It is not currently very easy to access sub-controllers from a root controller. This makes it difficult to use a controller to open and populate a dialog, for example.

The proposed solution is to map nested controller instances directly to including controller fields, like we currently do with namespace values. For example, if a main FXML document looks like this:

<VBox fx:controller="com.foo.MyMainController">
 ...
 <fx:include fx:id="myInclude" source="my_include.fxml"/>
 ...
</VBox>

The document's controller might look like this (the root element of the include is assumed to be a HBox with a controller of type MyIncludeController):

public class MyMainController extends Controller {
   @FXML private HBox myInclude;
   @FXML private MyIncludeController myIncludeController;

   ...
}

When initialize() is called, myInclude would contain the root element from "my_include.fxml", and myIncludeController would contain the include's controller, allowing the including code to easily interact with the included controller.