Exploring Headless Testing for XPF and WPF Applications

Introduction:

Developers have long struggled with the challenges and time constraints of testing applications with complex user interfaces, especially those built on Windows Presentation Foundation (WPF). Headless testing offers a new approach that addresses many of these challenges. This blog post explores how headless testing can improve UI testing for Avalonia XPF (a cross-platform extension of WPF), making it faster, more portable, and more efficient.

While we focus on Avalonia XPF here, it's worth noting that headless testing is a core feature of Avalonia itself, with approximately 90% of Avalonia's tests being headless. We've also applied this technique to Avalonia XPF's own testing process. For instance, we use headless testing to render various geometries into bitmaps and compare them with expected results, ensuring accurate visual output across platforms.

This approach not only streamlines the testing process but also opens up new possibilities for cross-platform development and testing. Whether you're working on a WPF application, developing with Avalonia XPF, or creating reusable UI controls, headless testing can significantly enhance your development workflow.

Benefits of Headless Testing

Headless testing provides several advantages for developers working with Avalonia and XPF applications:

Platform-Independent Testing

Headless testing doesn't rely on platform-specific input or windowing systems to test your UI. This means you can run tests without the need for complex packaging (like on macOS), certificates, or even a display. You can execute all your headless tests on a virtual machine without a graphical interface, which solves a major pain point often encountered with e2e automation tests.

Flexible Test Execution

While WPF itself is Windows-only, headless testing via XPF allows you to run your UI tests on any platform that supports .NET. This means you can leverage cheap Linux virtual machines for your test runs, significantly reducing infrastructure costs and increasing testing flexibility.

Optional UI Rendering

Headless testing allows for optional rendering of the UI. This means you can choose to render the UI when needed (for example, to capture screenshots or check layout), but you're not required to do so for every test. This flexibility can lead to faster test execution in many scenarios.

Simpler Setup

Headless testing eliminates the need for complex automation frameworks or separate testing environments. Tests run in the same process as the application, making it easier to interact with UI elements, send events, and access the application's output directly.

Enhanced Reliability

By removing the dependency on platform-specific display systems, headless tests are less affected by issues related to screen resolution, DPI settings, or other display variables that can cause traditional UI tests to fail unpredictably.

Earlier Issue Detection

The speed and simplicity of headless testing encourage more frequent testing, leading to earlier detection of UI-related issues. This can significantly reduce the cost and effort of fixing bugs later in the development cycle.

Improved Portability for UI Controls

Headless testing via XPF isn't limited to full XPF applications. It can also be used with normal WPF applications or class libraries (with an XPF license). This is particularly valuable for control library developers who want to ensure their components work correctly across various scenarios without the need for a full application context.

In the following sections, we'll explore how to implement headless testing in your XPF and discuss some best practices to get the most out of this approach.

How to Implement Headless Testing for XPF/WPF Applications:

Now that we understand the benefits, let's dive into how you can implement headless testing for your XPF applications:

Setting Up Your Testing Project:

First, you'll need to configure your testing project to support headless testing. Currently, XPF/Avalonia headless testing supports XUnit and NUnit. Here's how to set it up: a) Add the necessary NuGet package to your testing project:

<ItemGroup>
    <PackageReference Include="Avalonia.Headless.XUnit" Version="$(XpfAvaloniaVersion)" />
    <!-- or -->
    <PackageReference Include="Avalonia.Headless.NUnit" Version="$(XpfAvaloniaVersion)" />
</ItemGroup>

Note: $(XpfAvaloniaVersion) is a pre-defined constant in the Xpf.Sdk. If you're not using it, you can manually specify the latest version.

Add the AvaloniaUI.Xpf.LicenseKey to your project:

<ItemGroup>
    <RuntimeHostConfigurationOption Include="AvaloniaUI.Xpf.LicenseKey" Value="--Insert your key here--"/>
</ItemGroup>

Configuring the Testing Application:

While optional, configuring a custom AppBuilder can enhance your XPF testing experience. Here's an example of how to set it up:

[assembly: AvaloniaTestApplication(typeof(TestAppBuilder))]

public class TestAppBuilder
{
    public static AppBuilder BuildAvaloniaApp() => AppBuilder.Configure<DefaultXpfAvaloniaApplication>()
        .WithAvaloniaXpf()
        .UseSkia()
        .UseHeadless(new AvaloniaHeadlessPlatformOptions
        {
            UseHeadlessDrawing = false
        });
}

This configuration allows you to customize various aspects of the headless environment, such as enabling frame capturing for more advanced testing scenarios.

