When does setState trigger a rebuild in Flutter?

Your situation summarized

You currently have:

setState(() {
futureWait = "$fifteen second wait for both futures completed";
});
futureWaitBool = true;

You’re asking:

• Is there a race condition if you move futureWaitBool = true; inside setState?
• When exactly does the rebuild happen?

How does setState work internally?

1. You call setState(fn).

2. Flutter:

   • Marks the state object as needing build (dirty).
   • Executes fn() immediately, synchronously.

3. After the current Dart event loop completes (i.e. after your function returns), Flutter’s scheduler triggers a rebuild for all dirty widgets before the next frame is rendered.

When does the rebuild happen?

• The rebuild is scheduled when you call setState().
• The closure runs immediately within the setState call.
• The actual build happens after setState completes, before the next frame is rendered.

Is there a race condition in your original example?

setState(() {
  futureWait = "...";
});
futureWaitBool = true;

No race condition (because both statements run sequentially on the main isolate). However, you updated futureWaitBool outside of setState, so:

• The framework is not notified that futureWaitBool changed.
• Your UI will not rebuild to reflect the change until another setState is called later.

Best practice

Wrap all state changes that affect your UI within the same setState() call:

setState(() {
  futureWait = "$fifteen second wait for both futures completed";
  futureWaitBool = true;
});

This ensures:

• Single rebuild for related state changes.
• No missing updates.

Documentation interpretation

Official docs state:

“Calling setState notifies the framework that the internal state of this object has changed ... which causes the framework to schedule a build for this State object.”

Summary:

• The notification happens when you call setState().
• Your closure runs immediately within setState.
• The rebuild is scheduled after setState completes.

Further reading

• Flutter State.setState API docs
• Flutter’s source code for setState implementation.
• Articles on Flutter’s rendering pipeline for a deeper understanding.

Bottom line

No race conditions here. setState:

1. Runs your closure immediately and synchronously.
2. Marks the widget dirty.
3. Schedules a rebuild after your function completes.

Always update all UI-affecting state inside setState() for predictable behavior.