Finder byWidgetPredicate(WidgetPredicate predicate, { String description, bool skipOffstage: true })

Finds widgets using a widget predicate.

Example:

expect(tester, hasWidget(find.byWidgetPredicate(
  (Widget widget) => widget is Tooltip && widget.message == 'Back',
  description: 'widget with tooltip "Back"',
)));

If description is provided, then this uses it as the description of the Finder and appears, for example, in the error message when the finder fails to locate the desired widget. Otherwise, the description prints the signature of the predicate function.

If the skipOffstage argument is true (the default), then this skips nodes that are Offstage or that are from inactive Routes.

Source 

Finder byWidgetPredicate(WidgetPredicate predicate, { String description, bool skipOffstage: true }) {
  return new _WidgetPredicateFinder(predicate, description: description, skipOffstage: skipOffstage);
}