How can i minimize or close the app from a dialogue?

Question

I am new to Xamarin, and mobile development, i have experience with winforms and .net, but threading and MVVM is not something i am very familiar with.

My code works if you click back, select ok in the dialogue, and then click back again. But i would like to improve it by either:

• Programmatically call the backbutton to force the check again (with incremented counter)
• Close the app directly if you click OK (preferred)

The code below overrides the backbutton on android, and if it returns true. The commented out parts are my fumblings from earlier.

    /// <summary>
    /// Gives the user a warning that they are about to close the main page
    /// </summary>
    public override bool OnAndroidBackButtonPressed()
    {
        if (this.CloseAppAttempts == 0)
        {
            try
            {
                this.device.BeginInvokeOnMainThread(async () =>
                {
                    bool closeApp = await this.DisplayService.DisplayAlertAsync("Close app", "Click the OK button to close the app / or click the back button again to close the app", "Ok", "Cancel");
                    if (closeApp)
                    {
                        // this.DisplayService.CloseAsync();
                        // this.DisplayService.ExitAsync();
                        // this.DisplayService.pageStack.Pop();
                        // await IDisplayService.navigation.PopAsync();

                        this.CloseAppAttempts++;
                        // i would like to either call the backbutton programatically here, or close 
                    }
                });
            }
            catch (Exception ex)
            {
                System.Diagnostics.Debug.WriteLine("===== Debug Message - Closing the app did not work -  [ " + ex.Message + " ]");
            }

            return true;
        }
        else
        {
            return false;
        }
    }

Answer 1

Xamarin.Android

The following code overrides the back button and warns the user before going back:

public class MainActivity : Activity
{
    . . .

    bool pressed = false;

    public override void OnBackPressed()
    {
        if (!pressed)
        {
            Toast.Show("Press again to exit"); //Implement the Toast correctly
            pressed = true;
        }
        else
            base.OnBackPressed();
    }
}

(Add some logic to rearm this Toast, so if the user press the button after a while, it shows again)

---

The next snippet shows a message (I don't know how to show a message in Xamarin.Android, but the logic is there)

public class MainActivity : Activity
{
    . . .

    public override void OnBackPressed()
    {
        if (Alert("Exit?", "Yes", "No"))
            base.OnBackPressed();
    }
}

---

Xamarin.Forms

On Xamarin.Forms, you need to know what page is being shown, so you won't show the dialog at every page and subpage.

In this case, I used NavigationPage to control all the pages.

public class MainActivity : global::Xamarin.Forms.P . . .
{
    . . .

    bool pressed = false;

    public override void OnBackPressed()
    {
        if (Xamarin.Forms.Application.Current.MainPage.Navigation.NavigationStack.Count == 1)
        {
            if (Xamarin.Forms.Application.Current.MainPage.Navigation.NavigationStack[0].DisplayAlert(
                "Confirm exit",
                "Do you really want to exit?",
                "Yes", "No"))
                base.OnBackPressed();
        }
        else
            base.OnBackPressed();
    }
}

Answer 2

You can close the app on android with this command:

public void CloseApplication()
{
    var activity = (Activity)Forms.Context;
    activity.FinishAffinity();
}

Using Xamarin.Forms you can call it through a dependency service.