TouchSwipeMixin

Overview

Purpose: maps touch events to swipe gestures.

This mixin works at the beginning of the Elix render pipeline. In response to touch events, the mixin may call methods on the component or call setState directly.

events ➞ methods → setState → render DOM → post-render

Expects the component to provide:

• setState method compatible with ReactiveMixin's setState.
• swipeLeft and swipeRight methods, or swipeDown and swipeUp methods. If the component defines these, the mixin will call them if the user makes a flick (fast swipe) gesture to the left/right or down/up, respectively.

Provides the component with:

• [internal.state].swipeAxis member that is either "horizontal" (default) or "vertical", indicating in which axis swipes should be tracked.
• [internal.state].swipeFraction member expressing how far through a swipe the user is currently is. See below.

Usage

import TouchSwipeMixin from "elix/src/base/TouchSwipeMixin.js";
class MyElement extends TouchSwipeMixin(HTMLElement) {}

See also the related TrackpadSwipeMixin, which is like TouchSwipeMixin, but for trackpads instead of touch devices. Both TouchSwipeMixin and TrackpadSwipeMixin are designed to work with SwipeCommandsMixin to reveal commands behind list items when the user swipes.

Examples

[internal.state].swipeFraction

This mixin calculates a state member [internal.state].swipeFraction to track the state of a touch swipe operation. If no swipe is in progress, this value is null. If a swipe is in progress, then swipeFraction holds a real number expressing the distance the user has swiped, divided by the width of the element being swiped (which, by default is the element itself). This value is negative for swipes to the left or up, and positive for swipes to the right or down.

Example: If the user touches an element with TouchSwipeMixin, and drags to the left one half of the element's width, then swipeFraction will be equal to -0.5.

The related TrackpadSwipeMixin may also set [internal.state].swipeFraction, adhering to an identical definition.

Completing a swipe

When the user lifts their finger to complete a swipe, the mixin inspects the current value of [internal.state].swipeFraction:

• If the drag is over halfway to the left/up (swipeFraction less than -0.5), the mixin will invoke the component's swipeLeft/swipeUp method (if defined).
• Conversely, if the drag ends over halfway to the right/down (swipeFraction greater than 0.5), the mixin will invoke swipeRight/swipeDown (if defined).

In all cases, when the drag completes, the mixin will set [internal.state].swipeFraction to null.

Flick gestures

If the user ends a swipe at high speed, the gesture is interpreted as a flick. If the flick is to the left, the mixin will invoke the component's swipeLeft method (if defined). Conversely, if the flick is to the right, the mixin will invoke swipeRight (if defined).

You can use TouchSwipeMixin with SwipeDirectionMixin to map these swipeLeft and swipeRight calls to directional navigations. You can additionally map directional navigation operations to selection calls using DirectionCursorMixin. Elix's carousel components like SlidingPages use this chain of mixins to turn flick gestures into changes in the carousel's selection:

• internal.swipeLeft → internal.goRight → selectNext
• internal.swipeRight → internal.goLeft → selectPrevious