The image in this question includes a Flutter app interface. Which Flutter widget has been added to this app title bar to add this navigation button?

The following code is used to add a floating action button to a Flutter app interface.
Which of the following function or method can be used to add an action to this button if the app user taps this button?
Select the correct choice below to complete the following sentence:
SnackBar widget is used if you want to let your app pop up a message............... at the bottom of your app interface.
A Stateful widget is a dynamic widget which can change the appearance of its content in response to events triggered by user's interactions or when it receives data.
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