av-parser/tests/AvParser.UI.HeadlessTests/StyleTests.cs

using Avalonia;
using Avalonia.Controls;
using Avalonia.Controls.Presenters;
using Avalonia.Controls.Primitives;
using Avalonia.Headless.XUnit;
using Avalonia.Media;
using Avalonia.Styling;
using Avalonia.Threading;
using Avalonia.VisualTree;

namespace AvParser.UI.HeadlessTests;

public class StyleTests
{
    private static T Show<T>(T control, ThemeVariant variant)
        where T : Control
    {
        Application.Current!.RequestedThemeVariant = variant;

        var window = new Window
        {
            Width = 400,
            Height = 200,
            Content = control,
        };
        window.Show();
        Dispatcher.UIThread.RunJobs();

        return control;
    }

    private static Color TokenColor(string key, ThemeVariant variant)
    {
        Application.Current!.TryFindResource(key, variant, out var value).ShouldBeTrue();
        return value.ShouldBeOfType<SolidColorBrush>().Color;
    }

    private static Color? RenderedBackground(Control control) =>
        (
            control.GetVisualDescendants().OfType<ContentPresenter>().FirstOrDefault()?.Background
            ?? control.GetValue(TemplatedControl.BackgroundProperty)
        )
            is SolidColorBrush brush
            ? brush.Color
            : null;

    [AvaloniaTheory]
    [InlineData("Light")]
    [InlineData("Dark")]
    public void The_primary_button_is_filled_with_the_accent_colour(string variantName)
    {
        var variant = variantName == "Light" ? ThemeVariant.Light : ThemeVariant.Dark;
        var button = Show(new Button { Classes = { "primary" }, Content = "Go" }, variant);

        // If this fails the button paints itself white-on-white and simply disappears.
        RenderedBackground(button).ShouldBe(TokenColor("AppAccentBrush", variant));
    }

    [AvaloniaTheory]
    [InlineData("Light")]
    [InlineData("Dark")]
    public void A_card_is_distinguishable_from_the_page_behind_it(string variantName)
    {
        var variant = variantName == "Light" ? ThemeVariant.Light : ThemeVariant.Dark;

        var page = TokenColor("AppSurfaceBrush", variant);
        var card = TokenColor("AppSurfaceRaisedBrush", variant);
        var border = TokenColor("AppBorderBrush", variant);

        // Summed across three channels, 24 is roughly a 3% per-channel step — the point at which
        // a card stops being a rectangle you have to squint for. The first light palette here
        // scored 33 and was still visually indistinguishable, so the bar is deliberately high.
        Distance(page, card).ShouldBeGreaterThan(30);
        Distance(page, border).ShouldBeGreaterThan(24);
    }

    [AvaloniaTheory]
    [InlineData("Light")]
    [InlineData("Dark")]
    public void The_card_style_reaches_a_bare_border(string variantName)
    {
        var variant = variantName == "Light" ? ThemeVariant.Light : ThemeVariant.Dark;
        var card = Show(new Border { Classes = { "card" } }, variant);

        card.BorderThickness.ShouldBe(new Thickness(1));
        card.Padding.ShouldBe(new Thickness(16));

        // The value a DynamicResource actually resolved to for this element's variant — not what
        // TryFindResource can dig out when asked for a variant explicitly.
        (card.Background as SolidColorBrush)?.Color.ShouldBe(TokenColor("AppSurfaceRaisedBrush", variant));
    }

    private static int Distance(Color a, Color b) => Math.Abs(a.R - b.R) + Math.Abs(a.G - b.G) + Math.Abs(a.B - b.B);
}