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.

See also:

Inheritance
Mixes-in
Implemented by

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, read-only
context ScrollContext
Where the scrolling is taking place. [...]
final
debugLabel String
A label that is used in the toString output. Intended to aid with identifying animation controller instances in debug output.
final
haveDimensions bool
Whether viewportDimension, minScrollExtent, maxScrollExtent, outOfRange, and atEdge are available yet. [...]
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-only
minScrollExtent double
The minimum in-range value for pixels. [...]
read-only
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-only
viewportDimension double
The extent of the viewport along the axisDirection.
read-only
atEdge bool
Whether the pixels value is exactly at the minScrollExtent or the maxScrollExtent.
read-only, inherited
axis Axis
The axis in which the scroll view scrolls.
read-only, inherited
axisDirection AxisDirection
The direction in which the scroll view scrolls.
read-only, inherited
extentAfter double
The quantity of content conceptually "below" the currently visible content of the viewport in the scrollable. This is the content below the content described by extentInside.
read-only, inherited
extentBefore double
The quantity of content conceptually "above" the currently visible content of the viewport in the scrollable. This is the content above the content described by extentInside.
read-only, inherited
extentInside double
The quantity of visible content. [...]
read-only, inherited
hashCode int
The hash code for this object. [...]
read-only, inherited
outOfRange bool
Whether the pixels value is outside the minScrollExtent and maxScrollExtent.
read-only, inherited
runtimeType Type
A representation of the runtime type of the object.
read-only, inherited
userScrollDirection ScrollDirection
The direction in which the user is trying to change pixels, relative to the viewport's RenderViewport.axisDirection. [...]
read-only, inherited

Methods

absorb(ScrollPosition other) → void
Take any current applicable state from the given ScrollPosition. [...]
@mustCallSuper, @protected
animateTo(double to, { Duration duration, Curve curve }) Future<Null>
Animates the position from its current value to the given value. [...]
applyBoundaryConditions(double value) double
Returns the overscroll by applying the boundary conditions. [...]
@protected
applyContentDimensions(double minScrollExtent, double maxScrollExtent) bool
Called when the viewport's content extents are established. [...]
applyNewDimensions() → void
Notifies the activity that the dimensions of the underlying viewport or contents have changed. [...]
@mustCallSuper, @protected
applyViewportDimension(double viewportDimension) bool
Called when the viewport's extents are established. [...]
beginActivity(ScrollActivity newActivity) → void
Change the current activity, disposing of the old one and sending scroll notifications as necessary. [...]
correctBy(double correction) → void
Apply a layout-time correction to the scroll offset. [...]
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. [...]
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. [...]
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 and removeListener will throw after the object is disposed). [...]
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 }) Future<Null>
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
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. [...]
jumpToWithoutSettling(double value) → void
Deprecated. Use jumpTo or a custom ScrollPosition instead.
@Deprecated('This will lead to bugs.')
notifyListeners() → void
Call all the registered listeners. [...]
restoreScrollOffset() → void
Called whenever the ScrollPosition is created, to restore the scroll offset if possible. [...]
@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
setPixels(double newPixels) double
Update the scroll position (pixels) to a given pixel value. [...]
addListener(VoidCallback listener) → void
Register a closure to be called when the object changes. [...]
inherited
cloneMetrics() ScrollMetrics
Creates a ScrollMetrics that has the same properties as this object. [...]
inherited
noSuchMethod(Invocation invocation) → dynamic
Invoked when a non-existent method or property is accessed. [...]
inherited
removeListener(VoidCallback listener) → void
Remove a previously registered closure from the list of closures that are notified when the object changes. [...]
inherited
toString() String
Returns a string representation of this object.
inherited

Operators

operator ==(other) bool
The equality operator. [...]
inherited