Xamarin swap single ResourceDictionary programmatically - xamarin.forms

Question

I'm currently trying to get custom themes working with a Xamarin App and I've found a way I can make this work the way I want it to, but it seems terribly inefficient, so figured I'd ask to see if you lot had better answers.
So here's the issue, I want to build a screen where users can change themes (there will be many, not just light/dark), and I want to swap out the current theme ResourceDictionary and replace it with the new chosen theme's ResourceDictionary. The problem is I also want the App to have other global ResourceDictionaries, and every tutorial I find recommends calling Resources.MergedDictionaries.Clear(), but that then gets rid of my other ResourceDictionaries.
Here is my Xaml, I don't want to replace DefaultButton.xaml or LinkButton.xaml, I only want to replace LightTheme.xaml.
<?xml version="1.0" encoding="utf-8" ?>
<Application xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
x:Class="myTestApp.App">
<Application.Resources>
<ResourceDictionary Source="Themes/ResourceDictionaries/LightTheme.xaml" />
<ResourceDictionary Source="Styles/DefaultButton.xaml" />
<ResourceDictionary Source="Styles/LinkButton.xaml" />
</Application.Resources>
</Application>
Then in my App class I have
public void ChangeTheme(ThemeOptions theme)
{
ResourceDictionary newRes;
switch (theme)
{
case ThemeOptions.Light:
newRes = new LightTheme();
break;
case ThemeOptions.Dark:
newRes = new DarkTheme();
break;
default:
newRes = new LightTheme();
break;
}
Resources.MergedDictionaries.Clear();
Resources.Add(newRes);
}
Now obviously the DefaultButton.xaml and LinkButton.xaml ResourceDictionaries have now been cleared. I can add them back, but it seems terribly inefficient over just finding and removing the Theme. Is there an easy way to simply identify the ResourceDictionary I want to overwrite and replace that?

Answer 1

I believe I may have discovered the answer to my problem, but not to the original question. I created a "BundledSiteStyles" ResourceDictionary, and simply added that as a MergedDictionary to each of my themes. Then the App.xaml file only loads a theme and all the styling comes along for the ride.
So my App.xaml has no ResourceDictionaries added, then programmatically on startup I get the active theme and call my ChangeTheme function
public void ChangeTheme(ThemeOptions theme)
{
switch (theme)
{
case ThemeOptions.Light:
Resources = new LightTheme();
break;
case ThemeOptions.Dark:
Resources = new DarkTheme();
break;
default:
Resources = new LightTheme();
break;
}
}
Here is my LightTheme.xaml
<?xml version="1.0" encoding="utf-8" ?>
<ResourceDictionary xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
x:Class="myTestApp.Themes.ResourceDictionaries.LightTheme">
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="../../Styles/BundledSiteStyles.xaml" />
</ResourceDictionary.MergedDictionaries>
<Color x:Key="LinkButtonBackgroundColor">Transparent</Color>
<Color x:Key="LinkButtonTextColor">Blue</Color>
<!-- Add more colours here -->
</ResourceDictionary>
And here is my BundledSiteStyles.xaml
<ResourceDictionary xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
x:Class="myTestApp.Styles.BundledSiteStyles">
<ResourceDictionary.MergedDictionaries>
<!-- Add global ResourceDictionaries Here -->
<ResourceDictionary Source="DefaultButton.xaml" />
<ResourceDictionary Source="LinkButton.xaml" />
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
If there is a more efficient/correct way of doing this, please let me know, but for now it seems to work.

Answer 2

Assuming all the themes have the same keys, you can replace it by iterating through its keys, and replacing those entries in the merged dictionary.
I haven't tested, but should be something like this:
var resources = Application.Current.Resources;
foreach (var key in themes.Keys)
resources[key] = themes[key];