Handler for keys pressed when this object or one of its children has focus.
Key events are first given to the FocusNode that has primary focus, and if its onKey method return false, then they are given to each ancestor node up the focus hierarchy in turn. If an event reaches the root of the hierarchy, it is discarded.
This is not the way to get text input in the manner of a text field: it leaves out support for input method editors, and doesn't support soft keyboards in general. For text input, consider TextField, EditableText, or CupertinoTextField instead, which do support these things.
final FocusOnKeyCallback onKey