ScrollPositionWithSingleContext class

A scroll position that manages scroll activities for a single ScrollContext.

This class is a concrete subclass of ScrollPosition logic that handles a single ScrollContext, such as a Scrollable. An instance of this class manages ScrollActivity instances, which change what content is visible in the Scrollable's Viewport.

Accessing Scrolling Information

There are several ways to acquire information about scrolling and scrollable widgets, but each provides different types of information about the scrolling activity, the position, and the dimensions of the Viewport.

A ScrollController is a Listenable. It notifies its listeners whenever any of the attached ScrollPositions notify their listeners, such as when scrolling occurs. This is very similar to using a NotificationListener of type ScrollNotification to listen to changes in the scroll position, with the difference being that a notification listener will provide information about the scrolling activity. A notification listener can further listen to specific subclasses of ScrollNotification, like UserScrollNotification.

ScrollController does not notify its listeners when the list of ScrollPositions attached to the scroll controller changes. To listen to the attaching and detaching of scroll positions to the controller, use the ScrollController.onAttach and ScrollController.onDetach methods. This is also useful for adding a listener to the ScrollPosition.isScrollingNotifier when the position is created during the build method of the Scrollable.

At the time that a scroll position is attached, the ScrollMetrics, such as the ScrollMetrics.maxScrollExtent, are not yet available. These are not determined until the Scrollable has finished laying out its contents and computing things like the full extent of that content. ScrollPosition.hasContentDimensions can be used to know when the metrics are available, or a ScrollMetricsNotification can be used, discussed further below.

From a different context

When needing to access scrolling information from a context that is within the scrolling widget itself, use Scrollable.of to access the ScrollableState and the ScrollableState.position. This would be the same ScrollPosition attached to a ScrollController.

When needing to access scrolling information from a context that is not an ancestor of the scrolling widget, use ScrollNotificationObserver. This is used by AppBar to create the scrolled under effect. Since Scaffold.appBar is a separate subtree from the Scaffold.body, scroll notifications would not bubble up to the app bar. Use ScrollNotificationObserverState.addListener to listen to scroll notifications happening outside of the current context.

Dimension changes

Lastly, listening to a ScrollController or a ScrollPosition will not notify when the ScrollMetrics of a given scroll position changes, such as when the window is resized, changing the dimensions of the Viewport and the previously mentioned extents of the scrollable. In order to listen to changes in scroll metrics, use a NotificationListener of type ScrollMetricsNotification. This type of notification differs from ScrollNotification, as it is not associated with the activity of scrolling, but rather the dimensions of the scrollable area, such as the window size.

See also:

• ScrollPosition, which defines the underlying model for a position within a Scrollable but is agnostic as to how that position is changed.
• ScrollView and its subclasses such as ListView, which use ScrollPositionWithSingleContext to manage their scroll position.
• ScrollController, which can manipulate one or more ScrollPositions, and which uses ScrollPositionWithSingleContext as its default class for scroll positions.