Explorer

Explorer serves as a base class for implementing item selection user interface patterns that couple two synchronized elements that both support selection. The main "stage" element focuses the user's attention on a single item drawn from a list. A secondary "proxy list" element presents a set of smaller proxy elements, one for each item in the main list. The Explorer keeps the stage and proxy list elements in sync: if the user changes the selection in one element, the selection in the other element is updated to match.

Explorer serves as the basis for a variety of common user interface elements:

Mountain lake Terraced farm Winter trees Forest river Red panda
Demo: `Carousel` uses a set of dots as proxies, and a `SlidingStage` as its stage
Page one
Page two
Page three
Demo: `Tabs` uses `TabButton` elements as proxies and `Modes` as its stage
home
Home
search
Search
settings
Settings
Home page
Search page
Settings page
Demo: A `Tabs` variation for navigation using `SlidingStage` as the stage
Autumn foliage on mountainside Coastal mountains Dirt road lined with fallen leaves Forest river Hoodoo formations Iceberg Lake with milky water Mountain lake Palm trees Red panda River at sunrise River canyon Sandy beach Snowy peaks Snowy river Stone arch Sunset over rocky ocean pool Sunset through palm trees Terraced farm Winter trees
Demo: `ListExplorer` uses a `ListBox` for the proxy list and `Modes` for the stage

These components present different user interfaces, but they all possess a list synchronized with a stage. The specific form of the stage or proxy list may vary while remaining true to the pattern.

Customizing Explorer using element tags

You can customize Explorer or its subclasses by specifying which tags should be used to create various subelements:

You can create your own components to fill any of these roles. Generally speaking, any component that supports SingleSelectionMixin should suffice as the stage or proxy list.

Supplying items to an Explorer

Elements you place inside an Explorer become the list items navigated by the stage element. E.g., you could create the first Carousel demo above like this:

    <elix-carousel>
      <img src="image1.jpg">
      <img src="image2.jpg">
      <img src="image3.jpg">
      <img src="image4.jpg">
      <img src="image5.jpg">
    </elix-carousel>

For each item in the main list, Explorer will create an instance of the element specified by proxyTag. E.g., the Carousel above will instantiate a PageDot for each item (image).

Certain subclasses of Explorer will automatically set the content of the proxy element to reflect data in the corresponding list item. E.g., Tabs will use the aria-label or alt attribute of the list item.

If you want to programmatically manipulate the appearance or content of a proxy element, you can override the Explorer's proxyUpdates method.

Supplying proxies for the items

You can also create proxy elements yourself and slot them into the proxy slot. Instead of using the default PageDot proxies in a Carousel, you can supply your own proxies:

    <elix-carousel>
      <div slot="proxy">1</div>
      <div slot="proxy">2</div>
      <div slot="proxy">3</div>
      <div slot="proxy">4</div>
      <div slot="proxy">5</div>
      <img src="image1.jpg">
      <img src="image2.jpg">
      <img src="image3.jpg">
      <img src="image4.jpg">
      <img src="image5.jpg">
    </elix-carousel>
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2
3
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5
Mountain lake Terraced farm Winter trees Forest river Red panda
Demo: `Carousel` with custom proxies

Layout

Explorer manages the top/left/bottom/right visual positioning of the list in relation the stage. You can specify a position through the proxyListPosition property, and control whether the proxy list overlaps the stage with the proxyListOverlap property.

API

Ancestry: Explorer → ReactiveElement → HTMLElement

Built with mixins AttributeMarshallingMixin, LanguageDirectionMixin, ReactiveMixin, RenderUpdatesMixin, ShadowTemplateMixin, SingleSelectionMixin, and SlotItemsMixin.

Includes subelements ListBox and Modes.

Extended by classes Carousel, CarouselSlideshow, CarouselWithThumbnails, ListExplorer, and Tabs.

$ 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

canSelectNext property

True if the selection can be moved to the next item, false if not (the selected item is the last item in the list).

Type: boolean

Defined by SingleSelectionMixin

canSelectPrevious property

True if the selection can be moved to the previous item, false if not (the selected item is the first one in the list).

Type: boolean

Defined by SingleSelectionMixin

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

itemCalcs(item, index) method

Returns a set of calculations about the given item that can be derived from the component's current state.

The goal of the itemCalcs step is to ensure that all mixins/classes use a consistent definition for facts about an item that can be derived from component state. By default, itemCalcs includes a member index containing the index of the indicated item. Other mixins/classes can extend the result of itemCalcs to include additional facts.

