Flutter: Does it matter what code is in setState()?

You should do all your mutations inside the closure, and the computing outside of the closure.

According to the docs:

@protected

void setState (
    VoidCallback fn
)

Notify the framework that the internal state of this object has changed.

Whenever you change the internal state of a State object, make the change in a function that you pass to setState:

setState(() { _myState = newValue });

The provided callback is immediately called synchronously. It must not return a future (the callback cannot be async), since then it would be unclear when the state was actually being set.

And also

Generally it is recommended that the setState method only be used to wrap the actual changes to the state, not any computation that might be associated with the change.

It's doesn't explicitly say any thing about if it's different to call is in or outside the function. But it does recommend to put it inside. And in my opinion it's also more readable and logical if you put the changes in side the setState function