Flush effects after initial render

Flush any effects scheduled with `useEffect` using the `act` helper
on the initial render.

This means that in a test a developer can write:

```
const wrapper = mount(<ComponentThatUsesEffects/>)
```

And know that effects will have been run before they start interacting
with the result wrapper.

See enzymejs/enzyme#2034 for corresponding change
to the React adapter.

src/MountRenderer.ts

@@ -3,10 +3,9 @@ import { VNode, h } from 'preact';
 
 import { getNode as getNodeClassic } from './preact8-rst';
 import { getNode as getNodeV10 } from './preact10-rst';
-import { getDisplayName, isPreact10 } from './util';
+import { getDisplayName, isPreact10, withReplacedMethod } from './util';
 import { render } from './compat';
 import { getLastVNodeRenderedIntoContainer } from './preact10-internals';
-import { withReplacedMethod } from './util';
 
 type EventDetails = { [prop: string]: any };
 
@@ -18,6 +17,23 @@ export interface Options {
   container?: HTMLElement;
 }
 
+let actImpl: (cb: () => any) => void;
+if (isPreact10()) {
+  actImpl = require('preact/test-utils').act;
+}
+
+/**
+ * Invoke `callback` and then immediately flush any effects or pending renders
+ * which were scheduled during the callback.
+ */
+function act(callback: () => any) {
+  if (actImpl) {
+    actImpl(callback);
+  } else {
+    callback();
+  }
+}
+
 export default class MountRenderer implements EnzymeRenderer {
   private _container: HTMLElement;
   private _getNode: typeof getNodeClassic;
@@ -33,7 +49,9 @@ export default class MountRenderer implements EnzymeRenderer {
   }
 
   render(el: VNode, context?: any, callback?: () => any) {
-    render(el, this._container);
+    act(() => {
+      render(el, this._container);
+    });
     const rootNode = this._getNode(this._container);
 
     // Monkey-patch the component's `setState` to make it force an update after

test/MountRenderer_test.tsx

@@ -4,6 +4,7 @@ import { assert } from 'chai';
 import * as sinon from 'sinon';
 
 import MountRenderer from '../src/MountRenderer';
+import { isPreact10 } from '../src/util';
 
 describe('MountRenderer', () => {
   describe('#render', () => {
@@ -18,7 +19,7 @@ describe('MountRenderer', () => {
     it('renders the element into the provided container', () => {
       const container = document.createElement('div');
       const renderer = new MountRenderer({ container });
-      renderer.render(<button/>);
+      renderer.render(<button />);
       assert.ok(container.querySelector('button'));
     });
 
@@ -55,6 +56,49 @@ describe('MountRenderer', () => {
       const container = renderer.getNode()!.instance.base!;
       assert.equal(container.innerHTML, '1');
     });
+
+    if (isPreact10()) {
+      const { useEffect, useLayoutEffect } = require('preact/hooks');
+
+      it('executes effect hooks on initial render', () => {
+        let effectCount = 0;
+        let layoutEffectCount = 0;
+
+        function TestComponent() {
+          useLayoutEffect(() => {
+            ++layoutEffectCount;
+          });
+          useEffect(() => {
+            ++effectCount;
+          });
+          return null;
+        }
+
+        const renderer = new MountRenderer();
+        renderer.render(<TestComponent />);
+
+        assert.equal(layoutEffectCount, 1);
+        assert.equal(effectCount, 1);
+      });
+
+      it('executes hook cleanup on unmount', () => {
+        let effectRemoved = false;
+
+        function TestComponent() {
+          useEffect(() => {
+            return () => (effectRemoved = true);
+          });
+          return null;
+        }
+
+        const renderer = new MountRenderer();
+        renderer.render(<TestComponent />);
+
+        assert.equal(effectRemoved, false);
+        renderer.unmount();
+        assert.equal(effectRemoved, true);
+      });
+    }
   });
 
   describe('#unmount', () => {

test/init.ts

@@ -3,12 +3,14 @@ import minimist from 'minimist';
 
 // Setup DOM globals required by Preact rendering.
 function setupJSDOM() {
-  const dom = new JSDOM();
+  // Enable `requestAnimationFrame` which Preact 10 uses for scheduling hooks.
+  const dom = new JSDOM('', { pretendToBeVisual: true });
   const g = global as any;
   g.Event = dom.window.Event;
   g.Node = dom.window.Node;
   g.window = dom.window;
   g.document = dom.window.document;
+  g.requestAnimationFrame = dom.window.requestAnimationFrame;
 }
 setupJSDOM();