ScrollNotification class abstract
A Notification related to scrolling.
Scrollable widgets notify their ancestors about scrolling-related changes. The notifications have the following lifecycle:
- A ScrollStartNotification, which indicates that the widget has started scrolling.
- Zero or more ScrollUpdateNotifications, which indicate that the widget has changed its scroll position, mixed with zero or more OverscrollNotifications, which indicate that the widget has not changed its scroll position because the change would have caused its scroll position to go outside its scroll bounds.
- Interspersed with the ScrollUpdateNotifications and OverscrollNotifications are zero or more UserScrollNotifications, which indicate that the user has changed the direction in which they are scrolling.
- A ScrollEndNotification, which indicates that the widget has stopped scrolling.
- A UserScrollNotification, with a UserScrollNotification.direction of ScrollDirection.idle.
Notifications bubble up through the tree, which means a given NotificationListener will receive notifications for all descendant Scrollable widgets. To focus on notifications from the nearest Scrollable descendant, check that the depth property of the notification is zero.
When a scroll notification is received by a NotificationListener, the listener will have already completed build and layout, and it is therefore too late for that widget to call State.setState. Any attempt to adjust the build or layout based on a scroll notification would result in a layout that lagged one frame behind, which is a poor user experience. Scroll notifications are therefore primarily useful for paint effects (since paint happens after layout). The GlowingOverscrollIndicator and Scrollbar widgets are examples of paint effects that use scroll notifications.
To create a local project with this code sample, run:
flutter create --sample=widgets.ScrollNotification.1 mysample
To drive layout based on the scroll position, consider listening to the ScrollPosition directly (or indirectly via a ScrollController). This 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. 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.
- context → BuildContext?
The build context of the widget that fired this notification.
- depth → int
The number of viewports that this notification has bubbled through.
- hashCode → int
The hash code for this object.
- metrics → ScrollMetrics
A description of a Scrollable's contents, useful for modeling the state
of its viewport.
- runtimeType → Type
A representation of the runtime type of the object.
List< String>description) → void
Add additional information to the given description for use by toString.
BuildContext? target) → void
Start bubbling this notification at the given build context.
Invocation invocation) → dynamic
Invoked when a nonexistent method or property is accessed.
) → String
A string representation of this object.