Writing Your First Headless Test:

Now that your environment is set up, let's write a simple headless test:

[AvaloniaTest]
public void Should_Be_Able_To_Raise_Event()
{
    var window = new MainWindow();
    window.Show();
    var button = window.ClickingButton;

    Assert.That(button.Content, Is.EqualTo("Click me"));

    button.RaiseEvent(new RoutedEventArgs(ButtonBase.ClickEvent, button));

    Assert.That(button.Content, Is.EqualTo("Click count: 1"));
}

In this test, we're creating a window, showing it (which in the headless environment doesn't actually render it), and then interacting with a button. We assert the initial state, raise a click event, and then verify that the button's content has updated correctly.

Leveraging Avalonia Headless Extensions:

While the above test uses WPF-style event raising, you can also take advantage of Avalonia's headless extensions for more natural input simulation:

var avWindow = XpfWpfAbstraction.GetAvaloniaWindowForWindow(xpfWindow);
avWindow.KeyTextInput("Hello");

This allows you to simulate keyboard input, clicks, and other user interactions in a more intuitive way.

Testing Across Platforms:

One of the most powerful aspects of headless testing with XPF is the ability to run your tests on non-Windows platforms.

To enable this:

• Set your testing project's TargetFramework to net8.0-windows
• Set EnableWindowsTargeting to true in your project file

This configuration allows you to run your WPF/XPF tests on Linux and macOS, ensuring your UI behaves consistently across different operating systems.

Best Practices and Tips:

Keep your tests focused

Test one aspect of your UI at a time for clearer, more maintainable tests. Use meaningful assertions: Don't just check that an action occurred; verify that it had the expected effect on your UI.

Leverage setup methods

Use NUnit's [SetUp] or XUnit's constructor to create common UI elements, reducing duplication in your tests.

Explore Avalonia's headless capabilities

Many of Avalonia's headless testing features can be used with XPF, providing powerful tools for UI interaction and verification. Consider headless testing early: Integrate headless tests into your development process early to catch UI issues before they become entrenched.

Advanced Headless Testing Features

Headless testing in Avalonia and Avalonia XPF offers several advanced features that enhance the testing process. One key capability is input simulation, allowing you to simulate keyboard and mouse input for comprehensive interaction testing without actual hardware input. This is complemented by idle state simulation, which lets you test how your application behaves during periods of inactivity.

For visual regression testing and ensuring UI consistency across platforms, you can force the rendering of a frame and capture it. This feature is crucial for maintaining the visual integrity of your application across different environments.

An exciting feature currently in development is timer and animation control. While presently only tested with Avalonia, this feature would allow you to control application timers and animations, enabling you to skip time and significantly speed up test execution. This is particularly useful for scenarios involving long animations or timers, where waiting for real-time completion or individually mocking each timer can be time-consuming.

Instead, you could fast-forward through these time-dependent processes, dramatically reducing the time required for testing complex UI interactions and time-dependent behaviors. It's worth noting that while the timer control feature is not yet available in XPF and will require additional development, its potential for improving test efficiency is significant. By leveraging these advanced features, developers can create more thorough and efficient test suites, covering a wider range of scenarios in less time.

Conclusion:

Headless testing in Avalonia and Avalonia XPF offers a unique capability not found in other XAML-based .NET technologies. This approach significantly improves UI testing for applications powered by Avalonia and Avalonia XPF by providing platform independence, faster execution, and increased reliability.

If you're looking to extend your WPF applications to macOS and Linux while adding a robust testing strategy, consider trying Avalonia XPF. You can sign up for a trial to explore its capabilities and see how it can enhance your cross-platform development process.

For a deeper understanding of how to implement headless testing in your XPF projects, refer to our documentation on Headless testing for XPF. This resource provides detailed guidance on setup, writing tests, and best practices specific to XPF.

By adopting headless testing with Avalonia XPF, you can leverage a cutting-edge approach to UI testing that's currently unique in the .NET XAML ecosystem. This can give you a significant advantage in developing and maintaining high-quality, cross-platform applications.