LitElement

Node or ShadowRoot into which element DOM should be rendered. Defaults to an open shadowRoot.

Ensure this class is marked as finalized as an optimization ensuring it will not needlessly try to finalize.

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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.

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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.

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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.

Returns a list of attributes corresponding to the registered properties.

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.

参数

name

string

old

null | string

value

null | string

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.

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.

参数

_changedProperties

Map<string | number | symbol, unknown>

Map of changed properties with old values

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Setting properties inside this method will trigger the element to update again after this update cycle completes.

Override point for the updateComplete promise.

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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; } }

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.

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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.

参数

name?

PropertyKey

(optional) name of requesting property

oldValue?

unknown

(optional) old value of requesting property

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.

参数

name?

PropertyKey

oldValue?

unknown

options?

PropertyDeclaration<unknown, unknown>

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.

参数

_changedProperties

Map<string | number | symbol, unknown>

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.

参数

changedProperties

Map<string | number | symbol, unknown>

Invoked whenever the element is updated. Implement to perform post-updating tasks via DOM APIs, for example, focusing an element.

参数

_changedProperties

Map<string | number | symbol, unknown>

Map of changed properties with old values

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Setting properties inside this method will trigger the element to update again after this update cycle completes.

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.

参数

name

PropertyKey

options

PropertyDeclaration<unknown, unknown>

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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,

参数

name

PropertyKey

key

string | symbol

options

PropertyDeclaration<unknown, unknown>

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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(...).

参数

name

PropertyKey

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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.