How to get theme colors programatically - xamarin

Question

I want to set some widget color programmatically in C# code but I cannot figure out how to get the color when they are set in my app theme and differ for day/night mode.
What I have so far:
attrs.xml:
<?xml version="1.0" encoding="utf-8" ?>
<resources>
<declare-styleable name="MyThemeColors">
<attr format="color" name="colorOkay"/>
<attr format="color" name="colorOnOkay"/>
<attr format="color" name="colorWarning"/>
<attr format="color" name="colorOnWarning"/>
</declare-styleable>
</resources>
in my themes.xml this colors are set like
<item name="colorWarning">#color/color_warning</item>
where colorWarning is defined in color.xml, different for day and nicht mode. The same is done for colorSurface and so on. This works fine.
To get the color for colorWarning in C# code I tried:
Context context = Android.App.Application.Context;
TypedArray ta = context.Theme.ObtainStyledAttributes(Resource.Styleable.MyThemeColors);
var colorWarning = ta.GetColorStateList(Resource.Styleable.MyThemeColors_colorWarning);
ta.Recycle();
but colorWarnig is always null.
I also tried
Android.App.Application.Context.GetColor(...)
but this seemt to work for Android.Resource.Color.[...] colors only and not for MyAppNamespace.Resource.Color.[...] colors.

Answer 1

Xamarin.Forms applications can respond to system theme changes by consuming resources with the AppThemeBinding markup extension, and the SetAppThemeColor and SetOnAppTheme<T> extension methods.
You could use the code below to change the color which differ for day/night mode.
Label label = new Label();
label.SetAppThemeColor(Label.TextColorProperty, Color.Green, Color.Red);
For more details, you could check the MS docs with the code sample. https://learn.microsoft.com/en-us/xamarin/xamarin-forms/user-interface/theming/system-theme-changes#react-to-theme-changes
Updated:
The Dark theme is available in Android 10 (API level 29) and higher.
1. You could change your AppTheme in AndroidManifest.xml.
Change:
<style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">
<!-- Customize your theme here. -->
<item name="colorPrimary">#color/colorPrimary</item>
<item name="colorPrimaryDark">#color/colorPrimaryDark</item>
<item name="colorAccent">#color/colorAccent</item>
</style>
To:
<style name="AppTheme" parent="Theme.AppCompat.DayNight">
<!-- Customize your theme here. -->
<item name="colorPrimary">#color/colorPrimary</item>
<item name="colorPrimaryDark">#color/colorPrimaryDark</item>
<item name="colorAccent">#color/colorAccent</item>
</style>
And set the Theme in the MainActivity:
[Activity(Label = "#string/app_name", Theme = "#style/AppTheme", MainLauncher = true)]
2. You could change the Theme programmatically.
Delet the Theme = "#style/AppTheme" in ActivityAttribute.
Create another AppTheme2 in Styles.xml.
<style name="AppTheme2" parent="Theme.AppCompat.DayNight">
<!-- Customize your theme here. -->
<item name="colorPrimary">#color/colorPrimary</item>
<item name="colorPrimaryDark">#color/colorPrimaryDark</item>
<item name="colorAccent">#color/colorAccent</item>
Add the code in the OnCreate() method. Please note call the SetTheme before you load the layout.
protected override void OnCreate(Bundle savedInstanceState)
{
base.OnCreate(savedInstanceState);
Xamarin.Essentials.Platform.Init(this, savedInstanceState);
// Set our view from the "main" layout resource
SetTheme(Resource.Style.AppTheme2);
SetContentView(Resource.Layout.activity_main);
}