/**
 * @license
 * Copyright 2020 Google LLC
 * SPDX-License-Identifier: BSD-3-Clause
 */

import {directive, Directive, PartInfo, PartType} from '../directive.js';
import {noChange} from '../lit-html.js';

class TemplateContentDirective extends Directive {
  private _previousTemplate?: HTMLTemplateElement;

  constructor(partInfo: PartInfo) {
    super(partInfo);
    if (partInfo.type !== PartType.CHILD) {
      throw new Error('templateContent can only be used in child bindings');
    }
  }

  render(template: HTMLTemplateElement) {
    if (this._previousTemplate === template) {
      return noChange;
    }
    this._previousTemplate = template;
    return document.importNode(template.content, true);
  }
}

/**
 * Renders the content of a template element as HTML.
 *
 * Note, the template should be developer controlled and not user controlled.
 * Rendering a user-controlled template with this directive
 * could lead to cross-site-scripting vulnerabilities.
 */
export const templateContent = directive(TemplateContentDirective);

/**
 * The type of the class that powers this directive. Necessary for naming the
 * directive's return type.
 */
export type {TemplateContentDirective};