[drupalMedia] Unable to set default caption value on <drupal-media> tag

According to the InsertDrupalMediaCommand docs this should be possible:

/**
 * The insert media command.
 *
 * The command is registered by the `DrupalMediaEditing` plugin as
 * `insertDrupalMedia`.
 *
 * In order to insert media at the current selection position, execute the
 * command and pass the attributes desired in the drupal-media element:
 *
 * @example
 *    editor.execute('insertDrupalMedia', {
 *      'alt': 'Alt text',
 *      'data-align': 'left',
 *      'data-caption': 'Caption text',
 *      'data-entity-type': 'media',
 *      'data-entity-uuid': 'media-entity-uuid',
 *      'data-view-mode': 'default',
 *    });
 *
 * @private
 */
export default class InsertDrupalMediaCommand extends Command {

I wonder if this is a bug in DrupalMediaUI, where it does:

      this.listenTo(buttonView, 'execute', () => {
        openDialog(
          libraryURL,
          ({ attributes }) => {
            editor.execute('insertDrupalMedia', attributes);
          },
          dialogSettings,
        );
      });

👆 as you can see, it passes on all attributes it receives!

>2 weeks of silence: I'm assuming the snippet I provided in #2 unblocked @nyanmar.

I was able to get the data-caption attribute inserted in the source by adding it in the this.attrs and this.converterAttributes in the constructor of the DrupalMediaEditing plugin thanks to @rikki_iki but it isn't editable, and pressing the "Toggle caption on" button adds an extra caption, overriding the original.

Attaching a patch to demonstrate, and a screencast below.

I've used CKEditor Inspector to compare the model of a working media with caption with what I'm getting with the patch.

Working:

<drupalMedia drupalMediaEntityType="media" drupalMediaEntityUuid="..." drupalMediaIsImage="true" drupalMediaType="image">
  <caption>Test caption</caption>
</drupalMedia>

Not working:

<drupalMedia drupalMediaCaption="Test caption" drupalMediaEntityType="media" drupalMediaEntityUuid="..." drupalMediaIsImage="true" drupalMediaType="image">
</drupalMedia>

I believe the issue is that the upcast converter is doing a simple attributeToAttribute conversion whereas it needs special handling for the caption. I guess the DrupalMediaCaptionEditing should take care of that but it just needs to be triggered some how.

Updated the title and re-categories the issue. Let's update the IS with steps to reproduce.

The Needs Review Queue Bot → tested this issue.

While you are making the above changes, we recommend that you convert this patch to a merge request → . Merge requests are preferred over patches. Be sure to hide the old patch files as well. (Converting an issue to a merge request without other contributions to the issue will not receive credit.)