RenderViewport class

A render object that is bigger on the inside.

RenderViewport is the visual workhorse of the scrolling machinery. It displays a subset of its children according to its own dimensions and the given offset. As the offset varies, different children are visible through the viewport.

RenderViewport hosts a bidirectional list of slivers in a single shared Axis, anchored on a center sliver, which is placed at the zero scroll offset. The center widget is displayed in the viewport according to the anchor property.

Slivers that are earlier in the child list than center are displayed in reverse order in the reverse axisDirection starting from the center. For example, if the axisDirection is AxisDirection.down, the first sliver before center is placed above the center. The slivers that are later in the child list than center are placed in order in the axisDirection. For example, in the preceding scenario, the first sliver after center is placed below the center.

Most scroll views by default are ordered GrowthDirection.forward. Changing the default values of ScrollView.anchor, ScrollView.center, or both, can configure a scroll view for GrowthDirection.reverse.

This sample shows a CustomScrollView, with Radio buttons in the AppBar.bottom that change the AxisDirection to illustrate different configurations. The CustomScrollView.anchor and CustomScrollView.center properties are also set to have the 0 scroll offset positioned in the middle of the viewport, with GrowthDirection.forward and GrowthDirection.reverse illustrated on either side. The sliver that shares the CustomScrollView.center key is positioned at the CustomScrollView.anchor.

To create a local project with this code sample, run:
flutter create --sample=cupertino.GrowthDirection.1 mysample

RenderViewport cannot contain RenderBox children directly. Instead, use a RenderSliverList, RenderSliverFixedExtentList, RenderSliverGrid, or a RenderSliverToBoxAdapter, for example.

Constructors

RenderViewport({AxisDirection axisDirection = AxisDirection.down, required AxisDirection crossAxisDirection, required ViewportOffset offset, double anchor = 0.0, List<RenderSliver>? children, RenderSliver? center, double? cacheExtent, CacheExtentStyle cacheExtentStyle = CacheExtentStyle.pixel, Clip clipBehavior = Clip.hardEdge}): Creates a viewport for RenderSliver objects.

Properties

alwaysNeedsCompositing → bool: Whether this render object always needs compositing.
no setterinherited
anchor ↔ double: The relative position of the zero scroll offset.
getter/setter pair
attached → bool: Whether the render tree this render object belongs to is attached to a PipelineOwner.
no setterinherited
axis → Axis: The axis along which the viewport scrolls.
no setterinherited
axisDirection ↔ AxisDirection: The direction in which the SliverConstraints.scrollOffset increases.
getter/setter pairinherited
cacheExtent ↔ double?: The viewport has an area before and after the visible area to cache items that are about to become visible when the user scrolls.
getter/setter pairinherited
cacheExtentStyle ↔ CacheExtentStyle: Controls how the cacheExtent is interpreted.
getter/setter pairinherited
center ↔ RenderSliver?: The first child in the GrowthDirection.forward growth direction.
getter/setter pair
childCount → int: The number of children.
no setterinherited
childrenInHitTestOrder → Iterable<RenderSliver>: Provides an iterable that walks the children of the viewport, in the order that hit-testing should use.
no setteroverride
childrenInPaintOrder → Iterable<RenderSliver>: Provides an iterable that walks the children of the viewport, in the order that they should be painted.
no setteroverride
clipBehavior ↔ Clip: The content will be clipped (or not) according to this option.
getter/setter pairinherited
constraints → BoxConstraints: The box constraints most recently received from the parent.
no setterinherited
crossAxisDirection ↔ AxisDirection: The direction in which child should be laid out in the cross axis.
getter/setter pairinherited
debugCanParentUseSize → bool: Whether the parent render object is permitted to use this render object's size.
no setterinherited
debugCreator ↔ Object?: The object responsible for creating this render object.
getter/setter pairinherited
debugDisposed → bool?: Whether this has been disposed.
no setterinherited
debugDoingThisLayout → bool: Whether performLayout for this render object is currently running.
no setterinherited
debugDoingThisLayoutWithCallback → bool: Whether invokeLayoutCallback for this render object is currently running.
no setterinherited
debugDoingThisPaint → bool: Whether paint for this render object is currently running.
no setterinherited
debugDoingThisResize → bool: Whether performResize for this render object is currently running.
no setterinherited
debugLayer → ContainerLayer?: In debug mode, the compositing layer that this render object uses to repaint.
no setterinherited
debugLayoutParent → RenderObject?: The RenderObject that's expected to call layout on this RenderObject in its performLayout implementation.
no setterinherited
debugNeedsCompositedLayerUpdate → bool: Whether this render object's layer information is dirty.
no setterinherited
debugNeedsLayout → bool: Whether this render object's layout information is dirty.
no setterinherited
debugNeedsPaint → bool: Whether this render object's paint information is dirty.
no setterinherited
debugSemantics → SemanticsNode?: The semantics of this render object.
no setterinherited
depth → int: The depth of this render object in the render tree.
no setterinherited
firstChild → RenderSliver?: The first child in the child list.
no setterinherited
hashCode → int: The hash code for this object.
no setterinherited
hasSize → bool: Whether this render object has undergone layout and has a size.
no setterinherited
hasVisualOverflow → bool: Whether the contents of this viewport would paint outside the bounds of the viewport if paint did not clip.
no setteroverride
indexOfFirstChild → int: The index of the first child of the viewport relative to the center child.
no setteroverride
isRepaintBoundary → bool: Whether this render object repaints separately from its parent.
no setterinherited
lastChild → RenderSliver?: The last child in the child list.
no setterinherited
layer ↔ ContainerLayer?: The compositing layer that this render object uses to repaint.
getter/setter pairinherited
needsCompositing → bool: Whether we or one of our descendants has a compositing layer.
no setterinherited
offset ↔ ViewportOffset: Which part of the content inside the viewport should be visible.
getter/setter pairinherited
owner → PipelineOwner?: The owner for this render object (null if unattached).
no setterinherited
paintBounds → Rect: Returns a rectangle that contains all the pixels painted by this box.
no setterinherited
parent → RenderObject?: The parent of this render object in the render tree.
no setterinherited
parentData ↔ ParentData?: Data for use by the parent render object.
getter/setter pairinherited
runtimeType → Type: A representation of the runtime type of the object.
no setterinherited
semanticBounds → Rect: The bounding box, in the local coordinate system, of this object, for accessibility purposes.
no setterinherited
size ↔ Size: The size of this render box computed during layout.
getter/setter pairinherited
sizedByParent → bool: Whether the constraints are the only input to the sizing algorithm (in particular, child nodes have no impact).
no setteroverride

