How to make MainPage.xaml.cs talk to MainActivity.cs using interface

Question

I'm trying to do these classes share some data, I read that the right way to do that is through an interface but MainPage.xaml.cs doesn't recognizes the "link". What I need to add in MainPage.xaml.cs to make it work?

Class1.cs:

namespace interfaceWebnav
{
    public interface IPortableInterface
    {
        object Test();
    }
}

MediaService.cs

using interfaceWebnav;

namespace ComprarMusicas
{
    public class MediaService : Java.Lang.Object, IPortableInterface
    {
        public object Test()
        {
            //do something
        }
    }
}

MainActivity.cs:

...
...
using interfaceWebnav;

[assembly: Dependency(typeof(IPortableInterface))]
namespace MyApp.Droid
{
     [Activity(Label = "Comprar Músicas", Icon = "@mipmap/icon", Theme = "@style/MainTheme", MainLauncher = true, NoHistory = true,ConfigurationChanges = ConfigChanges.ScreenSize | ConfigChanges.Orientation)]
    public class MainActivity :  global::Xamarin.Forms.Platform.Android.FormsAppCompatActivity
    {
        protected override void OnCreate(Bundle savedInstanceState)
        {
            ............
            base.OnCreate(savedInstanceState);
            global::Xamarin.Forms.Forms.Init(this, savedInstanceState);
            LoadApplication(new App());
        }
   }

    public override void OnBackPressed()
    {
        // here I need to know what SomeMethod() from MainPage.xaml.cs is saying
    }

}

MainPage.xaml.cs (I commented out some of my tries of make it work):

using Xamarin.Forms;
// using interfaceWebnav;

// [assembly: Dependency(typeof(IPortableInterface))]

namespace MyApp
{
    public partial class MainPage : ContentPage
    {
        public MainPage()
        {
            InitializeComponent();
        }
    }

    void SomeMethod(object sender, EventArgs e)
    {
        object resultado = DependencyService.Get<IPortableInterface>().Test();
    }
}

At MainPage.xaml.cs, visual studio says about SomeMethod: "A namespace can't contain members as fields or methods directly."

About "Get IPortableInterface" it says:

"The name of type or namespace IPortableInterface can't be find(are you missing an using directive or an assembly referency?)"

Answer 1

Add Class says MediaService.cs in your android project inherit it from IPortableInterface

public class MediaService : Java.Lang.Object, IPortableInterface
{
    public void Test()
    {
        // your android platform specific implementation
    }

}

In the MainPage.cs file your invoking dependency service

namespace MyApp
{
    public partial class MainPage : ContentPage
    {
        public bool foo = true; //just an example

        public MainPage()
        {
            InitializeComponent();
        }
    }
    //I suppose invoking any event lets says button click calling interface dependency service of android project
    void Button_Clicked(object sender,EventArgs e)
    {
        object resultado = DependencyService.Get<IPortableInterface>().Test();
    }

}

I hope this will help you