Map trackpad events to swipe gestures


Purpose: maps trackpad events (or horizontal mouse wheel events) to swipe gestures. This allows, for example, users of laptops with trackpads to manipulate a view or selection with a trackpad swipe. (The exact user interface gesture depends on the OS and its configuration. A common OS configuration requires the user to drag on the trackpad with two fingers to perform a swipe.)

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.

eventsmethods → setState → render DOM → post-render

Expects the component to provide:

  • setState method compatible with ReactiveMixin's setState.
  • swipeLeft and swipeRight methods. If the component defines these, the mixin will call them if the user makes a full-width drag to the left or right, respectively.

Provides the component with:

  • [internal.state].swipeFraction member expressing how far through a swipe the user is currently is. See below.


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

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


This demo shows how a component can use TouchSwipeMixin and TrackpadSwipeMixin to listen to horizontal touch swipes and trackpad swipes, respectively. Swiping will show the current swipe distance as a fraction of the demo width. It will also translate the gray block by that fraction so the user feels like they are directly moving the block.
Demo: Swiping shows the fraction of the demo's width you have swiped
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Demo: TrackpadSwipeMixin reveals commands behind these list items on a trackpad swipe


This mixin calculates a state member [internal.state].swipeFraction to track the state of a trackpad 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, and positive for swipes to the right.

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

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

Full-width drag operations

Trackpad operations may complete while the user is still dragging. If the user drags an item a full element width to the left (swipeFraction is less or equal to -1) or to the right (swipeFraction is greater than or equal to 1), the mixin interprets this as completing a drag. The mixin will invoke the component's swipeLeft or swipeRight method (if defined).

You can use TrackpadSwipeMixin 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:

  • swipeLeftinternal.goRightselectNext
  • swipeRightinternal.goLeftselectPrevious

Trackpad timeouts

If the user slows or stops their trackpad drag operation, the operation will timeout. If the swipe is over halfway to the left or right, the mixin will invoke swipeLeft or swipeRight respectively. Otherwise, the mixin will cancel the drag and set [internal.state].swipeFraction to null.