Use and update dynamic resources

In the previous unit, you defined a resource in Extensible Application Markup Language (XAML) and used it as a static value. However, there are some cases where StaticResource isn't appropriate. Consider the following scenarios:

• Suppose you want to implement color themes that let the user change the look of the app at runtime. The StaticResource mark-up extension 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. The StaticResource mark-up extension throws an exception if the key isn't found in the dictionary.

This unit shows you how to handle issues such as these by using dynamic resources.

How to update resources at runtime

You store resources in a resource dictionary. You can write code to update these resources at runtime. You can even add new resources or remove existing resources.

Consider the following example:

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

Suppose you want to change the value of the PanelBackgroundColor resource while the application is running. You can add code to the code-behind file for the page, to access the Resources property. The following example updates the resource value from the previous XAML example to a different color.

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

What is DynamicResource?

DynamicResource is another mark-up 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 an 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.

To use the updated background color from the earlier example, you can apply the DynamicResource in your XAML code like this:

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

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

If the value of the PanelBackgroundColor resource changes, the BackgroundColor value of the StackLayout control is updated automatically.