FindViewById on Android Subpage (using Xamarin.Forms)

Question

I'm developing a cross-platform app using Xamarin.Forms. As I want to set the background of my TabLayouts to a gradient, which isn't natively supported by XF, I've written a custom renderer for my TabbedPages:

using Android.App;
using Android.Graphics;
using Android.Support.Design.Widget;
using Xamarin.Forms;
using Xamarin.Forms.Platform.Android;
using Xamarin.Forms.Platform.Android.AppCompat;
using MyApp.Droid.Renderer;

[assembly: ExportRenderer(typeof(TabbedPage), typeof(CustomTabbedPageRenderer))]
namespace MyApp.Droid.Renderer
{
    public class CustomTabbedPageRenderer : TabbedPageRenderer
    {
        private Activity _activity;

        protected override void OnElementChanged(ElementChangedEventArgs<TabbedPage> e)
        {
            base.OnElementChanged(e);

            _activity = this.Context as FormsAppCompatActivity;
        }

        protected override void DispatchDraw(Canvas canvas)
        {
            base.DispatchDraw(canvas);

            TabLayout tabs = _activity.FindViewById<TabLayout>(Resource.Id.sliding_tabs);

            var gradient = new MyGradient();

            tabs.SetBackground(gradient);
        }
    }
}

Now as intended the renderer gets triggered every time the app loads a TabbedPage. However, _activity.FindViewById<TabLayout>(Resource.Id.sliding_tabs) always just returns the TabLayout of my MainPage (also properly setting MyGradient as background) and I can't figure out how to get the TabLayout on one of my subpages to modify it as well.

Not sure if this is a problem with Xamarin or Android...

Hope you guys can help me.

Thanks!

Jan

EDIT: here's the styles.xml (I hope that's what @Code-Apprentice was referring to):

<resources>
  <style name="MainTheme" parent="MainTheme.Base">
  </style>
  <!-- Base theme applied no matter what API -->
  <style name="MainTheme.Base" parent="Theme.AppCompat.Light.DarkActionBar">
    <!--If you are using revision 22.1 please use just windowNoTitle. Without android:-->
    <item name="windowNoTitle">true</item>
    <!--We will be using the toolbar so no need to show ActionBar-->
    <item name="windowActionBar">false</item>
    <!-- Set theme colors from http://www.google.com/design/spec/style/color.html#color-color-palette -->
    <!-- colorPrimary is used for the default action bar background -->
    <item name="colorPrimary">#2196F3</item>
    <!-- colorPrimaryDark is used for the status bar -->
    <!--<item name="colorPrimaryDark">#1976D2</item>-->
    <item name="colorPrimaryDark">#FF8630</item>
    <!--<item name="android:windowTranslucentStatus">true</item>-->
    <!-- colorAccent is used as the default value for colorControlActivated
     which is used to tint widgets -->
    <item name="colorAccent">#FF4081</item>
    <!-- You can also set colorControlNormal, colorControlActivated
     colorControlHighlight and colorSwitchThumbNormal. -->
    <item name="windowActionModeOverlay">true</item>

    <item name="android:datePickerDialogTheme">@style/AppCompatDialogStyle</item>
  </style>

  <style name="AppCompatDialogStyle" parent="Theme.AppCompat.Light.Dialog">
    <item name="colorAccent">#FF4081</item>
  </style>
</resources>

Also a screenshot of the MainPage's top: http://i.imgur.com/VyX3YX7.png

And it's subpage ( MainPage.Navigation.PushAsync(new SubPage()); ): http://i.imgur.com/pRmecjE.png

Answer 1

For anyone else wondering, here's how I get the result I want now (even though I guess it's not the most elegant way of doing this):

using Android.Runtime;
using Android.Support.Design.Widget;
using Android.Views;
using Xamarin.Forms;
using Xamarin.Forms.Platform.Android;
using Xamarin.Forms.Platform.Android.AppCompat;
using MyApp.Droid.Renderer;

[assembly: ExportRenderer(typeof(TabbedPage), typeof(CustomTabbedPageRenderer))]
namespace MyApp.Droid.Renderer
{
    public class CustomTabbedPageRenderer : TabbedPageRenderer
    {
        protected override void OnVisibilityChanged(Android.Views.View changedView, [GeneratedEnum] ViewStates visibility)
        {
            base.OnVisibilityChanged(changedView, visibility);

            if (visibility == ViewStates.Visible)
            {
                var tabs = changedView.FindViewById<TabLayout>(Resource.Id.sliding_tabs);
                var gradient = new MyGradient();
                tabs.SetBackground(gradient);
            }
        }
    }
}