For example, the SingleSelectionMixin tracks selection at the component level through a state member state.selectedIndex. When rendering a specific item, a component generally wants to know, "Is this specific item the one which is selected?". SingleSelectionMixin does this with a defintion for itemCalcs that looks like this:

itemCalcs(item, index) {
  const base = super.itemCalcs ? super.itemCalcs(item, index) : null;
  return Object.assign({}, base, {
    selected: index === this.selectedIndex
  });
}


This ensures that any other aspect of the component that wants to inspect the selected state of a given item uses a consistent definition for selection.

Parameters:
  • item: Elementthe item being considered
  • index: numberthe item's position in the list

Defined by ContentItemsMixin inherited from SlotItemsMixin

items property

The current set of items drawn from the element's current state.

Defined by ContentItemsMixin inherited from SlotItemsMixin

itemUpdates(item, calcs, original) method

Determine what updates should be applied to an item to reflect the current state, using the format defined by the updates helpers.

By default, this returns an empty object. You should override this method (or use mixins that override this method) to indicate what updates should be applied to the given item during rendering.

Example: AriaListMixin uses code similar to the following to have an item's aria-selected attribute reflect its selection state:

itemUpdates(item, calcs, original) {
  const base = super.itemUpdates ? super.itemUpdates(item, calcs, original) : {};
  return merge(base, {
    attributes: {
      'aria-selected': calcs.selected
    },
  });
}


This code fragment is intended for use with SingleSelectionMixin, which provides the calcs.selected member.

Parameters:
  • item: Elementthe item to be updated
  • calcs: objectper-item calculations derived from element state
  • original: objectthe item's original HTML attributes, classes, and style

Returns: object the DOM updates that should be applied to the item

Defined by ContentItemsMixin inherited from SlotItemsMixin

proxies property

The current set of proxy elements that correspond to the component's main items. If you have assigned elements to the proxy slot, this returns the collection of those elements. Otherwise, this will return a collection of default proxies generated by the component, one for each item.

Type: Array.

proxyListOverlap property

True if the list of proxies should overlap the stage, false if not.

Type: boolean

Default: {false}

proxyListPosition property

The position of the proxy list relative to the stage.

The start and end values refer to text direction: in left-to-right languages such as English, these are equivalent to left and right, respectively.

Type: 'bottom', 'end', 'left', 'right', 'start', 'top'

Default: 'start'

proxyListTag property

The tag used to create the Explorer's list of proxies.

Type: string

Default: 'div'

proxyTag property

The tag used to create default proxies for the list items.

Type: string

Default: 'div'

proxyUpdates(proxy, calcs) method

Determine what updates should be applied to a proxy to reflect the state of the corresponding item, using the format defined by the updates helpers.

By default, this returns an empty object. You should override this method (or use mixins that override this method) to indicate what updates should be applied to the given proxy during rendering.

The calcs parameter is an object with the following members:

  • index: the index of this proxy in the list.
  • isDefaultProxy: true if this proxy was generated by the Explorer, false if the proxy was assigned to the Explorer's proxy slot.
  • item: the list item corresponding to this proxy. E.g., for a tab button, the item is the corresponding tab panel.

Parameters:
  • proxy: Elementthe proxy to be updated
  • calcs: objectper-proxy calculations derived from element state

Returns: object the DOM updates that should be applied to the item

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

selected-index-changed event

Raised when the selectedIndex property changes.

Defined by SingleSelectionMixin

selectedIndex property

The index of the currently-selected item, or -1 if no item is selected.

Type: number

Defined by SingleSelectionMixin

selectedItem property

The currently-selected item, or null if no item is selected.

Type: Element

Defined by SingleSelectionMixin

selectFirst() method

Select the first item in the list.

Returns: Boolean True if the selection changed, false if not.

Defined by SingleSelectionMixin

selectionRequired property

True if the list should always have a selection (if it has items).

Type: boolean

Default: false

Defined by SingleSelectionMixin

selectionWraps property

True if selection navigations wrap from last to first, and vice versa.

Type: boolean

Default: false

Defined by SingleSelectionMixin

selectLast() method

Select the last item in the list.

Returns: Boolean True if the selection changed, false if not.

Defined by SingleSelectionMixin

selectNext() method

Select the next item in the list.

If the list has no selection, the first item will be selected.

Returns: Boolean True if the selection changed, false if not.

Defined by SingleSelectionMixin

selectPrevious() method

Select the previous item in the list.

If the list has no selection, the last item will be selected.

Returns: Boolean True if the selection changed, false if not.

Defined by SingleSelectionMixin

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

stageTag property

The tag used to create the main "stage" element showing a single item at a time.

Type: string

Default: 'elix-modes'

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

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