ScrollPosition class Null safety

Determines which portion of the content is visible in a scroll view.

The pixels value determines the scroll offset that the scroll view uses to select which part of its content to display. As the user scrolls the viewport, this value changes, which changes the content that is displayed.

The ScrollPosition applies physics to scrolling, and stores the minScrollExtent and maxScrollExtent.

Scrolling is controlled by the current activity, which is set by beginActivity. ScrollPosition itself does not start any activities. Instead, concrete subclasses, such as ScrollPositionWithSingleContext, typically start activities in response to user input or instructions from a ScrollController.

This object is a Listenable that notifies its listeners when pixels changes.

Subclassing ScrollPosition

Over time, a Scrollable might have many different ScrollPosition objects. For example, if Scrollable.physics changes type, Scrollable creates a new ScrollPosition with the new physics. To transfer state from the old instance to the new instance, subclasses implement absorb. See absorb for more details.

Subclasses also need to call didUpdateScrollDirection whenever userScrollDirection changes values.

Constructors

ScrollPosition({required ScrollPhysics physics, required ScrollContext context, bool keepScrollOffset = true, ScrollPosition? oldPosition, String? debugLabel}): Creates an object that determines which portion of the content is visible in a scroll view.

Properties

activity → ScrollActivity?: The currently operative ScrollActivity.
protected">@protectedvisibleForTesting">@visibleForTestingread-only
allowImplicitScrolling → bool: Whether a viewport is allowed to change pixels implicitly to respond to a call to RenderObject.showOnScreen.
read-onlyoverride
atEdge → bool: Whether the pixels value is exactly at the minScrollExtent or the maxScrollExtent.
read-onlyinherited
axis → Axis: The axis in which the scroll view scrolls.
read-onlyinherited
axisDirection → AxisDirection: The direction in which the scroll view scrolls.
read-onlyinherited
context → ScrollContext: Where the scrolling is taking place.
final
debugLabel → String?: A label that is used in the toString output.
final
extentAfter → double: The quantity of content conceptually "below" the viewport in the scrollable. This is the content below the content described by extentInside.
read-onlyinherited
extentBefore → double: The quantity of content conceptually "above" the viewport in the scrollable. This is the content above the content described by extentInside.
read-onlyinherited
extentInside → double: The quantity of content conceptually "inside" the viewport in the scrollable.
read-onlyinherited
hasContentDimensions → bool: Whether the minScrollExtent and the maxScrollExtent properties are available.
read-onlyoverride
hashCode → int: The hash code for this object.
read-onlyinherited
hasListeners → bool: Whether any listeners are currently registered.
protected">@protectedread-onlyinherited
hasPixels → bool: Whether the pixels property is available.
read-onlyoverride
hasViewportDimension → bool: Whether the viewportDimension property is available.
read-onlyoverride
haveDimensions → bool: Whether viewportDimension, minScrollExtent, maxScrollExtent, outOfRange, and atEdge are available.
read-only
isScrollingNotifier → ValueNotifier<bool>: This notifier's value is true if a scroll is underway and false if the scroll position is idle.
final
keepScrollOffset → bool: Save the current scroll offset with PageStorage and restore it if this scroll position's scrollable is recreated.
final
maxScrollExtent → double: The maximum in-range value for pixels.
read-onlyoverride
minScrollExtent → double: The minimum in-range value for pixels.
read-onlyoverride
outOfRange → bool: Whether the pixels value is outside the minScrollExtent and maxScrollExtent.
read-onlyinherited
physics → ScrollPhysics: How the scroll position should respond to user input.
final
pixels → double: The number of pixels to offset the children in the opposite of the axis direction.
read-onlyoverride
runtimeType → Type: A representation of the runtime type of the object.
read-onlyinherited
userScrollDirection → ScrollDirection: The direction in which the user is trying to change pixels, relative to the viewport's RenderViewportBase.axisDirection.
read-onlyinherited
viewportDimension → double: The extent of the viewport along the axisDirection.
read-onlyoverride

