Decouple public, internal act implementation

In the next major release, we intend to drop support for using the `act`
testing helper in production. (It already fires a warning.) The
rationale is that, in order for `act` to work, you must either mock the
testing environment or add extra logic at runtime. Mocking the testing
environment isn't ideal because it requires extra set up for the user.
Extra logic at runtime is fine only in development mode — we don't want
to slow down the production builds.

Since most people only run their tests in development mode, dropping
support for production should be fine; if there's demand, we can add it
back later using a special testing build that is identical to the
production build except for the additional testing logic.

One blocker for removing production support is that we currently use
`act` to test React itself. We must test React in both development and
production modes.

So, the solution is to fork `act` into separate public and
internal implementations:

- *public implementation of `act`* – exposed to users, only works in
  development mode, uses special runtime logic, does not support partial
  rendering
- *internal implementation of `act`* – private, works in both
  development and productionm modes, only used by the React Core test
  suite, uses no special runtime logic, supports partial rendering (i.e.
  `toFlushAndYieldThrough`)

The internal implementation should mostly match the public
implementation's behavior, but since it's a private API, it doesn't have
to match exactly. It works by mocking the test environment: it uses a
mock build of Scheduler to flush rendering tasks, and Jest's mock timers
to flush Suspense placeholders.

packages/react-debug-tools/src/__tests__/ReactHooksInspectionIntegration-test.js

@@ -22,7 +22,7 @@ describe('ReactHooksInspectionIntegration', () => {
     React = require('react');
     ReactTestRenderer = require('react-test-renderer');
     Scheduler = require('scheduler');
-    act = ReactTestRenderer.act;
+    act = ReactTestRenderer.unstable_concurrentAct;
     ReactDebugTools = require('react-debug-tools');
   });
 

packages/react-devtools-shared/src/__tests__/inspectedElementContext-test.js

@@ -33,6 +33,9 @@ describe('InspectedElementContext', () => {
   let TestUtils;
   let TreeContextController;
 
+  let TestUtilsAct;
+  let TestRendererAct;
+
   beforeEach(() => {
     utils = require('./utils');
     utils.beforeEachProfiling();
@@ -47,7 +50,9 @@ describe('InspectedElementContext', () => {
     ReactDOM = require('react-dom');
     PropTypes = require('prop-types');
     TestUtils = require('react-dom/test-utils');
+    TestUtilsAct = TestUtils.unstable_concurrentAct;
     TestRenderer = utils.requireTestRenderer();
+    TestRendererAct = TestUtils.unstable_concurrentAct;
 
     BridgeContext = require('react-devtools-shared/src/devtools/views/context')
       .BridgeContext;
@@ -999,8 +1004,8 @@ describe('InspectedElementContext', () => {
     expect(inspectedElement).toMatchSnapshot('1: Initially inspect element');
 
     inspectedElement = null;
-    TestUtils.act(() => {
-      TestRenderer.act(() => {
+    TestUtilsAct(() => {
+      TestRendererAct(() => {
         getInspectedElementPath(id, ['props', 'nestedObject', 'a']);
         jest.runOnlyPendingTimers();
       });
@@ -1009,8 +1014,8 @@ describe('InspectedElementContext', () => {
     expect(inspectedElement).toMatchSnapshot('2: Inspect props.nestedObject.a');
 
     inspectedElement = null;
-    TestUtils.act(() => {
-      TestRenderer.act(() => {
+    TestUtilsAct(() => {
+      TestRendererAct(() => {
         getInspectedElementPath(id, ['props', 'nestedObject', 'a', 'b', 'c']);
         jest.runOnlyPendingTimers();
       });
@@ -1021,8 +1026,8 @@ describe('InspectedElementContext', () => {
     );
 
     inspectedElement = null;
-    TestUtils.act(() => {
-      TestRenderer.act(() => {
+    TestUtilsAct(() => {
+      TestRendererAct(() => {
         getInspectedElementPath(id, [
           'props',
           'nestedObject',
@@ -1041,8 +1046,8 @@ describe('InspectedElementContext', () => {
     );
 
