Skip to content

Testing

Microsoft.UI.Reactor (Reactor)'s render loop is deterministic and synchronous. A Component mounted in a unit test renders, runs effects, accepts state updates, re-renders, and disposes through the same code path the WinUI host uses — minus the WinUI tree at the bottom. That makes the unit layer fast (an xUnit run for the framework's 800+ tests is seconds), and it keeps test bodies focused on the component's behavior instead of the windowing chrome.

This page covers the four test layers Reactor uses internally:

class Counter : Component
{
    public override Element Render()
    {
        var (count, setCount) = UseState(0);
        return VStack(8,
            TextBlock($"Count: {count}").FontSize(20).Bold(),
            Button("Increment", () => setCount(count + 1))
        ).Padding(16);
    }
}

Counter component used as the running fixture target

Reference

Test layer Project When to reach for it
Unit tests/Reactor.Tests/ Hook semantics, reducer logic, modifier chains, analyzer rules. Fastest loop.
Self-test tests/Reactor.SelfTests/ + Reactor.AppTests.Host Component renders into a real WinUI tree; assertions via VisualTreeHelper.
App E2E tests/Reactor.AppTests/ WinAppDriver-driven full apps; click, type, screenshot diff.
Doc docs/_pipeline/apps/ Compile + snapshot capture for the docs pipeline.

Unit-level fixtures

Reactor's component lifecycle (BeginRender → Render → FlushEffects → RunCleanups) is exposed through the framework's internal API and used directly by ComponentModelIntegrationTests in tests/Reactor.Tests/. The pattern is small enough to fit in a helper inside your test class:

private readonly ContextScope _scope = new();   // internal

private Element Mount(Component component,
    Dictionary<ContextBase, object?>? ctx = null)
{
    if (ctx is { Count: > 0 }) _scope.Push(ctx);
    component.Context.BeginRender(onReRender: () => { }, _scope);
    var el = component.Render();
    component.Context.FlushEffects();
    return el;
}

private Element Rerender(Component c) => Mount(c);
private void Unmount(Component c) => c.Context.RunCleanups();

Mount returns the root element. The test then drives state via the component's own public surface (a property the component exposes, or a captured setter from UseState), calls Rerender, and asserts. Unmount runs the cleanup chain so effects with dispose lambdas release their resources before the next test starts.

The full pattern is in ComponentModelIntegrationTests.cs — that file mounts a component with state + context + effects, drives 5 distinct lifecycle transitions, and asserts the effect log after each. Use it as the template when adding a unit fixture for a new hook.

Effect-aware async tests

UseEffect does not fire during render. It fires when the component's context flushes effects — which the unit Mount helper above does inline. Tests that exercise effect ordering must observe the log between mount and the next render, not during render:

// Effect-aware component used as a fixture target. UseEffect fires on the
// next flush, not during render — tests must wait for the flush before
// observing the side effect's log entry (see testing.md, "Async patterns").
class EffectfulCounter : Component
{
    public List<string> Log { get; } = new();

    public override Element Render()
    {
        var (count, setCount) = UseState(0);
        UseEffect(() =>
        {
            Log.Add($"effect:{count}");
            return () => Log.Add($"cleanup:{count}");
        }, count);
        return Button($"count={count}", () => setCount(count + 1));
    }
}

A test for EffectfulCounter mounts the component, asserts Log = ["effect:0"], increments state, re-renders, and asserts Log = ["effect:0", "cleanup:0", "effect:5"]. The cleanup from the previous effect runs before the new effect's body — that's the contract tests/Reactor.Tests/ComponentModelIntegrationTests.cs codifies.

For genuinely async effects (timer fires, HTTP callback), use UseAsync with a CancellationToken so the test can deterministically flush by awaiting the completion of a task the component exposes — or inject a fake clock via UseContext of a clock interface and tick it forward by hand. Avoid Thread.Sleep in tests; it leaks wall-clock time into the suite and makes CI flaky.

