onKey property Null safety
Called if this focus node receives a key event while focused (i.e. when hasFocus returns true).
Key Event Propagation
The FocusManager receives key events from RawKeyboard and HardwareKeyboard and will pass them to the focused nodes. It starts with the node with the primary focus, and will call the onKey or onKeyEvent callback for that node. If the callback returns KeyEventResult.ignored, indicating that it did not handle the event, the FocusManager will move to the parent of that node and call its onKey or onKeyEvent. If that onKey or onKeyEvent returns KeyEventResult.handled, then it will stop propagating the event. If it reaches the root FocusScopeNode, FocusManager.rootScope, the event is discarded.