Gutenberg Page Template Switcher

gutenberg-page-template-switcher.js

import gql from 'graphql-tag'
import { client } from '../utils/apollo-client'
const { parse } = wp.blocks;
const { select, subscribe, dispatch } = wp.data;
const GET_PAGE_BLOCK_TEMPLATE = gql`
  query GET_PAGE_BLOCK_TEMPLATE($id: ID!, $template: String) {
    page( id: $id ) {
      blockTemplate( pageTemplate: $template )
    }
  }
`;

class PageTemplateSwitcher {

  constructor() {
    this.template = null;
  }

  init() {

    subscribe( () => {

      const newTemplate = select( 'core/editor' ).getEditedPostAttribute( 'template' );

      if ( newTemplate && newTemplate !== this.template ) {
        this.template = newTemplate;
        this.changeTemplate();
      }

    } );

  }

  changeTemplate() {

    const { resetBlocks } = dispatch('core/editor');
    const postType = select( 'core/editor' ).getEditedPostAttribute( 'type' );
    const id = select( 'core/editor' ).getEditedPostAttribute( 'id' );

    if ( postType && id ) {

      const globalId = postType && id ? btoa( postType + ':' + id ) : null;

      client.query({
        query: GET_PAGE_BLOCK_TEMPLATE,
        variables: {
          id: globalId,
          template: this.template
        }
      }).then( result => {

        console.log( result );

        if ( result.data && result.data.page && result.data.page.blockTemplate ) {
          resetBlocks( parse( result.data.page.blockTemplate ) );
        }

      });

    }

  }
}

new PageTemplateSwitcher().init();

Also relevant, using React:
WordPress/gutenberg#8162 (comment)
https://discourse.roots.io/t/gutenberg-body-class-elements-by-template/15310