Getting notifications about sizeHint changes from internal widgets

Question

Small picture to depict what the problem is:

external_widget
+--------------------------------------------------+
|                                                  |
|                                                  |
|     layout_widget                                |
|     +-------------------------------+            |
|     |                               |            |
|     |     [internal_layout]         |            |
|     |                               |            |
|     |                               |            |
|     |   [Some Another Widget]       |            |
|     +-------------------------------+            |
|                                                  |
+--------------------------------------------------+

layout_widget - just QWidget with vertical layout. It contains another internal_layout and another widgets. internal_layout - layout where the widgets will be added and deleted in future. external_widget - just QWidget with no layout, I control the position and size of layout widget and other widgets manually.

When I add some widgets in internal_layout - it sends LayoutRequest event (as far as I know, I saw this in Qt source codes). This event is catched by parent widget and parent widget tells his layout that something is changed which forces layout to recalculate layout_widget sizeHint() etc. Do I understand this process right?

The problem is that sizeHint() is changed, but my external_widget doesn't know anything about that. I want to get notification about layout_widget sizeHint() changes. Is it possible?

I even tried to do this:

external widget ctor:
{
    layout_widget->installEventFilter(this);
}

and tried to catch them:

void ...eventFilter(QObject* obj, QEvent* event )
{
    if (obj == layout_widget && event == QEvent::LayoutRequest)
    {
        // TODO;
    }
}

and I never got into "TODO" block.

Do layout_widget send some events or notifications that something was changed inside him and it should be resized, i.e. his sizeHint() has been changed?