     inspectedElement = null;
-    TestUtils.act(() => {
-      TestRenderer.act(() => {
+    TestUtilsAct(() => {
+      TestRendererAct(() => {
         getInspectedElementPath(id, ['hooks', 0, 'value']);
         jest.runOnlyPendingTimers();
       });
@@ -1051,8 +1056,8 @@ describe('InspectedElementContext', () => {
     expect(inspectedElement).toMatchSnapshot('5: Inspect hooks.0.value');
 
     inspectedElement = null;
-    TestUtils.act(() => {
-      TestRenderer.act(() => {
+    TestUtilsAct(() => {
+      TestRendererAct(() => {
         getInspectedElementPath(id, ['hooks', 0, 'value', 'foo', 'bar']);
         jest.runOnlyPendingTimers();
       });
@@ -1108,8 +1113,8 @@ describe('InspectedElementContext', () => {
     expect(inspectedElement).toMatchSnapshot('1: Initially inspect element');
 
     inspectedElement = null;
-    TestUtils.act(() => {
-      TestRenderer.act(() => {
+    TestUtilsAct(() => {
+      TestRendererAct(() => {
         getInspectedElementPath(id, ['props', 'set_of_sets', 0]);
         jest.runOnlyPendingTimers();
       });
@@ -1179,23 +1184,23 @@ describe('InspectedElementContext', () => {
     expect(inspectedElement).toMatchSnapshot('1: Initially inspect element');
 
     inspectedElement = null;
-    TestRenderer.act(() => {
+    TestRendererAct(() => {
       getInspectedElementPath(id, ['props', 'nestedObject', 'a']);
       jest.runOnlyPendingTimers();
     });
     expect(inspectedElement).not.toBeNull();
     expect(inspectedElement).toMatchSnapshot('2: Inspect props.nestedObject.a');
 
     inspectedElement = null;
-    TestRenderer.act(() => {
+    TestRendererAct(() => {
       getInspectedElementPath(id, ['props', 'nestedObject', 'c']);
       jest.runOnlyPendingTimers();
     });
     expect(inspectedElement).not.toBeNull();
     expect(inspectedElement).toMatchSnapshot('3: Inspect props.nestedObject.c');
 
-    TestRenderer.act(() => {
-      TestUtils.act(() => {
+    TestRendererAct(() => {
+      TestUtilsAct(() => {
         ReactDOM.render(
           <Example
             nestedObject={{
@@ -1221,7 +1226,7 @@ describe('InspectedElementContext', () => {
       });
     });
 
-    TestRenderer.act(() => {
+    TestRendererAct(() => {
       inspectedElement = null;
       jest.advanceTimersByTime(1000);
     });
@@ -1281,7 +1286,7 @@ describe('InspectedElementContext', () => {
     expect(inspectedElement).not.toBeNull();
     expect(inspectedElement).toMatchSnapshot('1: Initially inspect element');
 
-    TestUtils.act(() => {
+    TestUtilsAct(() => {
       ReactDOM.render(
         <Example
           nestedObject={{
@@ -1300,8 +1305,8 @@ describe('InspectedElementContext', () => {
 
     inspectedElement = null;
 
-    TestRenderer.act(() => {
-      TestUtils.act(() => {
+    TestRendererAct(() => {
+      TestUtilsAct(() => {
         getInspectedElementPath(id, ['props', 'nestedObject', 'a']);
         jest.runOnlyPendingTimers();
       });

packages/react-devtools-shared/src/__tests__/storeComponentFilters-test.js

@@ -20,7 +20,7 @@ describe('Store component filters', () => {
   let utils;
 
   const act = (callback: Function) => {
-    TestUtils.act(() => {
+    TestUtils.unstable_concurrentAct(() => {
       callback();
     });
     jest.runAllTimers(); // Flush Bridge operations

packages/react-dom/src/__tests__/ReactDOMFiberAsync-test.js

@@ -28,7 +28,7 @@ describe('ReactDOMFiberAsync', () => {
     container = document.createElement('div');
     React = require('react');
     ReactDOM = require('react-dom');
-    act = require('react-dom/test-utils').act;
+    act = require('react-dom/test-utils').unstable_concurrentAct;
     Scheduler = require('scheduler');
 
     document.body.appendChild(container);

