Use and update dynamic resources

  • 3 minutes

Note

.NET MAUI is the next evolution of Xamarin and what we recommend you develop mobile and desktop apps with, and you can learn more about .NET MAUI in several training modules. This Xamarin training module will not be maintained going forward.

In the previous unit, you defined a resource in XAML and used it as a static value. There are some cases where StaticResource isn't sufficient:

  • You want to implement color themes that let the user change the look of the app at runtime. StaticResource does the dictionary lookup only once, so it can't update the UI dynamically.

  • You store user preferences on a web server and load them when the application starts. StaticResource throws an exception if the key isn't found in the dictionary.

This unit shows you how to deal with both of these issues.

How to update resources at runtime

Your resources are stored in a dictionary. If you can access the dictionary, you can write code to update those values at runtime. You can even add new resources or remove existing resources.

Recall that your ResourceDictionary object is stored in the Resources property of your page. The following code shows a typical case of a dictionary defined in XAML.

<Page.Resources>
    <Color x:Key="PanelBackgroundColor">Blue</Color>
</Page.Resources>

Suppose you want to change the value of the PanelBackgroundColor resource while the application is running. You'd write code in your page's code-behind file to access the Resources property. The following example shows how you'd update the resource value to a different color.

this.Resources["PanelBackgroundColor"] = Color.Green;

What is DynamicResource?

DynamicResource is a markup extension for looking up resources in a resource dictionary. It's similar to StaticResource in that it does a dictionary lookup when the target object is created. But it also listens for changes to the resource in the dictionary. If the resource value in the dictionary changes, DynamicResource automatically updates the UI.

DynamicResource has another advantage over StaticResource. If DynamicResource can't find the key in the dictionary, it leaves the property unset. Unlike StaticResource, a missing key isn't an error and it doesn't throw an exception.

Let's say you want to use the updated background color from the earlier example. Your definition will look like this XAML:

<ContentPage ...>
    <Page.Resources>
        <Color x:Key="PanelBackgroundColor">Blue</Color>
    </Page.Resources>

    <StackLayout BackgroundColor="{DynamicResource PanelBackgroundColor}">
    ...
    </StackLayout>
</ContentPage>

If the value of the PanelBackgroundColor resource changes, the BackgroundColor value of StackLayout will be updated automatically.