如何在Xamarin表单Android中实现警报框

Question

I implemented a Dependency Service to display alert box in my xamarin forms app.My app crashes when I call the alert box in android.

Here is My code

Android.App.AlertDialog.Builder _dialog = new AlertDialog.Builder(Android.App.Application.Context);
AlertDialog _alertDialog = _dialog.Create();
_alertDialog.SetTitle("Unauthorized");
_alertDialog.SetMessage("Please login again to continue using the      App);
_alertDialog.SetButton("OK", (c, ev) => { CloseApp(); });
_alertDialog.Show();

It throws an exception:-Unable to add window -- token null is not for an application in android.

How to fix this Please help me

Answer 1

Unable to add window -- token null is not valid; is your activity running?

You are using a Application context and you need to use an Activity based one.

So you need the current Activity's context within your Forms' dependancy class which you can obtain that via multiple methods; A static var on the MainActivity, using the "CurrentActivityPlugin", etc...

As a quick fix, add a static Context variable to your MainActivity class and set it in the OnResume override.

public static Context context;
protected override void OnResume()
{
    context = this;
    base.OnResume();
}

Then change your context to that static one:

Android.App.AlertDialog.Builder _dialog = new Android.App.AlertDialog.Builder(MainActivity.context);