view-locator.md

description
CONCEPTS

The View Locator

While the use of the View Locator comes as part of the default templates, it's important to note that it's not a mandatory requirement. It's an optional tool provided to help you structure your Avalonia application using the Model-View-ViewModel (MVVM) design pattern.

The View Locator is a mechanism in Avalonia that is used to resolve the view (user interface) that corresponds to a specific ViewModel. This is a core part of the MVVM (Model-View-ViewModel) pattern, which is a design pattern that separates the development of the graphical user interface from the development of the business logic or back-end logic.

How it works

The View Locator uses naming conventions to map ViewModel types to view types. By default, it replaces every occurrence of the string "ViewModel" within the fully-qualified ViewModel type name with "View".

For example, given a ViewModel named MyApplication.ViewModels.ExampleViewModel, the View Locator will look for a View named MyApplication.Views.ExampleView.

The View Locator is typically used in conjunction with the DataContext property, which is used to link a view to its ViewModel.

Here's a simple usage example:

public class ViewLocator : IDataTemplate
{
    public bool SupportsRecycling => false;

    public Control Build(object data)
    {
        var name = data.GetType().FullName.Replace("ViewModel", "View");
        var type = Type.GetType(name);

        if (type != null)
        {
            return (Control)Activator.CreateInstance(type);
        }
        else
        {
            return new TextBlock { Text = "Not Found: " + name };
        }
    }

    public bool Match(object data)
    {
        return data is ViewModelBase;
    }
}

In this example, the View Locator is implemented as an IDataTemplate. The Build method creates the view for the ViewModel, and the Match method checks if the data object is a ViewModel that this locator knows how to handle. If you do not have a ViewModelBase class, at a minimum your ViewModel must implement INotifyPropertyChanged, and the comparison in Match should be changed accordingly.

Customizing the View Locator

You can customize the View Locator to use different conventions. For example, you might want to look for views in a different assembly, or use a different naming convention. To do this, you can implement your own View Locator by creating a class that implements the IDataTemplate interface, and replace the default View Locator with your own.

Using the View Locator

By default, the View Locator is referenced in App.axaml as a DataTemplate, in the content of the Application.DataTemplates XAML tag. Ensure that its appropriate 'using' statement is in the xmlns:local property of the Application root tag.

<Application xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             x:Class="LearningAvalonia.App"
             xmlns:local="using:LearningAvalonia"
             RequestedThemeVariant="Default">
             <!-- "Default" ThemeVariant follows system theme variant. "Dark" or "Light" are other available options. -->
    <Application.DataTemplates>
        <local:ViewLocator />
    </Application.DataTemplates>

    <Application.Styles>
        <FluentTheme />
    </Application.Styles>
</Application>