Dynamically change App font size in style

Question

I have some styles defined in my App.xaml, such as:

<x:Double x:Key="defaultFontSize">14</x:Double>

<Style TargetType="Label">
    <Setter Property="FontSize" Value="{StaticResource defaultFontSize}" />
</Style>

<Style TargetType="Button">
    <Setter Property="FontSize" Value="{StaticResource defaultFontSize}" />
</Style>

<Style TargetType="Entry">
    <Setter Property="FontSize" Value="{StaticResource defaultFontSize}" />
</Style>

<Style TargetType="Editor">
    <Setter Property="FontSize" Value="{StaticResource defaultFontSize}" />
</Style>

<Style TargetType="Picker">
    <Setter Property="FontSize" Value="{StaticResource defaultFontSize}" />
</Style>

<Style TargetType="AnotherControl">
    <Setter Property="FontSize" Value="{StaticResource defaultFontSize}" />
</Style>

Now the users want to be able to increase/decrease the FontSize of every control throughout the app.
So I made a Preferences page where they can pick their own preferred FontSize and I save it in Xamarin.Essentials.Preferences.Set("FontSize", value).

The problem now is, I would like to remove this line from my style:

<x:Double x:Key="defaultFontSize">14</x:Double>

And replace it with some code behind equivalent that reads the value from the Preferences and dynamically sets the defaultFontSize style.

How can I achieve something like that?

Answer 1

Hi Stesvis-5434,

Welcome to our Microsoft Q&A platform!

You can use DynamicResource in App.xaml instead, then you can modify the font size dynamically. Here is the documentation you can refer to: Dynamic Styles in Xamarin.Forms.

The following is a simple demo using DynamicResource.

App.xaml

<x:Double x:Key="defaultFontSize">14</x:Double>  
<Style TargetType="Label">  
    <Setter Property="FontSize" Value="{DynamicResource defaultFontSize}" />  
</Style>  
<Style TargetType="Button">  
    <Setter Property="FontSize" Value="{DynamicResource defaultFontSize}" />  
</Style>  
<Style TargetType="Entry">  
    <Setter Property="FontSize" Value="{DynamicResource defaultFontSize}" />  
</Style>  

xaml.cs

protected override void OnAppearing()  
{  
    base.OnAppearing();  
    App.Current.Resources["defaultFontSize"] = Preferences.Get("FontSize", 14);  
}  
  
private void ButtonSave_Clicked(object sender, EventArgs e)  
{  
    int value = Convert.ToInt32(fontEnrty.Text);  
    Preferences.Set("FontSize", value);  
    App.Current.Resources["defaultFontSize"] = Preferences.Get("FontSize", 14);  
}  

Regards,
Kyle

---

If the response is helpful, please click "Accept Answer" and upvote it.

Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.