Content select (in modal) and cross window communication

It is often need to have a possibility to select a content value while editing another content. Usually this is achieved by use of a modal window. However, when the modal content is an iframe it introduces extra complexity related to cross window communication.

It may tempting to use direct access to a parent from an iframe window.parent['field_id'].value='selected value';. And it is what Joomla also used to do. Such approach is insecure, not reliable and often does not work with field in multiple mode (like SubForm field).

A better approach is to use a message based communication: Window: postMessage() method, which provide a reliable and secure way to transfer any data from children window (iframe) to parent.

How it works:

• Initiator creates a dialog/modal, and add a listener for message event of current window window.addEventListener('message', ... );
• Child window handle user selection, and post a message with selected value(s) to parent window.parent.postMessage();
• Initiator receive the message, set the values, removes message listener, and closes a dialog/modal;

Joomla provide modal-content-select, modal-content-select-field assets and Joomla\CMS\Form\Field\ModalSelectField, that can be used as is or as base for your field.

modal-content-select-field asset loaded with ModalSelect field, creates a dialog when requested, and listen to message.

modal-content-select asset, a helper which should be loaded within the iframe. It will listen on click on any element with data-content-select attribute, and will post message with messageType: 'joomla:content-select' and the element's dataset to parent. Additionally, when scriptOptions contains content-select-on-load value, the helper will post this value to parent on iframe load event. Example of the "selectable" elements:

<button data-content-select data-id="1" data-title="Article 1">Select Article 1</button>
<button data-content-select data-id="31" data-title="Article 31">Select Article 31</button>

It is not limited to only id and title, it can be extended as needs.

The child window can post following messages:

• joomla:content-select which used to post a value to parent, example window.parent.postMessage({messageType: 'joomla:content-select', id: 1, title: 'A title'});
• joomla:cancel which cancel any action, example window.parent.postMessage({messageType: 'joomla:cancel'});

Dialog buttons and advanced usage

It is recommended that all interactive buttons (like save, cancel etc) be embedded within dialog content (within iframe), instead of rendering them with parent window. This reduce complexity and makes code more reliable.

When using Create/Edit/Cancel the controller usually do redirect to a complete page. For dialog window you can do redirect to layout=modalreturn, and on this page set the final state to scriptOptions then modal-content-select script will check it, and post the result to parent. Example:

$doc->addScriptOptions('content-select-on-load', [
  'id' => 1,
  'title' => 'A title',
]);