android - How to change navigation page back button in xamarin forms

Question

I am trying to change back arrow image in navigation page. For this in android app i created navigation page renderer and then using method toolbar.SetNavigationIcon and its not working, but when i use toolbar.SetLogo its working fine.

    protected override void OnElementChanged(ElementChangedEventArgs<NavigationPage> e)
    {

        base.OnElementChanged(e);
        toolbar.SetNavigationIcon(Resource.Drawable.arrow); 
        toolbar.SetLogo(Resource.Drawable.arrow);
    }
    public override void OnViewAdded(Android.Views.View child)
    {
        base.OnViewAdded(child);
        if (child.GetType() == typeof(Android.Support.V7.Widget.Toolbar))
        {
            toolbar = (Android.Support.V7.Widget.Toolbar)child;
            toolbar.ChildViewAdded += Toolbar_ChildViewAdded;
        }
    }

I also tried add image to app:navigationIcon in toolbar.axml, and it shows great in designer my arrow

But, when i starting my app i have the same standart arrow icon enter image description here

Answer 1

If your MainActivity is FormsApplicationActivity, you could refer to this example :

https://github.com/jessejiang0214/ChangeBackIconInXF/tree/master/Droid

If your MainActivity type is FormsAppCompatActivity, you could custom a PageRenderer and change the Toolbar's NavigationIcon.

For example :

[assembly: ExportRenderer(typeof(ContentPage), typeof(NavigationPageRendererDroid))]
...
public class NavigationPageRendererDroid : PageRenderer
{
    protected override void OnElementChanged(ElementChangedEventArgs<Xamarin.Forms.Page> e)
    {
        base.OnElementChanged(e);

        var context = (Activity)Xamarin.Forms.Forms.Context;
        var toolbar = context.FindViewById<Android.Support.V7.Widget.Toolbar>(Droid.Resource.Id.toolbar);

        toolbar.NavigationIcon = Android.Support.V4.Content.ContextCompat.GetDrawable(context, Resource.Drawable.Back);
    }
}

Usage :

MainPage = new NavigationPage(new MainPage());
...
//When click a button in MainPage, navigate to another page
private async void Button_Clicked(object sender, EventArgs e)
{
    await Navigation.PushAsync(new TestPage());
}

Effect.

---

Update :

When you use Navigation.PushAsync() method navigate to another page, the system will automatically update the Toolbar's icon, you could find in the source code :

protected virtual Task<bool> OnPushAsync(Page view, bool animated)
{
    return SwitchContentAsync(view, animated);
}

Task<bool> SwitchContentAsync(Page page, bool animated, bool removed = false, bool popToRoot = false)
{
    ...
    UpdateToolbar();
    ...
}

void UpdateToolbar()
{
    ...
    bool isNavigated = ((INavigationPageController)Element).StackDepth > 1;
    if (isNavigated)
    {
        ...
        if (NavigationPage.GetHasBackButton(Element.CurrentPage))
        {
             //Set the navigation back  icon < =================== !!! =-=
             var icon = new DrawerArrowDrawable(activity.SupportActionBar.ThemedContext);
             icon.Progress = 1;
             bar.NavigationIcon = icon;
        }
    }
    ...
}

Solution :

So we have no choice but to custom a NavigationPageRenderer, override the OnPushAsync method to set the Toolbar's icon.

using AToolbar = Android.Support.V7.Widget.Toolbar;
[assembly: ExportRenderer(typeof(CustomNavigationPage), typeof(NavigationPageRendererDroid))] // APPCOMP
...
public class NavigationPageRendererDroid : Xamarin.Forms.Platform.Android.AppCompat.NavigationPageRenderer // APPCOMP
{
    public AToolbar toolbar;
    public Activity context;

    protected override Task<bool> OnPushAsync(Page view, bool animated)
    {
        var retVal = base.OnPushAsync(view, animated);

        context = (Activity)Xamarin.Forms.Forms.Context;
        toolbar = context.FindViewById<Android.Support.V7.Widget.Toolbar>(Droid.Resource.Id.toolbar);

        if (toolbar != null)
        {
            if (toolbar.NavigationIcon != null)
            {
                toolbar.NavigationIcon = Android.Support.V4.Content.ContextCompat.GetDrawable(context, Resource.Drawable.Back);
                //toolbar.SetNavigationIcon(Resource.Drawable.Back);
            }
        }
        return retVal;
    }
}

The CustomNavigationPage are defined in PCL :

public class CustomNavigationPage : NavigationPage
{
    public CustomNavigationPage(Page startupPage) : base(startupPage)
    {
    }
}

Usage :

public App()
{
    InitializeComponent();

    MainPage = new CustomNavigationPage(new MainPage());
}
...
// In MainPage
private async void Button_Clicked(object sender, EventArgs e)
{
    await Navigation.PushAsync(new TestPage());
}