How do I communicate a page(modal dialog) with its sibling (sidebar)?

I'm converting a script of mine to be an add on. One of the needs is to configure a template, so I have programmed a sidebar that launchs a field picker. As the sidebar does not have enough room for the picker, I have to launch it from a modal dialog that I create from the sidebar, by calling this code in the server side:

var html = HtmlService.createHtmlOutputFromFile('TemplatePicker.html')
  .setWidth(600).setHeight(425);
SpreadsheetApp.getUi().showModalDialog(html, 'Select the file with the template');

My problem is that once the user picks the file, when I have the id of the chosen file, I'm not able to pass that id to the sidebar. I tried invoking someJSFunctionOfSidebar(id) and parent.someJSFunctionOfSidebar(id), but it didn't work, so I finally ended passing the value to the server side and reloading the sidebar from there, but it's very slow and the resulting effect is ugly.

My question is:

Is there a way to pass a value at client level from a modal dialog created with SpreadsheetApp.getUi().showModalDialog to its parent? Perhaps it's not really its parent and that's the reason for it not working.

Perhaps it's not really its parent and that's the reason for it not working.

Right - there isn't actually a DOM parent / child relationship in play here. Both the sidebar and the modal dialog were launched from server-side scripts, and are independent. They can both communicate with the server, though, so you can use a store-and-forward technique to get the result from your picker to the sidebar.

Basic idea:

• The sidebar will start polling the server for the picker's result as soon as it requests the picker's launch.
• The picker's result will be sent to the server, using google.script.run.
• The server will store the result temporarily - this could be as simple as a global variable, depending on your situation.
• Once there is a result, the next poll will retrieve it.

Have a look at How to poll a Google Doc from an add-on for the basic idea of a poller.