I have a standard xamarin forms project (both iOS and Android) using shell for navigation. I have implemented themes using merged dictionaries. This is the method I use to apply the theme:

public class MyShellRenderer : ShellRenderer
    {

protected override IShellBottomNavViewAppearanceTracker CreateBottomNavViewAppearanceTracker(Xamarin.Forms.ShellItem shellItem)
        {
            return new MyShellBottomNavViewAppearanceTracker(this);
        }
    }

internal class MyShellBottomNavViewAppearanceTracker : IShellBottomNavViewAppearanceTracker
    {
        private MyShellRenderer myShellRenderer;

        public MyShellBottomNavViewAppearanceTracker(MyShellRenderer myShellRenderer)
        {
            this.myShellRenderer = myShellRenderer;
        }


Task SetAppTheme(PreferredTheme preferredTheme)
        {
            if (Application.Current is null)
                return Task.CompletedTask;

            BaseTheme defaultTheme;
            if (Application.Current.RequestedTheme == OSAppTheme.Dark)
                defaultTheme = new DarkTheme();
            else
                defaultTheme = new LightTheme();

            return _mainThread.InvokeOnMainThreadAsync(() =>
            {
                BaseTheme theme = preferredTheme switch
                {
                    PreferredTheme.Dark => new DarkTheme(),
                    PreferredTheme.Light => new LightTheme(),
                    PreferredTheme.Default => defaultTheme
                };

                CurrentTheme = theme;

                ICollection<ResourceDictionary> mergedDictionaries = Application.Current.Resources.MergedDictionaries;
                if (mergedDictionaries != null)
                {
                    mergedDictionaries.Clear();
                    mergedDictionaries.Add(theme);
                }

                
                OnPreferenceChanged(preferredTheme);
            });
        }

This works for iOS but on Android the ItemText color is not changing. This is a problem when the background color of the dark mode theme is the same as (or close to) the color of the deselected Tab. Icons render as expected.

I have tried implementing a custom render on android like so:

public void ResetAppearance(BottomNavigationView bottomView)
        {

            //Get and set background color from theme
            Xamarin.Forms.Color xfcolor = (Xamarin.Forms.Color)App.Current.Resources["PageBackgroundColor"];
            Android.Graphics.Color acolor = xfcolor.ToAndroid();
            bottomView.SetBackgroundColor(acolor);

            //Get and set state colors for menu items text
            Xamarin.Forms.Color xfSelectedColor = (Xamarin.Forms.Color)App.Current.Resources["ColorBlue"];
            Android.Graphics.Color aSelectedColor = xfSelectedColor.ToAndroid();

            Xamarin.Forms.Color xfDeselectedColor = (Xamarin.Forms.Color)App.Current.Resources["ColorGray"];
            Android.Graphics.Color aDeselectedColor = xfDeselectedColor.ToAndroid();

            int[][] states = new int[][]
            {
                new int[] {-Android.Resource.Attribute.Checked }, // unchecked
                new int[] { Android.Resource.Attribute.Checked }  // pressed
            };

            int[] colors = new int[]
            {
                aSelectedColor,
                aDeselectedColor
            };
            ColorStateList colorStateList = new ColorStateList(states, colors);

            bottomView.ItemTextColor = colorStateList;
...

This partly works in that the entire text is getting the aSelectedColor applied.

Is this a bug in the SDK or am I simply just doing something wrong?

CodePudding user response：

Found a solution (Thanks to James Montemagno!!!)

Inspiration found here

So fore some odd reason using mergedDictionaries does not work for Android.

Instead I did the following rewrite based on the Hanselman.Forms sample:

 Task SetAppTheme(PreferredTheme preferredTheme)
    {
        if (Application.Current is null)
            return Task.CompletedTask;

        BaseTheme defaultTheme;
        if (Application.Current.RequestedTheme == OSAppTheme.Dark)
            defaultTheme = new DarkTheme();
        else
            defaultTheme = new LightTheme();

        return _mainThread.InvokeOnMainThreadAsync(() =>
        {
            BaseTheme theme = preferredTheme switch
            {
                PreferredTheme.Dark => new DarkTheme(),
                PreferredTheme.Light => new LightTheme(),
                PreferredTheme.Default => defaultTheme
            };

            CurrentTheme = theme;

            //Previous code
            /*ICollection<ResourceDictionary> mergedDictionaries = Application.Current.Resources.MergedDictionaries;
            if (mergedDictionaries != null)
            {
                mergedDictionaries.Clear();
                mergedDictionaries.Add(theme);
            }*/

            //New code
            var applicationResourceDictionary = Application.Current.Resources;
            ManuallyCopyThemes(theme, applicationResourceDictionary);

            
            OnPreferenceChanged(preferredTheme);
        });
    }


    void ManuallyCopyThemes(ResourceDictionary fromResource, ResourceDictionary toResource)
    {
        foreach (var item in fromResource.Keys)
        {
            if (toResource.ContainsKey(item))
                toResource[item] = fromResource[item];
            else
                toResource.Add(item, fromResource[item]);
        }
    }