refactor: flush microtask queue implementation (#1374)

* refactor: flush microtask queue implementation

* refactor: code review changes

* refactor: restore original flush function as legacy version

* chore: rename file

src/flush-micro-tasks.ts

@@ -0,0 +1,30 @@
+import { setImmediate } from './helpers/timers';
+
+export function flushMicroTasks() {
+  return new Promise((resolve) => setImmediate(resolve));
+}
+
+/**
+ * @deprecated To be removed in the next major release.
+ */
+type Thenable<T> = { then: (callback: () => T) => unknown };
+
+/**
+ * This legacy implementation of `flushMicroTasks` is used for compatibility with
+ * older versions of React Native (pre 0.71) which uses Promise polyfil.
+ *
+ * For users with older version of React Native there is a workaround of using our own
+ * Jest preset instead the `react-native` one, but requiring such change would be a
+ * breaking change for existing users.
+ *
+ * @deprecated To be removed in the next major release.
+ */
+export function flushMicroTasksLegacy(): Thenable<void> {
+  return {
+    // using "thenable" instead of a Promise, because otherwise it breaks when
+    // using "modern" fake timers
+    then(resolve) {
+      setImmediate(resolve);
+    },
+  };
+}

src/index.ts

@@ -1,5 +1,5 @@
 import { cleanup } from './pure';
-import { flushMicroTasks } from './flushMicroTasks';
+import { flushMicroTasksLegacy } from './flush-micro-tasks';
 import { getIsReactActEnvironment, setReactActEnvironment } from './act';
 
 if (typeof process === 'undefined' || !process.env?.RNTL_SKIP_AUTO_CLEANUP) {
@@ -11,7 +11,7 @@ if (typeof process === 'undefined' || !process.env?.RNTL_SKIP_AUTO_CLEANUP) {
   if (typeof afterEach === 'function') {
     // eslint-disable-next-line no-undef
     afterEach(async () => {
-      await flushMicroTasks();
+      await flushMicroTasksLegacy();
       cleanup();
     });
   }

src/waitFor.ts

@@ -1,12 +1,11 @@
 /* globals jest */
 import act, { setReactActEnvironment, getIsReactActEnvironment } from './act';
 import { getConfig } from './config';
-import { flushMicroTasks } from './flushMicroTasks';
+import { flushMicroTasks, flushMicroTasksLegacy } from './flush-micro-tasks';
 import { ErrorWithStack, copyStackTrace } from './helpers/errors';
 import {
   setTimeout,
   clearTimeout,
-  setImmediate,
   jestFakeTimersAreEnabled,
 } from './helpers/timers';
 import { checkReactVersionAtLeast } from './react-versions';
@@ -89,7 +88,7 @@ function waitForInternal<T>(
         // of parallelization so we're fine.
         // https://stackoverflow.com/a/59243586/971592
         // eslint-disable-next-line no-await-in-loop
-        await new Promise((resolve) => setImmediate(resolve));
+        await flushMicroTasks();
       }
     } else {
       overallTimeoutTimer = setTimeout(handleTimeout, timeout);
@@ -207,7 +206,7 @@ export default async function waitFor<T>(
     try {
       const result = await waitForInternal(expectation, optionsWithStackTrace);
       // Flush the microtask queue before restoring the `act` environment
-      await flushMicroTasks();
+      await flushMicroTasksLegacy();
       return result;
     } finally {
       setReactActEnvironment(previousActEnvironment);