Methods

absorb(ScrollPosition other) → void: Take any current applicable state from the given ScrollPosition.
mustCallSuper">@mustCallSuperprotected">@protected
addListener(VoidCallback listener) → void: Register a closure to be called when the object changes.
inherited
animateTo(double to, {required Duration duration, required Curve curve}) → Future<void>: Animates the position from its current value to the given value.
override
applyBoundaryConditions(double value) → double: Returns the overscroll by applying the boundary conditions.
protected">@protected
applyContentDimensions(double minScrollExtent, double maxScrollExtent) → bool: Called when the viewport's content extents are established.
override
applyNewDimensions() → void: Notifies the activity that the dimensions of the underlying viewport or contents have changed.
mustCallSuper">@mustCallSuperprotected">@protected
applyViewportDimension(double viewportDimension) → bool: Called when the viewport's extents are established.
override
beginActivity(ScrollActivity? newActivity) → void: Change the current activity, disposing of the old one and sending scroll notifications as necessary.
copyWith({double? minScrollExtent, double? maxScrollExtent, double? pixels, double? viewportDimension, AxisDirection? axisDirection}) → ScrollMetrics: Creates a ScrollMetrics that has the same properties as this object.
inherited
correctBy(double correction) → void: Apply a layout-time correction to the scroll offset.
override
correctForNewDimensions(ScrollMetrics oldPosition, ScrollMetrics newPosition) → bool: Verifies that the new content and viewport dimensions are acceptable.
protected">@protected
correctPixels(double value) → void: Change the value of pixels to the new value, without notifying any customers.
debugFillDescription(List<String> description) → void: Add additional information to the given description for use by toString.
override
didEndScroll() → void: Called by beginActivity to report when an activity has ended.
didOverscrollBy(double value) → void: Called by setPixels to report overscroll when an attempt is made to change the pixels position. Overscroll is the amount of change that was not applied to the pixels value.
didStartScroll() → void: Called by beginActivity to report when an activity has started.
didUpdateScrollDirection(ScrollDirection direction) → void: Dispatches a notification that the userScrollDirection has changed.
didUpdateScrollMetrics() → void: Dispatches a notification that the ScrollMetrics have changed.
didUpdateScrollPositionBy(double delta) → void: Called by setPixels to report a change to the pixels position.
dispose() → void: Discards any resources used by the object. After this is called, the object is not in a usable state and should be discarded (calls to addListener will throw after the object is disposed).
override
drag(DragStartDetails details, VoidCallback dragCancelCallback) → Drag: Start a drag activity corresponding to the given DragStartDetails.
ensureVisible(RenderObject object, {double alignment = 0.0, Duration duration = Duration.zero, Curve curve = Curves.ease, ScrollPositionAlignmentPolicy alignmentPolicy = ScrollPositionAlignmentPolicy.explicit, RenderObject? targetRenderObject}) → Future<void>: Animates the position such that the given object is as visible as possible by just scrolling this position.
forcePixels(double value) → void: Change the value of pixels to the new value, and notify any customers, but without honoring normal conventions for changing the scroll offset.
protected">@protected
hold(VoidCallback holdCancelCallback) → ScrollHoldController: Stop the current activity and start a HoldScrollActivity.
jumpTo(double value) → void: Jumps the scroll position from its current value to the given value, without animation, and without checking if the new value is in range.
override
jumpToWithoutSettling(double value) → void: Deprecated. Use jumpTo or a custom ScrollPosition instead.
Deprecated('This will lead to bugs.')">@Deprecated('This will lead to bugs.')
moveTo(double to, {Duration? duration, Curve? curve, bool? clamp = true}) → Future<void>: Calls jumpTo if duration is null or Duration.zero, otherwise animateTo is called.
override
noSuchMethod(Invocation invocation) → dynamic: Invoked when a non-existent method or property is accessed.
inherited
notifyListeners() → void: Call all the registered listeners.
override
pointerScroll(double delta) → void: Changes the scrolling position based on a pointer signal from current value to delta without animation and without checking if new value is in range, taking min/max scroll extent into account.
recommendDeferredLoading(BuildContext context) → bool: Provides a heuristic to determine if expensive frame-bound tasks should be deferred.
removeListener(VoidCallback listener) → void: Remove a previously registered closure from the list of closures that are notified when the object changes.
inherited
restoreOffset(double offset, {bool initialRestore = false}) → void: Called by context to restore the scroll offset to the provided value.
restoreScrollOffset() → void: Called whenever the ScrollPosition is created, to restore the scroll offset if possible.
protected">@protected
saveOffset() → void: Called whenever scrolling ends, to persist the current scroll offset for state restoration purposes.
protected">@protected
saveScrollOffset() → void: Called whenever scrolling ends, to store the current scroll offset in a storage mechanism with a lifetime that matches the app's lifetime.
protected">@protected
setPixels(double newPixels) → double: Update the scroll position (pixels) to a given pixel value.
toString() → String: A string representation of this object.
inherited

Operators

operator ==(Object other) → bool: The equality operator.
inherited