If indeed all that is needed is a callback, without all the complexity (or
flexibility) of Actions and Shortcuts, you can already use a Focus widget
for this. For example, here’s the implementation of Flutter’s simple
CallbackShortcuts widget that takes a map of activators and executes 
callbacks for them:

This may be all that is needed for some apps.

Shortcuts

As you’ll see below, actions are useful on their own, but the most common use
case involves binding them to a keyboard shortcut. This is what the Shortcuts
widget is for.

The ShortcutManager

The shortcut manager, a longer-lived object than the Shortcuts widget, passes
on key events when it receives them. It contains the logic for deciding how to
handle the keys, the logic for walking up the tree to find other shortcut
mappings, and maintains a map of key combinations to intents.