (Exception ex)
{
var Error = ex.Message;
}
});
At last, you could show your toast using Navigation.PushModalAsync(new RegisterPage());