1. 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.

Calling setState notifies the framework that the internal state of this object has changed in a way that might impact the user interface in this subtree, which causes the framework to schedule a build for this State object.

If you just change the state directly without calling setState, the framework might not schedule a build and the user interface for this subtree might not be updated to reflect the new state.

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. For example, here a value used by the build function is incremented, and then the change is written to disk, but only the increment is wrapped in the setState:

Future<Null> _incrementCounter() async {
  setState(() {
    _counter++;
  });
  final String dir = await PathProvider.getApplicationDocumentsDirectory();
  await new File('$dir/counter.txt').writeAsString('$_counter');
  return null;
}

It is an error to call this method after the framework calls dispose. You can determine whether it is legal to call this method by checking whether the mounted property is true.

Source

@protected
void setState(VoidCallback fn) {
  assert(fn != null);
  assert(() {
    if (_debugLifecycleState == _StateLifecycle.defunct) {
      throw new FlutterError(
        'setState() called after dispose(): $this\n'
        'This error happens if you call setState() on a State object for a widget that '
        'no longer appears in the widget tree (e.g., whose parent widget no longer '
        'includes the widget in its build). This error can occur when code calls '
        'setState() from a timer or an animation callback. The preferred solution is '
        'to cancel the timer or stop listening to the animation in the dispose() '
        'callback. Another solution is to check the "mounted" property of this '
        'object before calling setState() to ensure the object is still in the '
        'tree.\n'
        'This error might indicate a memory leak if setState() is being called '
        'because another object is retaining a reference to this State object '
        'after it has been removed from the tree. To avoid memory leaks, '
        'consider breaking the reference to this object during dispose().'
      );
    }
    return true;
  });
  dynamic result = fn() as dynamic;
  assert(() {
    if (result is Future) {
      throw new FlutterError(
        'setState() callback argument returned a Future.\n'
        'The setState() method on $this was called with a closure or method that '
        'returned a Future. Maybe it is marked as "async".\n'
        'Instead of performing asynchronous work inside a call to setState(), first '
        'execute the work (without updating the widget state), and then synchronously '
        'update the state inside a call to setState().'
      );
    }
    // We ignore other types of return values so that you can do things like:
    //   setState(() => x = 3);
    return true;
  });
  _element.markNeedsBuild();
}