packages/react-dom/src/__tests__/ReactDOMServerIntegrationHooks-test.js

@@ -17,6 +17,7 @@ let React;
 let ReactDOM;
 let ReactDOMServer;
 let ReactTestUtils;
+let act;
 let Scheduler;
 let useState;
 let useReducer;
@@ -43,6 +44,7 @@ function initModules() {
   ReactDOMServer = require('react-dom/server');
   ReactTestUtils = require('react-dom/test-utils');
   Scheduler = require('scheduler');
+  act = ReactTestUtils.unstable_concurrentAct;
   useState = React.useState;
   useReducer = React.useReducer;
   useEffect = React.useEffect;
@@ -1063,7 +1065,7 @@ describe('ReactDOMServerHooks', () => {
       expect(domNode.children.length).toEqual(1);
       expect(oldClientId).not.toBeNull();
 
-      await ReactTestUtils.act(async () => _setShowId(true));
+      await act(async () => _setShowId(true));
 
       expect(domNode.children.length).toEqual(2);
       expect(domNode.children[0].getAttribute('aria-labelledby')).toEqual(
@@ -1281,7 +1283,7 @@ describe('ReactDOMServerHooks', () => {
       const oldServerId = container.children[0].children[0].getAttribute('id');
       expect(oldServerId).not.toBeNull();
 
-      await ReactTestUtils.act(async () => {
+      await act(async () => {
         _setShowDiv(true);
       });
       expect(container.children[0].children.length).toEqual(2);
@@ -1322,7 +1324,7 @@ describe('ReactDOMServerHooks', () => {
       const oldServerId = container.children[0].children[0].getAttribute('id');
       expect(oldServerId).not.toBeNull();
 
-      await ReactTestUtils.act(async () => {
+      await act(async () => {
         _setShowDiv(true);
       });
       expect(container.children[0].children.length).toEqual(2);
@@ -1356,12 +1358,12 @@ describe('ReactDOMServerHooks', () => {
       document.body.append(container);
       container.innerHTML = ReactDOMServer.renderToString(<App />);
       const root = ReactDOM.unstable_createRoot(container, {hydrate: true});
-      ReactTestUtils.act(() => {
+      act(() => {
         root.render(<App />);
       });
       expect(Scheduler).toHaveYielded(['App', 'App']);
       // The ID goes from not being used to being added to the page
-      ReactTestUtils.act(() => {
+      act(() => {
         _setShow(true);
       });
       expect(Scheduler).toHaveYielded(['App', 'App']);
@@ -1391,7 +1393,7 @@ describe('ReactDOMServerHooks', () => {
       ReactDOM.hydrate(<App />, container);
       expect(Scheduler).toHaveYielded(['App', 'App']);
       // The ID goes from not being used to being added to the page
-      ReactTestUtils.act(() => {
+      act(() => {
         _setShow(true);
       });
       expect(Scheduler).toHaveYielded(['App']);
@@ -1418,12 +1420,12 @@ describe('ReactDOMServerHooks', () => {
       document.body.append(container);
       container.innerHTML = ReactDOMServer.renderToString(<App />);
       const root = ReactDOM.unstable_createRoot(container, {hydrate: true});
-      ReactTestUtils.act(() => {
+      act(() => {
         root.render(<App />);
       });
 
       // The ID goes from not being used to being added to the page
-      ReactTestUtils.act(() => {
+      act(() => {
         ReactDOM.flushSync(() => {
           _setShow(true);
         });
@@ -1518,7 +1520,7 @@ describe('ReactDOMServerHooks', () => {
       expect(child1Ref.current).toBe(null);
       expect(Scheduler).toHaveYielded([]);
 
-      ReactTestUtils.act(() => {
+      act(() => {
         _setShow(true);
 
         // State update should trigger the ID to update, which changes the props
@@ -1603,7 +1605,7 @@ describe('ReactDOMServerHooks', () => {
 
       suspend = true;
       const root = ReactDOM.unstable_createRoot(container, {hydrate: true});
-      await ReactTestUtils.act(async () => {
+      await act(async () => {
         root.render(<App />);
       });
       jest.runAllTimers();
@@ -1616,7 +1618,7 @@ describe('ReactDOMServerHooks', () => {
         container.children[0].children[0].getAttribute('id'),
       ).not.toBeNull();
 
