VisibilityDetector

A VisibilityDetector widget wraps an existing Flutter widget and fires a callback when the widget's visibility changes. (It actually reports when the visibility of the VisibilityDetector itself changes, and its visibility is expected to be identical to that of its child.)

Callbacks are not fired immediately on visibility changes. Instead, callbacks are deferred and coalesced such that the callback for each VisibilityDetector will be invoked at most once per VisibilityDetectorController.updateInterval (unless forced by VisibilityDetectorController.notifyNow()). Callbacks for all VisibilityDetector widgets are fired together synchronously between frames.

VisibilityDetectorController.notifyNow() may be used to force triggering pending visibility callbacks; this might be desirable just prior to tearing down the widget tree (such as when switching views or when exiting the application).

For more details, see the documentation to the VisibilityDetector, VisibilityInfo, and VisibilityDetectorController classes.

Example usage

@override
Widget build(BuildContext context) {
  return VisibilityDetector(
    key: Key('my-widget-key'),
    onVisibilityChanged: (visibilityInfo) {
      var visiblePercentage = visibilityInfo.visibleFraction * 100;
      debugPrint(
          'Widget ${visibilityInfo.key} is ${visiblePercentage}% visible');
    },
    child: someOtherWidget,
  );
}

See the example/ directory for a sample application. To build it, first create the default Flutter project files:

cd example
flutter create .

and then it can be run with flutter run.

Widget tests

Widget tests that use VisibilityDetectors usually should set:

VisibilityDetectorController.instance.updateInterval = Duration.zero;

This will have two effects:

1. Visibility changes will be reported immediately, which can be less surprising for automated tests.

2. It avoids the following assertion when tearing down the widget tree:

   The following assertion was thrown running a test:
   A Timer is still pending even after the widget tree was disposed.

   See flutter/flutter#24166 for details.

If setting updateInterval = Duration.zero is undesirable, to address each of the corresponding issues above, tests alternatively can:

1. Wait sufficiently long for callbacks to fire:

   await tester.pump(VisibilityDetectorController.instance.updateInterval);

2. Avoid the "Timer is still pending..." assertion by explicitly destroying the widget tree before the test completes:

   await tester.pumpWidget(Placeholder());

See test/widget_test.dart for examples.

Known limitations

• VisibilityDetector considers only its bounding box. It does not take widget opacity into account.

• The reported visibleFraction might not account for overlapping widgets that obscure the VisbilityDetector.