Purpose: manages state changes that depend upon completion of CSS transitions.

This mixin primarily works at the beginning of the Elix render pipeline:

eventsmethods → setState → updates → render DOM → post-render

The mixin also performs work after the DOM has been updated.

Expects the component to provide:

  • setState and render methods compatible with ReactiveMixin's setState.
  • state.effect member indicating the name of an effect to perform.

Provides the component with:

  • state.effectPhase member indicating the current phase of the effect: "before", "during", or "after". See below.
  • effect-phase-changed event when state.effectPhase has changed.


import TransitionEffectMixin from 'elix/src/TransitionEffectMixin.js';
class MyElement extends TransitionEffectMixin(HTMLElement) {}

You can use TransitionEffectMixin if your component has CSS transitions that effect or determine state. This is particularly useful for elements that need to perform work after an effect has completed.

TransitionEffectMixin can interoperate with OpenCloseMixin for elements that display asynchronous CSS transitions when opening and/or closing. Among other things, OpenCloseMixin ensures the element's closeFinished property returns false until the close effect has finished. That in turn allows mixins like OverlayMixin to wait until an overlay has closed before hiding it.


The Drawer and Toast components are overlays with CSS transition effects for their open/close operations. Overlays are generally hidden when closed, which presents a particular challenge if you wish to create an overlay with an asynchronous closing effect: the overlay must remain visible until the closing effect has completed. Otherwise the overlay will immediately disappear, preventing the closing effect from rendering.

Drawer elements go here...
Demo: `Drawer` uses `TransitionEffectMixin` to perform work after it closes

By using TransitionEffectMixin, Drawer ensures that OverlayMixin will not hide the drawer until it the "close" effect has finished.

Effect phases

TransitionEffectMixin defines an effect timing model with three phases: before, during, and after.

  • before: In this phase, the element can prepare for the application of a CSS transition. E.g., before an opening effect, the element might ensure that it's visible.
  • during: The element applies CSS classes or attributes to itself or shadow elements sufficient to trigger the application of one or more CSS transitions.
  • after: The CSS transitions have completed, so the element can perform any necessary clean-up. E.g., after a closing effect, the element might hide itself.

The mixin tracks the effect phase as a string value in state.effectPhase. When an effect is started via the mixin's startEffect method, the mixin sets state.effectPhase to "before". When that state has rendered and componentDidUpdate is called, the mixin will immediately move to the "during" phase. The component will then rerender. The component can then apply whatever CSS transitions constitute the desired visual effect.

The second phase transition, from "during" to "after", is asynchronous, and will occur when one of the elements with defined CSS transitions raises the transitionend event. By default, the mixin listens to the host element for transitionend. If, however, you want to apply CSS transitions to shadow elements, you can override the symbols.elementsWithTransitions property and return an array of the elements that have transitions. Once transitionend is received, the mixins sets state.effectPhase to "after", and the component can rerender if necessary.


Used by classes Drawer, ExpandablePanel, and Toast.

effect-phase-changed event

Raised when state.effect or state.effectPhase changes.

Note: In general, Elix components do not raise events in response to outside manipulation. (See symbols.raiseChangeEvents.) However, for a component using TransitionEffectMixin, a single invocation of the startEffect method will cause the element to pass through multiple visual states. This makes it hard for external hosts of this component to know what visual state the component is in. Accordingly, the mixin raises the effect-phase-changed event whenever the effect or phase changes, even if symbols.raiseChangeEvents is false.

symbols.elementsWithTransitions property

Return the elements that use CSS transitions to provide visual effects.

By default, this assumes the host element itself will have a CSS transition applied to it, and so returns an array containing the element. If you will be applying CSS transitions to other elements, override this property and return an array containing the implicated elements.

See symbols.elementsWithTransitions for details.

Type: Array.