-      await ReactTestUtils.act(async () => {
+      await act(async () => {
         suspend = false;
         resolve();
         await promise;
@@ -1703,7 +1705,7 @@ describe('ReactDOMServerHooks', () => {
 
       suspend = false;
       const root = ReactDOM.unstable_createRoot(container, {hydrate: true});
-      await ReactTestUtils.act(async () => {
+      await act(async () => {
         root.render(<App />);
       });
       jest.runAllTimers();
@@ -1968,7 +1970,7 @@ describe('ReactDOMServerHooks', () => {
       expect(Scheduler).toHaveYielded([]);
       expect(Scheduler).toFlushAndYield([]);
 
-      ReactTestUtils.act(() => {
+      act(() => {
         _setShow(false);
       });
 

packages/react-dom/src/__tests__/ReactDOMServerPartialHydration-test.internal.js

@@ -79,7 +79,7 @@ describe('ReactDOMServerPartialHydration', () => {
 
     React = require('react');
     ReactDOM = require('react-dom');
-    act = require('react-dom/test-utils').act;
+    act = require('react-dom/test-utils').unstable_concurrentAct;
     ReactDOMServer = require('react-dom/server');
     Scheduler = require('scheduler');
     Suspense = React.Suspense;

packages/react-dom/src/__tests__/ReactDOMServerSelectiveHydration-test.internal.js

@@ -17,6 +17,7 @@ let ReactDOMServer;
 let ReactTestUtils;
 let Scheduler;
 let Suspense;
+let act;
 
 function dispatchMouseHoverEvent(to, from) {
   if (!to) {
@@ -101,6 +102,7 @@ describe('ReactDOMServerSelectiveHydration', () => {
     ReactDOM = require('react-dom');
     ReactDOMServer = require('react-dom/server');
     ReactTestUtils = require('react-dom/test-utils');
+    act = ReactTestUtils.unstable_concurrentAct;
     Scheduler = require('scheduler');
     Suspense = React.Suspense;
   });
@@ -880,7 +882,7 @@ describe('ReactDOMServerSelectiveHydration', () => {
     const spanC = container.getElementsByTagName('span')[4];
 
     const root = ReactDOM.createRoot(container, {hydrate: true});
-    ReactTestUtils.act(() => {
+    act(() => {
       root.render(<App a="A" />);
 
       // Hydrate the shell.

packages/react-dom/src/__tests__/ReactDOMServerSuspense-test.internal.js

@@ -15,6 +15,7 @@ let React;
 let ReactDOM;
 let ReactDOMServer;
 let ReactTestUtils;
+let act;
 
 function initModules() {
   // Reset warning cache.
@@ -24,6 +25,7 @@ function initModules() {
   ReactDOM = require('react-dom');
   ReactDOMServer = require('react-dom/server');
   ReactTestUtils = require('react-dom/test-utils');
+  act = ReactTestUtils.unstable_concurrentAct;
 
   // Make them available to the helpers.
   return {
@@ -124,7 +126,7 @@ describe('ReactDOMServerSuspense', () => {
     expect(divB.tagName).toBe('DIV');
     expect(divB.textContent).toBe('B');
 
-    ReactTestUtils.act(() => {
+    act(() => {
       const root = ReactDOM.createBlockingRoot(parent, {hydrate: true});
       root.render(example);
     });

packages/react-dom/src/__tests__/ReactDOMSuspensePlaceholder-test.js

@@ -28,7 +28,7 @@ describe('ReactDOMSuspensePlaceholder', () => {
     ReactCache = require('react-cache');
     ReactTestUtils = require('react-dom/test-utils');
     Scheduler = require('scheduler');
-    act = ReactTestUtils.act;
+    act = ReactTestUtils.unstable_concurrentAct;
     Suspense = React.Suspense;
     container = document.createElement('div');
     document.body.appendChild(container);

packages/react-dom/src/__tests__/ReactErrorBoundaries-test.internal.js

@@ -44,7 +44,7 @@ describe('ReactErrorBoundaries', () => {
     ReactFeatureFlags.replayFailedUnitOfWorkWithInvokeGuardedCallback = false;
     ReactDOM = require('react-dom');
     React = require('react');
-    act = require('react-dom/test-utils').act;
+    act = require('react-dom/test-utils').unstable_concurrentAct;
     Scheduler = require('scheduler');
 
     BrokenConstructor = class extends React.Component {