ckeditor_controller.js

import UploadAdapter from './upload_adapter';

function CustomUploadAdapterPlugin( editor ) {
    editor.plugins.get( 'FileRepository' ).createUploadAdapter = ( loader ) => {
        // Configure the URL to the upload script in your back-end here!
        return new UploadAdapter( loader, editor );
    };
}

import { Controller } from "stimulus"

export default class extends Controller {
 connect(){
  ClassicEditor
      .create( this.element,
          {
            //...
            extraPlugins: [ CustomUploadAdapterPlugin ],
            customUpload: {
              directUploadUrl: this.element.dataset.directUploadUrl,
              blobUrlTemplate: this.element.dataset.blobUrlTemplate
            }
            //...
          }
      )
      .then( editor => {
        console.log( editor );
      } )
      .catch( error => {
        console.error( error );
      });
   }
}

form_view.html.haml

// ...

= form.text_area :content, data: { "controller" => "ckeditor", "direct-upload-url" => rails_direct_uploads_url, "blob-url-template" => rails_service_blob_url(":signed_id", ":filename") }

upload_adapter.js

import { DirectUpload } from "@rails/activestorage"

export default class UploadAdapter {
  constructor( loader, editor ) {
    this.loader = loader;
    this.editor = editor;
    this.blobUrlTemplate = this.editor.config.get("customUpload.blobUrlTemplate")
    this.directUploadUrl = this.editor.config.get("customUpload.directUploadUrl")
  }

  upload() {
    return this.loader.file
      .then( file => new Promise( ( resolve, reject ) => {
        this.resolve = resolve;
        this.reject = reject;
        const directUpload = new DirectUpload(file, this.directUploadUrl, this)
        directUpload.create(this.directUploadDidComplete.bind(this))
      }));
  }

  directUploadWillStoreFileWithXHR(xhr) {
    this.xhr = xhr;

    xhr.upload.addEventListener("progress", event => {
      if ( event.lengthComputable ) {
        this.loader.uploadTotal = event.total;
        this.loader.uploaded = event.loaded;
      }
    })
  }

  abort() {
    if ( this.xhr ) {
      this.xhr.abort();
    }
  }

  directUploadDidComplete(error, attributes) {
    if (error) {
      this.reject(`Something went wrong with uploading the image. Please try again!`);
    }

    const url = this.createBlobUrl(attributes.signed_id, attributes.filename)

    this.resolve({ default: url });
  }

  createBlobUrl(signedId, filename) {
    return this.blobUrlTemplate
      .replace(":signed_id", signedId)
      .replace(":filename", encodeURIComponent(filename))
  }
}