Node or ShadowRoot into which element DOM should be rendered. Defaults to an open shadowRoot.
Marks class as having finished creating properties.
Ensure this class is marked as finalized as an optimization ensuring
it will not needlessly try to finalize.
Note this property name is a string to prevent breaking Closure JS Compiler optimizations. See updating-element.ts for more information.
User-supplied object that maps property names to PropertyDeclaration
objects containing options for configuring the property.
Reference to the underlying library method used to render the element's
DOM. By default, points to the render method from lit-html's shady-render
module.
Most users will never need to touch this property.
This property should not be confused with the render instance method,
which should be overridden to define a template for the element.
Advanced users creating a new base class based on LitElement can override
this property to point to a custom render method with a signature that
matches shady-render's render
method.
Array of styles to apply to the element. The styles should be defined
using the css tag function or via constructible stylesheets.
Returns a Promise that resolves when the element has completed updating.
The Promise value is a boolean that is true if the element completed the
update without triggering another update. The Promise result is false if
a property was set inside updated(). If the Promise is rejected, an
exception was thrown during the update.
To await additional asynchronous work, override the _getUpdateComplete
method. For example, it is sometimes useful to await a rendered element
before fulfilling this Promise. To do this, first await
super._getUpdateComplete(), then any subsequent state.
The Promise returns a boolean that indicates if the update resolved without triggering another update.
Returns a list of attributes corresponding to the registered properties.
Override point for the updateComplete promise.
It is not safe to override the updateComplete getter directly due to a
limitation in TypeScript which means it is not possible to call a
superclass getter (e.g. super.updateComplete.then(...)) when the target
language is ES5 (https://github.com/microsoft/TypeScript/issues/338).
This method should be overridden instead. For example:
class MyElement extends LitElement { async _getUpdateComplete() { await super._getUpdateComplete(); await this._myChild.updateComplete; } }
Applies styling to the element shadowRoot using the styles
property. Styling will apply using shadowRoot.adoptedStyleSheets where
available and will fallback otherwise. When Shadow DOM is polyfilled,
ShadyCSS scopes styles and adds them to the document. When Shadow DOM
is available but adoptedStyleSheets is not, styles are appended to the
end of the shadowRoot to mimic spec
behavior.
Synchronizes property values when attributes change.
Returns the node into which the element should render and by default
creates and returns an open shadowRoot. Implement to customize where the
element's DOM is rendered. For example, to render into the element's
childNodes, return this.
Returns a node into which to render.
Allows for super.disconnectedCallback() in extensions while
reserving the possibility of making non-breaking feature additions
when disconnecting at some point in the future.
Invoked when the element is first updated. Implement to perform one time work on the element after update.
Setting properties inside this method will trigger the element to update again after this update cycle completes.
Map of changed properties with old values
Performs element initialization. By default this calls
createRenderRoot to create the element renderRoot node and
captures any pre-set values for registered properties.
Performs an element update. Note, if an exception is thrown during the
update, firstUpdated and updated will not be called.
You can override this method to change the timing of updates. If this
method is overridden, super.performUpdate() must be called.
For instance, to schedule updates to occur just before the next frame:
protected async performUpdate(): Promise<unknown> {
await new Promise((resolve) => requestAnimationFrame(() => resolve()));
super.performUpdate();
}Invoked on each update to perform rendering tasks. This method may return
any value renderable by lit-html's NodePart - typically a
TemplateResult. Setting properties inside this method will not trigger
the element to update.
Requests an update which is processed asynchronously. This should
be called when an element should update based on some state not triggered
by setting a property. In this case, pass no arguments. It should also be
called when manually implementing a property setter. In this case, pass the
property name and oldValue to ensure that any configured property
options are honored. Returns the updateComplete Promise which is resolved
when the update completes.
(optional) name of requesting property
(optional) old value of requesting property
A Promise that is resolved when the update completes.
This protected version of requestUpdate does not access or return the
updateComplete promise. This promise can be overridden and is therefore
not free to access.
Controls whether or not update should be called when the element requests
an update. By default, this method always returns true, but this can be
customized to control when to update.
Map of changed properties with old values
Updates the element. This method reflects property values to attributes
and calls render to render DOM via lit-html. Setting properties inside
this method will not trigger another update.
Invoked whenever the element is updated. Implement to perform post-updating tasks via DOM APIs, for example, focusing an element.
Setting properties inside this method will trigger the element to update again after this update cycle completes.
Map of changed properties with old values
Creates a property accessor on the element prototype if one does not exist
and stores a PropertyDeclaration for the property with the given options.
The property setter calls the property's hasChanged property option
or uses a strict identity check to determine whether or not to request
an update.
This method may be overridden to customize properties; however,
when doing so, it's important to call super.createProperty to ensure
the property is setup correctly. This method calls
getPropertyDescriptor internally to get a descriptor to install.
To customize what properties do when they are get or set, override
getPropertyDescriptor. To customize the options for a property,
implement createProperty like this:
static createProperty(name, options) { options = Object.assign(options, {myOption: true}); super.createProperty(name, options); }
Creates property accessors for registered properties and ensures any superclasses are also finalized.
Returns a property descriptor to be defined on the given named property. If no descriptor is returned, the property will not become an accessor. For example,
class MyElement extends LitElement { static getPropertyDescriptor(name, key, options) { const defaultDescriptor = super.getPropertyDescriptor(name, key, options); const setter = defaultDescriptor.set; return { get: defaultDescriptor.get, set(value) { setter.call(this, value); // custom action. }, configurable: true, enumerable: true } } }
Returns the property options associated with the given property.
These options are defined with a PropertyDeclaration via the properties
object or the @property decorator and are registered in
createProperty(...).
Note, this method should be considered "final" and not overridden. To
customize the options for a given property, override createProperty.
Return the array of styles to apply to the element. Override this method to integrate into a style management system.
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Base element class that manages element properties and attributes, and renders a lit-html template.
To define a component, subclass
LitElementand implement arendermethod to provide the component's template. Define properties using thepropertiesproperty or thepropertydecorator.