# `dom-event-testing-library`

A library for unit testing events via high-level interactions, e.g., `pointerdown`,

that produce realistic and complete DOM event sequences.

There are number of challenges involved in unit testing modules that work with

DOM events.

1. Gesture recognizers may need to support environments with and without support for

   the `PointerEvent` API.

2. Gesture recognizers may need to support various user interaction modes including

   mouse, touch, and pen use.

3. Gesture recognizers must account for the actual event sequences browsers produce

   (e.g., emulated touch and mouse events.)

4. Gesture recognizers must work with "virtual" events produced by tools like

   screen-readers.

Writing unit tests to cover all these scenarios is tedious and error prone. This

event testing library is designed to solve these issues by allowing developers to

more easily dispatch events in unit tests, and to more reliably test pointer

interactions using a high-level API based on `PointerEvent`. Here's a basic example:

```js

import {

  describeWithPointerEvent,

  testWithPointerType,

  createEventTarget,

  setPointerEvent,

  resetActivePointers

} from 'dom-event-testing-library';

describeWithPointerEvent('useTap', hasPointerEvent => {

  beforeEach(() => {

    // basic PointerEvent mock

    setPointerEvent(hasPointerEvent);

  });

  afterEach(() => {

    // clear active pointers between test runs

    resetActivePointers();

  });

  // test all the pointer types supported by the environment

  testWithPointerType('pointer down', pointerType => {

    const ref = createRef(null);

    const onTapStart = jest.fn();

    render(() => {

      useTap(ref, { onTapStart });

      return <div ref={ref} />

    });

    // create an event target

    const target = createEventTarget(ref.current);

    // dispatch high-level pointer event

    target.pointerdown({ pointerType });

    expect(onTapStart).toBeCalled();

  });

});

```

This tests the interaction in multiple scenarios. In each case, a realistic DOM

event sequence–with complete mock events–is produced. When running in a mock

environment without the `PointerEvent` API, the test runs for both `mouse` and

`touch` pointer types. When `touch` is the pointer type it produces emulated mouse

events. When running in a mock environment with the `PointerEvent` API, the test

runs for `mouse`, `touch`, and `pen` pointer types.

It's important to cover all these scenarios because it's very easy to introduce

bugs – e.g., double calling of callbacks – if not accounting for emulated mouse

events, differences in target capturing between `touch` and `mouse` pointers, and

the different semantics of `button` across event APIs.

Default values are provided for the expected native events properties. They can

also be customized as needed in a test.

```js

target.pointerdown({

  button: 0,

  buttons: 1,

  pageX: 10,

  pageY: 10,

  pointerType,

  // NOTE: use x,y instead of clientX,clientY

  x: 10,

  y: 10

});

```

Tests that dispatch multiple pointer events will dispatch multi-touch native events

on the target.

```js

// first pointer is active

target.pointerdown({pointerId: 1, pointerType});

// second pointer is active

target.pointerdown({pointerId: 2, pointerType});

```