@lit/reactive-element - 1.0.0

Major Changes

• @lit/reactive-element is a new package that factors out the base class that provides the reactive update lifecycle based on property/attribute changes to LitElement (what was previously called UpdatingElement) into a separate package. LitElement now extends ReactiveElement to add lit-html rendering via the render() callback. See ReactiveElement API for more details.
• UpdatingElement has been renamed to ReactiveElement.
• The updating-element package has been renamed to @lit/reactive-element.
• The @internalProperty decorator has been renamed to @state.
• For consistency, renamed _getUpdateComplete to getUpdateComplete.
• When a property declaration is reflect: true and its toAttribute function returns undefined the attribute is now removed where previously it was left unchanged (#872).
• Errors that occur during the update cycle were previously squelched to allow subsequent updates to proceed normally. Now errors are re-fired asynchronously so they can be detected. Errors can be observed via an unhandledrejection event handler on window.
• ReactiveElement's renderRoot is now created when the element's connectedCallback is initially run.
• Removed requestUpdateInternal. The requestUpdate method is now identical to this method and should be used instead.
• The initialize method has been removed. This work is now done in the element constructor.

Minor Changes

• Adds static addInitializer for adding a function which is called with the element instance when is created. This can be used, for example, to create decorators which hook into element lifecycle by creating a reactive controller (#1663).
• Added ability to add a controller to an element. A controller can implement callbacks that tie into element lifecycle, including connectedCallback, disconnectedCallback, willUpdate, update, and updated. To ensure it has access to the element lifecycle, a controller should be added in the element's constructor. To add a controller to the element, call addController(controller).
• Added removeController(controller) which can be used to remove a controller from a ReactiveElement.
• Added willUpdate(changedProperties) lifecycle method to UpdatingElement. This is called before the update method and can be used to compute derived state needed for updating. This method is intended to be called during server side rendering and should not manipulate element DOM.