of method

OverlayState of (BuildContext context, { Widget debugRequiredFor })

The state from the closest instance of this class that encloses the given context.

In checked mode, if the debugRequiredFor argument is provided then this function will assert that an overlay was found and will throw an exception if not. The exception attempts to explain that the calling Widget (the one given by the debugRequiredFor argument) needs an Overlay to be present to function.

Typical usage is as follows:

OverlayState overlay = Overlay.of(context);

Implementation 

static OverlayState of(BuildContext context, { Widget debugRequiredFor }) {
  final OverlayState result = context.ancestorStateOfType(const TypeMatcher<OverlayState>());
  assert(() {
    if (debugRequiredFor != null && result == null) {
      final String additional = context.widget != debugRequiredFor
        ? '\nThe context from which that widget was searching for an overlay was:\n  $context'
        : '';
      throw FlutterError(
        'No Overlay widget found.\n'
        '${debugRequiredFor.runtimeType} widgets require an Overlay widget ancestor for correct operation.\n'
        'The most common way to add an Overlay to an application is to include a MaterialApp or Navigator widget in the runApp() call.\n'
        'The specific widget that failed to find an overlay was:\n'
        '  $debugRequiredFor'
        '$additional'
      );
    }
    return true;
  }());
  return result;
}