Dynamic Resource

If you want to update a xaml resource at run time, this can be done using a {DynamicResouce} binding. take a look at our view

<?xml version="1.0" encoding="utf-8" ?>

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"

             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"

             xmlns:local="clr-namespace:pav00.DynamicResouceExample"

             x:Class="pav00.DynamicResouceExample.MainPage">

    <ContentPage.Resources>

        <ResourceDictionary>

            <Color x:Key="HighlightColor">Red</Color>

        </ResourceDictionary>

    </ContentPage.Resources>

    <StackLayout>

        <Label Text="Welcome to Xamarin.Forms!" 

               BackgroundColor="{DynamicResource HighlightColor}"

               HorizontalOptions="Center"

               VerticalOptions="CenterAndExpand" />

        <Button x:Name="Red_Button" Clicked="Color_Toggled" Text="Red"

                TextColor="{DynamicResource HighlightColor}" />

        <Button x:Name="Green_Button" Clicked="Color_Toggled" Text="Green" />

        <Button x:Name="Blue_Button" Clicked="Color_Toggled" Text="Blue"  />

    </StackLayout>

</ContentPage>

pretty straight forward, notice that we create a normal resource dictionary with one value for Color in it, we then do a {DynamicResource} binding instead of a {StaticResource} binding, this is what will allow us to then update the value of our HighlighteColor resource from code.

public partial class MainPage : ContentPage

{

    public MainPage() => InitializeComponent();

    private void Color_Toggled(object sender, ToggledEventArgs e)

    {

        var button = sender as Button;

        var buttons = new[] { Red_Button, Green_Button, Blue_Button };

        switch (button.Text.ToLower())

        {

            case "red":

                this.Resources["HighlightColor"] = new Color(255, 0, 0);

                break;

            case "green":

                this.Resources["HighlightColor"] = new Color(0, 255, 0);

                break;

            case "blue":

                this.Resources["HighlightColor"] = new Color(0, 0, 255);

                break;

        }

        foreach (var btn in buttons)

        {

            btn.TextColor = Color.Default;

            btn.RemoveDynamicResource(Button.TextColorProperty);

        }

        button.SetDynamicResource(Button.TextColorProperty, "HighlightColor");

    }

}

and that's all that there's to it,

• we can update the resource dictionary by simply referring to the item inside it by the key and assigning it a different value.
• we can remove a reference to the resource using the RemoveDynamicResouce attached property 
• we can set a resource using the SetDynamicResouce attached property.