feat: add waitForElement (#20)

* feat: add waitForElement

* add docs

* update docs

Author: thymikee

Diff for: README.md

@@ -254,6 +254,32 @@ const { getByTestId } = render(
 fireEvent.scroll(getByTestId('scroll-view'), eventData);
 ```
 
+## `waitForElement`
+
+Defined as:
+
+```jsx
+function waitForExpect<T: ReactTestInstance>(
+  expectation: () => T,
+  timeout: number = 4500,
+  interval: number = 50
+): Promise<T> {}
+```
+
+Wait for non-deterministic periods of time until your element appears or times out. `waitForExpect` periodically calls `expectation` every `interval` milliseconds to determine whether the element appeared or not.
+
+```jsx
+import { render, waitForElement } from 'react-testing-library';
+
+test('waiting for an Banana to be ready', async () => {
+  const { getByText } = render(<Banana />);
+
+  await waitForElement(() => getByText('Banana ready'));
+});
+```
+
+If you're using Jest's [Timer Mocks](https://jestjs.io/docs/en/timer-mocks#docsNav), remember not to use `async/await` syntax as it will stall your tests.
+
 ## `debug`
 
 Log prettified shallowly rendered component or test instance (just like snapshot) to stdout.

Diff for: src/__tests__/fireEvent.test.js

@@ -7,8 +7,7 @@ import {
   ScrollView,
   TextInput,
 } from '../__mocks__/reactNativeMock';
-import fireEvent from '../fireEvent';
-import { render } from '..';
+import { render, fireEvent } from '..';
 
 const OnPressComponent = ({ onPress }) => (
   <View>

Diff for: src/__tests__/waitForElement.test.js

@@ -0,0 +1,92 @@
+// @flow
+/* eslint-disable react/no-multi-comp */
+import React from 'react';
+import { View, Text, TouchableOpacity } from '../__mocks__/reactNativeMock';
+import { render, fireEvent, waitForElement } from '..';
+
+class Banana extends React.Component<*, *> {
+  changeFresh = () => {
+    this.props.onChangeFresh();
+  };
+
+  render() {
+    return (
+      <View>
+        {this.props.fresh && <Text testID="fresh">Fresh</Text>}
+        <TouchableOpacity onPress={this.changeFresh} type="primary">
+          Change freshness!
+        </TouchableOpacity>
+      </View>
+    );
+  }
+}
+
+class BananaContainer extends React.Component<*, *> {
+  state = { fresh: false };
+
+  onChangeFresh = async () => {
+    await new Promise(resolve => setTimeout(resolve, 300));
+    this.setState({ fresh: true });
+  };
+
+  render() {
+    return (
+      <Banana onChangeFresh={this.onChangeFresh} fresh={this.state.fresh} />
+    );
+  }
+}
+
+test('waits for element until it stops throwing', async () => {
+  const { getByTestId, getByName, queryByTestId } = render(<BananaContainer />);
+
+  fireEvent.press(getByName('TouchableOpacity'));
+
+  expect(queryByTestId('fresh')).toBeNull();
+
+  const freshBananaText = await waitForElement(() => getByTestId('fresh'));
+
+  expect(freshBananaText.props.children).toBe('Fresh');
+});
+
+test('waits for element until timeout is met', async () => {
+  const { getByTestId, getByName } = render(<BananaContainer />);
+
+  fireEvent.press(getByName('TouchableOpacity'));
+
+  await expect(
+    waitForElement(() => getByTestId('fresh'), 100)
+  ).rejects.toThrow();
+});
+
+test('waits for element with custom interval', async () => {
+  const mockFn = jest.fn(() => {
+    throw Error('test');
+  });
+
+  try {
+    await waitForElement(() => mockFn(), 400, 200);
+  } catch (e) {
+    // suppress
+  }
+
+  expect(mockFn).toBeCalledTimes(3);
+});
+
+test('works with fake timers', async () => {
+  jest.useFakeTimers();
+
+  const mockFn = jest.fn(() => {
+    throw Error('test');
+  });
+
+  try {
+    waitForElement(() => mockFn(), 400, 200);
+  } catch (e) {
+    // suppress
+  }
+  jest.runTimersToTime(400);
+
+  expect(mockFn).toBeCalledTimes(3);
+
+  jest.useRealTimers();
+});

Diff for: src/index.js

@@ -4,9 +4,11 @@ import shallow from './shallow';
 import flushMicrotasksQueue from './flushMicrotasksQueue';
 import debug from './debug';
 import fireEvent from './fireEvent';
+import waitForElement from './waitForElement';
 
 export { render };
 export { shallow };
 export { flushMicrotasksQueue };
 export { debug };
 export { fireEvent };
+export { waitForElement };

Diff for: src/waitForElement.js

@@ -0,0 +1,26 @@
+// @flow
+export default function waitForExpect<T: ReactTestInstance>(
+  expectation: () => T,
+  timeout: number = 4500,
+  interval: number = 50
+) {
+  const startTime = Date.now();
+  return new Promise<T>((resolve, reject) => {
+    const rejectOrRerun = error => {
+      if (Date.now() - startTime >= timeout) {
+        reject(error);
+        return;
+      }
+      setTimeout(runExpectation, interval);
+    };
+    function runExpectation() {
+      try {
+        const result = expectation();
+        resolve(result);
+      } catch (error) {
+        rejectOrRerun(error);
+      }
+    }
+    setTimeout(runExpectation, 0);
+  });
+}