| 1 | # `dom-event-testing-library` |
| 2 | |
| 3 | A library for unit testing events via high-level interactions, e.g., `pointerdown`, |
| 4 | that produce realistic and complete DOM event sequences. |
| 5 | |
| 6 | There are number of challenges involved in unit testing modules that work with |
| 7 | DOM events. |
| 8 | |
| 9 | 1. Gesture recognizers may need to support environments with and without support for |
| 10 | the `PointerEvent` API. |
| 11 | 2. Gesture recognizers may need to support various user interaction modes including |
| 12 | mouse, touch, and pen use. |
| 13 | 3. Gesture recognizers must account for the actual event sequences browsers produce |
| 14 | (e.g., emulated touch and mouse events.) |
| 15 | 4. Gesture recognizers must work with "virtual" events produced by tools like |
| 16 | screen-readers. |
| 17 | |
| 18 | Writing unit tests to cover all these scenarios is tedious and error prone. This |
| 19 | event testing library is designed to solve these issues by allowing developers to |
| 20 | more easily dispatch events in unit tests, and to more reliably test pointer |
| 21 | interactions using a high-level API based on `PointerEvent`. Here's a basic example: |
| 22 | |
| 23 | ```js |
| 24 | import { |
| 25 | describeWithPointerEvent, |
| 26 | testWithPointerType, |
| 27 | createEventTarget, |
| 28 | setPointerEvent, |
| 29 | resetActivePointers |
| 30 | } from 'dom-event-testing-library'; |
| 31 | |
| 32 | describeWithPointerEvent('useTap', hasPointerEvent => { |
| 33 | beforeEach(() => { |
| 34 | // basic PointerEvent mock |
| 35 | setPointerEvent(hasPointerEvent); |
| 36 | }); |
| 37 | |
| 38 | afterEach(() => { |
| 39 | // clear active pointers between test runs |
| 40 | resetActivePointers(); |
| 41 | }); |
| 42 | |
| 43 | // test all the pointer types supported by the environment |
| 44 | testWithPointerType('pointer down', pointerType => { |
| 45 | const ref = createRef(null); |
| 46 | const onTapStart = jest.fn(); |
| 47 | render(() => { |
| 48 | useTap(ref, { onTapStart }); |
| 49 | return <div ref={ref} /> |
| 50 | }); |
| 51 | |
| 52 | // create an event target |
| 53 | const target = createEventTarget(ref.current); |
| 54 | // dispatch high-level pointer event |
| 55 | target.pointerdown({ pointerType }); |
| 56 | |
| 57 | expect(onTapStart).toBeCalled(); |
| 58 | }); |
| 59 | }); |
| 60 | ``` |
| 61 | |
| 62 | This tests the interaction in multiple scenarios. In each case, a realistic DOM |
| 63 | event sequence–with complete mock events–is produced. When running in a mock |
| 64 | environment without the `PointerEvent` API, the test runs for both `mouse` and |
| 65 | `touch` pointer types. When `touch` is the pointer type it produces emulated mouse |
| 66 | events. When running in a mock environment with the `PointerEvent` API, the test |
| 67 | runs for `mouse`, `touch`, and `pen` pointer types. |
| 68 | |
| 69 | It's important to cover all these scenarios because it's very easy to introduce |
| 70 | bugs – e.g., double calling of callbacks – if not accounting for emulated mouse |
| 71 | events, differences in target capturing between `touch` and `mouse` pointers, and |
| 72 | the different semantics of `button` across event APIs. |
| 73 | |
| 74 | Default values are provided for the expected native events properties. They can |
| 75 | also be customized as needed in a test. |
| 76 | |
| 77 | ```js |
| 78 | target.pointerdown({ |
| 79 | button: 0, |
| 80 | buttons: 1, |
| 81 | pageX: 10, |
| 82 | pageY: 10, |
| 83 | pointerType, |
| 84 | // NOTE: use x,y instead of clientX,clientY |
| 85 | x: 10, |
| 86 | y: 10 |
| 87 | }); |
| 88 | ``` |
| 89 | |
| 90 | Tests that dispatch multiple pointer events will dispatch multi-touch native events |
| 91 | on the target. |
| 92 | |
| 93 | ```js |
| 94 | // first pointer is active |
| 95 | target.pointerdown({pointerId: 1, pointerType}); |
| 96 | // second pointer is active |
| 97 | target.pointerdown({pointerId: 2, pointerType}); |
| 98 | ``` |