docs: expand testing doc (#60)

Author: vonovak

docs/testing.md

@@ -4,9 +4,9 @@ sidebar_position: 50
 
 # Testing
 
-## Setting up a mock
+## Setting up the mock
 
-If you use Jest for testing, you may need to mock the functionality of the native module - this is because the native code cannot run in Node environment.
+If you want to write JS-level tests that depend on Google Sign In, you need to mock the functionality of the native module - this is because the native code cannot run in Node environment.
 
 This library ships with a Jest mock that you can add to the `setupFiles` array in your Jest config.
 
@@ -20,6 +20,54 @@ By default, the mock behaves as if the calls were successful and returns mock us
 }
 ```
 
-[//]: # '### Writing tests'
-[//]: #
-[//]: # 'You can use [`@testing-library/react-native`](https://callstack.github.io/react-native-testing-library/) to write tests for React components that use React Native Google Sign In.'
+## Writing tests
+
+You can use [`@testing-library/react-native`](https://callstack.github.io/react-native-testing-library/) to write tests for React components that use React Native Google Sign In. Minimal example (make sure to [set up the mock](#setting-up-the-mock) first):
+
+```jsx title="App.test.js"
+import {
+  GoogleOneTapSignIn,
+  OneTapResponse,
+} from '@react-native-google-signin/google-signin';
+import * as React from 'react';
+import { render, screen, fireEvent } from '@testing-library/react-native';
+import { Button, Text } from 'react-native';
+import { useState } from 'react';
+
+function GoogleSignInComponent() {
+  const [userInfo, setUserInfo] = useState<OneTapResponse | undefined>();
+
+  return (
+    <>
+      <Button
+        title="Sign in with Google"
+        onPress={async () => {
+          GoogleOneTapSignIn.configure({
+            webClientId: 'autoDetect',
+          });
+          const userInfo = await GoogleOneTapSignIn.signIn();
+          setUserInfo(userInfo);
+        }}
+      />
+      {userInfo && <Text>{userInfo.data?.user.name}</Text>}
+    </>
+  );
+}
+
+it('GoogleSignInComponent should display user name after signing in', async () => {
+  // Render the component
+  render(<GoogleSignInComponent />);
+
+  const expectedUserName = 'mockFullName';
+  // Verify user name is not displayed initially
+  expect(screen.queryByText(expectedUserName)).toBeNull();
+
+  // Find and press the sign-in button
+  const signInButton = screen.getByText('Sign in with Google');
+  fireEvent.press(signInButton);
+
+  // verify the user name is displayed
+  const userName = await screen.findByText(expectedUserName);
+  expect(userName).toBeTruthy();
+});
+```