xamarin - Android change theme programmatically - xamarin

Question

I have a multi-tenant app, and have to switch the theme after the user logs in.
What I have so far is that when the User is already logged in and starts the app, I call setTheme() on the MainActivity OnCreate Method.
This works fine and the app is shown in the correct theme.
But when the user is not logged in, the app will start with the default theme. After login i can recognize the user and have to change the theme. But how can I do this. The login process is not in the android project, and from there I have no access to the MainActivity.
How can I set the new theme and recreate the app?
I am very grateful for help.
UPDATE
The log in method is at the moment in the MC.Core (Shared) (.NET Standard 2.0) library and the MainActivity is in the MC.Android library. So there is an Plugin which you can access the actual activity over projects, but I can't use it, becouse it does support .Net Standard.
And I'm not shure, if I can subscribe an event over two project. If this would be possible, how can I to this?
My very last solution is to move the login method form my core project to the Android project. But in this case I have to implement this for every platform.

Answer 1

To call a method in your MainActivity from the net standard project, you need to pass a reference to your MainActivity to the standard project. The best way to do this is to pass a reference to the shared net standard project App constructor, which is invoked from MainActivity. Of course, you cannot declare a parameter in your App constructor of type MainActivity, because your net standard project cannot reference the Android project, and because, if you will in the future implement iOS and / or UWP version of your app, you need a common type between all these different project.
So, you have to define an interface in your net standard project:
public interface IThemeChanger
{
void ApplyTheme(string newTheme);
}
Then, in the Android project, make your MainActivity implement this interface:
public class MainActivity : global::Xamarin.Forms.Platform.Android.FormsAppCompatActivity, IThemeChanger
{
public void ApplyTheme(string newTheme)
{
if (newTheme?.ToLower() == "dark")
{
SetTheme(Resource.Style.Base_Theme_AppCompat);
}
else
{
SetTheme(Resource.Style.Base_Theme_AppCompat_Light);
}
}
and make it pass a reference to itself in the constructor of the app class:
protected override void OnCreate(Bundle savedInstanceState)
{
TabLayoutResource = Resource.Layout.Tabbar;
ToolbarResource = Resource.Layout.Toolbar;
base.OnCreate(savedInstanceState);
global::Xamarin.Forms.Forms.Init(this, savedInstanceState);
LoadApplication(new App(this));
}
Then, in the net standard App class, change the constructor so that it accepts an argument of type IThemeChanger, and store a reference to it in a private or public field (depending if you need or not to access it from outside App class):
public readonly IThemeChanger ThemeChanger;
public App(IThemeChanger themeChanger)
{
InitializeComponent();
this.ThemeChanger = themeChanger;
MainPage = new MainPage();
}
Then, in your login page, after the user login successfully, change the theme accordingly, for example:
((App.Current) as App).ThemeChanger.ApplyTheme("Dark");
If you do not instantiate your App class directly but use a dependency injection container, then register the current instance of MainActivity as an implementer of the IThemeChanger interface with your container, and simply request an IThemeChanger instance in the constructor of your ViewModel. Of course syntax vary depending on which DI Container you use, here is an example with Caliburn.Micro SimpleContainer:
public class MainActivity : global::Xamarin.Forms.Platform.Android.FormsAppCompatActivity, IThemeChanger
{
protected override void OnCreate(Bundle savedInstanceState)
{
TabLayoutResource = Resource.Layout.Tabbar;
ToolbarResource = Resource.Layout.Toolbar;
base.OnCreate(savedInstanceState);
var container = IoC.Get<SimpleContainer>();
if (container.HasHandler<IThemeChanger>())
{
container.UnregisterHandler<IThemeChanger>();
}
container.Instance<IThemeChanger>(this);
global::Xamarin.Forms.Forms.Init(this, savedInstanceState);
LoadApplication(container.GetInstance<App>());
}
In this case, please note that a new instance of MainActivity can be created during app resume, so you need to check for a potential previous registration of the interface and unregister it.
In your ViewModel:
public class LoginViewmModel{
private readonly IThemeChanger themeChanger;
public LoginViewModel(IThemeChanger themeChanger){
this.themeChanger = themChanger;
}
private void ApplyTheme{
themeChanger.ApplyTheme("Dark");
}
}