Class TemplateInstance

packages/lit-html/src/lit-html.ts:1187–1288

* An updateable instance of a Template. Holds references to the Parts used to * update the template instance.

 * update the template instance.
 */
class TemplateInstance implements Disconnectable {
  _$template: Template;
  _$parts: Array<Part | undefined> = [];

  /** @internal */
  _$parent: ChildPart;
  /** @internal */
  _$disconnectableChildren?: Set<Disconnectable> = undefined;

  constructor(template: Template, parent: ChildPart) {
    this._$template = template;
    this._$parent = parent;
  }

  // Called by ChildPart parentNode getter
  get parentNode() {
    return this._$parent.parentNode;
  }

  // See comment in Disconnectable interface for why this is a getter
  get _$isConnected() {
    return this._$parent._$isConnected;
  }

  // This method is separate from the constructor because we need to return a
  // DocumentFragment and we don't want to hold onto it with an instance field.
  _clone(options: RenderOptions | undefined) {
    const {
      el: {content},
      parts: parts,
    } = this._$template;
    const fragment = (options?.creationScope ?? d).importNode(content, true);
    walker.currentNode = fragment;

    let node = walker.nextNode()!;
    let nodeIndex = 0;
    let partIndex = 0;
    let templatePart = parts[0];

    while (templatePart !== undefined) {
      if (nodeIndex === templatePart.index) {
        let part: Part | undefined;
        if (templatePart.type === CHILD_PART) {
          part = new ChildPart(
            node as HTMLElement,
            node.nextSibling,
            this,
            options
          );
        } else if (templatePart.type === ATTRIBUTE_PART) {
          part = new templatePart.ctor(
            node as HTMLElement,
            templatePart.name,
            templatePart.strings,
            this,
            options
          );
        } else if (templatePart.type === ELEMENT_PART) {