ExpandablePanel

A panel which expands/collapses vertically with an animated transition.

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Demo: A region that can be expanded and collapsed

This component combines OpenCloseMixin, TransitionEffectMixin and a simple CSS height animation.

Note that animating an element's height may not always produce the smoothest results. However, animating height does have the advantage of letting you set the height of the panel's collapsed state by setting the panel's min-height.

This component handles only the duties of collapsing and expanding. It does not provide a user interface for the user to trigger the change in state; you must provide that user interface yourself.

API

Ancestry: ExpandablePanel → ReactiveElement → HTMLElement

Built with mixins AttributeMarshallingMixin, OpenCloseMixin, ReactiveMixin, RenderUpdatesMixin, ShadowTemplateMixin, and TransitionEffectMixin.

$ property

The collection of references to the elements with IDs in the component's Shadow DOM subtree.

Example: if component's template contains a shadow element <button id="foo">, you can use the reference this.$.foo to obtain the corresponding button in the component instance's shadow tree.

Such references simplify a component's access to its own elements. In exchange, this mixin trades off a one-time cost of querying all elements in the shadow tree instead of paying an ongoing cost to query for an element each time the component wants to inspect or manipulate it.

These shadow element references are established the first time you read the $ property. They are not updated if you subsequently modify the shadow tree yourself (to replace one item with another, to add new items with id attributes, etc.).

Type: object

Defined by ShadowTemplateMixin inherited from ReactiveElement

close(result) method

Close the component (if not already closed).

Some components like AlertDialog want to indicate why or how they were closed. To support such scenarios, you can supply a value to the optional result parameter. This result will be made available in the whenClosed promise and the state.result member.

Parameters:
  • result: objectan indication of how or why the element closed

Defined by OpenCloseMixin

closed property

True if the element is currently closed.

Type: boolean

Defined by OpenCloseMixin

closeFinished property

True if the element has completely closed.

For components not using asynchronous open/close effects, this property returns the same value as the closed property. For elements that have a true value of state.openCloseEffects (e.g., elements using TransitionEffectMixin), this property returns true only if state.effect is "close" and state.effectPhase is "after".

Type: boolean

Defined by OpenCloseMixin

defaultState property

The default state for the component. This can be extended by mixins and classes to provide additional default state.

Type: object

Defined by ReactiveMixin inherited from ReactiveElement

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.

Defined by TransitionEffectMixin

open() method

Open the element (if not already opened).

Defined by OpenCloseMixin

opened property

True if the element is currently closed.

Type: boolean

Defined by OpenCloseMixin

opened-changed event

Raised when the opened/closed state of the component changes.

Defined by OpenCloseMixin

refineState(state) method

Apply changes to a proposed new state for the component to enforce necessary consistency between state members. See Refining state for details.

Parameters:
  • state: objecta proposed new state for the component

Returns: boolean - true if the state is already acceptable as it is

Defined by ReactiveMixin inherited from ReactiveElement

render() method

Render the component to the DOM.

This method does nothing if the state has not changed since the last render call.

This method invokes all internal render methods. It then invoked componentDidMount (for first render) or componentDidUpdate (for subsequent renders).

Defined by ReactiveMixin inherited from ReactiveElement

setState(changes) method

Update the component's state by merging the specified changes on top of the existing state. If the component is connected to the document, and the new state has changed, this returns a promise to asynchronously render the component. Otherwise, this returns a resolved promise.

Parameters:
  • changes: objectthe changes to apply to the element's state

Returns: Promise - resolves when the new state has been rendered

Defined by ReactiveMixin inherited from ReactiveElement

shouldComponentUpdate(nextState) method

Return true if the component should update.

The default implementation does a shallow check of property values like React's PureComponent. This seems adequate for most web components. You can override this to always return true (like React's base Component class), or to perform more specific, deeper checks for changes in state.

Parameters:
  • nextState: objectthe proposed new state for the element

Returns: boolean - true if the component should update (rerender)

Defined by ReactiveMixin inherited from ReactiveElement

state property

The component's current state. The returned state object is immutable. To update it, invoke setState.

Type: object

Defined by ReactiveMixin inherited from ReactiveElement

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.

Defined by TransitionEffectMixin

toggle(opened) method

Toggle the open/close state of the element.

Parameters:
  • opened: booleantrue if the element should be opened, false if closed.

Defined by OpenCloseMixin

updates property

The attributes and properies that should be applied to the component on render. By default, this is an empty plain object. Your mixin or component can extend this to identify the properties to set on the host element or elements in the shadow subtree.

Type: object

Defined by RenderUpdatesMixin inherited from ReactiveElement

whenClosed() method

This method can be used as an alternative to listening to the "opened-changed" event, particularly in situations where you want to only handle the next time the component is closed.

Returns: Promise A promise that resolves when the element has completely closed, including the completion of any asynchronous opening effect.

Defined by OpenCloseMixin