Snapshot tests

A snapshot test takes the rendered element tree, serializes it to a deterministic string, and compares against a checked-in golden file. Reactor's Element records implement ToString() for compact descriptions, and the reconciler's ElementDescription helper produces a stable JSON shape. Place goldens next to the test file as *.snapshot.txt and diff with Assert.Equal:

[Fact]
public void Card_renders_three_slots()
{
    var card = new ProfileCard("Ada", "Lovelace", 36);
    var rendered = Mount(card);
    var actual = ElementDescription.Of(rendered);
    var expected = File.ReadAllText("ProfileCard.snapshot.txt");
    Assert.Equal(expected.Trim(), actual.Trim());
}

Update goldens when an intended UI change makes them stale — the diff of the snapshot file is the review surface. Don't snapshot anything that includes a timestamp, a randomly generated id, or a hash; either normalize those fields before comparison or factor them out of the component under test.

Accessibility scanner integration

AccessibilityScanner.Scan(root) walks the rendered tree and returns a List<A11yDiagnostic> with the WCAG criterion, fix suggestion, and source context for each finding. In a test, mount the component and assert the scan returns empty:

// AccessibilityScanner fixture target. The scanner walks the element tree
// post-render and returns one A11yDiagnostic per finding; an icon-only
// button without an accessible name is the canonical positive case.
class IconOnlyButton : Component
{
    public override Element Render() =>
        Button("", () => { });   // no accessible name → diagnostic
}

[Fact]
public void IconOnlyButton_flags_missing_accessible_name()
{
    var rendered = Mount(new IconOnlyButton());
    var diagnostics = AccessibilityScanner.Scan(rendered);

    Assert.Contains(diagnostics, d => d.Rule == "ButtonName");
}

[Fact]
public void NamedButton_passes_a11y_scan()
{
    var rendered = Mount(new NamedButton());
    Assert.Empty(AccessibilityScanner.Scan(rendered));
}

The same scanner backs the in-app dev menu's "Run accessibility scan" button, so a fixture that passes here is the same shape that passes in the running app. Treat scanner-clean as the standing bar for every new component you ship.

Self-tests (real WinUI tree)

Reactor.SelfTests is the layer between the unit suite (pure C#, no WinUI) and the full E2E suite (WinAppDriver). A self-test mounts a real fixture into the Reactor.AppTests.Host window, walks the WinUI visual tree, and emits TAP. The xUnit wrapper in SelfTestBatch.cs parses the TAP and surfaces one test method per fixture.

To add a self-test:

  1. Add a new fixture file under tests/Reactor.AppTests.Host/Fixtures/ returning the component under test wrapped in a small assertion harness.
  2. Register the fixture name in FixtureRegistry.
  3. The xUnit wrapper picks it up at discovery time via --list-fixtures; no code change needed on the test runner side.

Reach for a self-test when the unit layer can't see the answer — e.g. when the WinUI control's measured size affects the component's behavior, or when an automation peer's role depends on the materialized XAML control class.

Tips

Don't drive the unit fixture from Task.Delay. If an effect schedules async work, expose its completion task so the test can await it. Wall-clock delays leak into the suite and make CI flaky.

Snapshot the tree, not the rendered pixels. ElementDescription.Of gives you a deterministic string; an actual rendered bitmap depends on font rendering, DPI, and platform Composition — none of which belong in a unit test.

Run the accessibility scan in every fixture's teardown. It's cheap, it catches regressions the moment they land, and it puts the analyzer's output next to the test that introduced the problem.

Use Reactor.AppTests only where xUnit and self-tests can't reach. WinAppDriver is the slow lane; reserve it for keyboard navigation, focus order, and click sequences that depend on Composition or input routing.

Next Steps

  • Hooks — Previous in the learning path: the primitives a fixture exercises.
  • Effects — UseEffect lifecycle and cleanup, including the flush ordering tested above.
  • Accessibility — The scanner's rules and how to extend it with project-specific checks.
  • Dev Tooling — mur CLI, preview mode, and the doc-pipeline harness that powers the screenshots on this page.
  • Components — Render purity rules that make the unit layer worth the investment.