fix(jest): keep mocked hooks working after jest.resetAllMocks()

src/jest/__tests__/mock-test.tsx

@@ -0,0 +1,64 @@
+import { beforeEach, describe, expect, it, jest } from '@jest/globals';
+import { render, screen } from '@testing-library/react-native';
+import * as React from 'react';
+import { Text } from 'react-native';
+import type { Metrics } from '../../SafeArea.types';
+import mockSafeAreaContext from '../mock';
+
+// This is the setup consuming apps are told to use in the README.
+jest.mock('react-native-safe-area-context', () => mockSafeAreaContext);
+
+const { SafeAreaProvider, useSafeAreaFrame, useSafeAreaInsets } =
+  require('react-native-safe-area-context') as typeof import('../../index');
+
+const TEST_METRICS: Metrics = {
+  insets: { top: 1, left: 2, right: 3, bottom: 4 },
+  frame: { x: 0, y: 0, width: 100, height: 200 },
+};
+
+const PrintMetricsTestView = () => {
+  const insets = useSafeAreaInsets();
+  const frame = useSafeAreaFrame();
+  return <Text>{`${insets.top} ${frame.width}`}</Text>;
+};
+
+const renderTestView = () =>
+  render(
+    <SafeAreaProvider initialMetrics={TEST_METRICS}>
+      <PrintMetricsTestView />
+    </SafeAreaProvider>,
+  );
+
+describe('jest mock', () => {
+  beforeEach(() => {
+    // Consuming apps commonly do this in a global setup file, which used to
+    // strip the mock implementations and make the hooks return undefined.
+    jest.resetAllMocks();
+  });
+
+  it('keeps returning metrics after jest.resetAllMocks()', () => {
+    renderTestView();
+
+    expect(screen.getByText('1 100')).toBeDefined();
+  });
+
+  it('still lets tests override the hooks after jest.resetAllMocks()', () => {
+    jest.mocked(useSafeAreaInsets).mockReturnValue({
+      top: 42,
+      left: 0,
+      right: 0,
+      bottom: 0,
+    });
+
+    renderTestView();
+
+    expect(screen.getByText('42 100')).toBeDefined();
+  });
+
+  it('still records calls after jest.resetAllMocks()', () => {
+    renderTestView();
+
+    expect(jest.mocked(useSafeAreaInsets)).toHaveBeenCalledTimes(1);
+    expect(jest.mocked(useSafeAreaFrame)).toHaveBeenCalledTimes(1);
+  });
+});

src/jest/mock.tsx

@@ -27,16 +27,34 @@ const RNSafeAreaContext = jest.requireActual<{
   SafeAreaFrameContext: typeof SafeAreaFrameContext;
 }>('react-native-safe-area-context');
 
+// `jest.resetAllMocks()`, which apps commonly call from a global `beforeEach`,
+// strips the implementation off every mock function. That would make these
+// hooks return `undefined` and break every component reading insets or frame.
+// Reinstall the default implementation when it has been stripped, so the hooks
+// keep working while tests can still override them with `mockReturnValue` and
+// friends.
+function mockHook<T extends (...args: never[]) => unknown>(implementation: T) {
+  const hook = jest.fn(implementation);
+  return new Proxy(hook, {
+    apply(target, thisArg, args) {
+      if (target.getMockImplementation() == null) {
+        target.mockImplementation(implementation);
+      }
+      return Reflect.apply(target, thisArg, args);
+    },
+  });
+}
+
 export default {
   ...RNSafeAreaContext,
   initialWindowMetrics: MOCK_INITIAL_METRICS,
-  useSafeAreaInsets: jest.fn(() => {
+  useSafeAreaInsets: mockHook(() => {
     return (
       useContext(RNSafeAreaContext.SafeAreaInsetsContext) ??
       MOCK_INITIAL_METRICS.insets
     );
   }),
-  useSafeAreaFrame: jest.fn(() => {
+  useSafeAreaFrame: mockHook(() => {
     return (
       useContext(RNSafeAreaContext.SafeAreaFrameContext) ??
       MOCK_INITIAL_METRICS.frame