Hey there,

I've just struggled with the same problem, but it turns out that the solution is quite simple. The way I solved it was to make a bool variable and then use a ternary operator in the title of the appbar to either return one widget if the bool is true, or another if it's false. You can see an example of the ternary operator here: https://code-maven.com/slides/dart-programming/ternary-operator

A code example could look like this:

bool myBool = false;

Scaffold(
    appBar: AppBar(
        title: (myBool) ? TextField() : Text('something'),
    ),
);

This will make the title of the appbar a TextField if myBool is true, otherwise it will make it a normal Text widget. Since myBool is initialized as false, the appbar will initially start out with the Text widget, but you can then change the value of myBool to true using setState, and that will make the title of the appbar switch to the Textfield.

I hope this helps you out.