Methods

add(RenderSliver child) → void: Append child to the end of this render object's child list.
inherited
addAll(List<RenderSliver>? children) → void: Add all the children to the end of this render object's child list.
inherited
adoptChild(RenderObject child) → void: Called by subclasses when they decide a render object is a child.
inherited
applyPaintTransform(covariant RenderObject child, Matrix4 transform) → void: Multiply the transform from the parent's coordinate system to this box's coordinate system into the given transform.
override
assembleSemanticsNode(SemanticsNode node, SemanticsConfiguration config, Iterable<SemanticsNode> children) → void: Assemble the SemanticsNode for this RenderObject.
inherited
attach(PipelineOwner owner) → void: Mark this render object as attached to the given owner.
inherited
childAfter(RenderSliver child) → RenderSliver?: The next child after the given child in the child list.
inherited
childBefore(RenderSliver child) → RenderSliver?: The previous child before the given child in the child list.
inherited
clearSemantics() → void: Removes all semantics from this render object and its descendants.
inherited
computeAbsolutePaintOffset(RenderSliver child, double layoutOffset, GrowthDirection growthDirection) → Offset: The offset at which the given child should be painted.
inherited
computeChildMainAxisPosition(RenderSliver child, double parentMainAxisPosition) → double: Converts the parentMainAxisPosition into the child's coordinate system.
override
computeDistanceToActualBaseline(TextBaseline baseline) → double?: Returns the distance from the y-coordinate of the position of the box to the y-coordinate of the first given baseline in the box's contents, if any, or null otherwise.
inherited
computeDryBaseline(covariant BoxConstraints constraints, TextBaseline baseline) → double?: Computes the value returned by getDryBaseline.
inherited
computeDryLayout(covariant BoxConstraints constraints) → Size: Computes the value returned by getDryLayout. Do not call this function directly, instead, call getDryLayout.
override
computeMaxIntrinsicHeight(double width) → double: Computes the value returned by getMaxIntrinsicHeight. Do not call this function directly, instead, call getMaxIntrinsicHeight.
inherited
computeMaxIntrinsicWidth(double height) → double: Computes the value returned by getMaxIntrinsicWidth. Do not call this function directly, instead, call getMaxIntrinsicWidth.
inherited
computeMinIntrinsicHeight(double width) → double: Computes the value returned by getMinIntrinsicHeight. Do not call this function directly, instead, call getMinIntrinsicHeight.
inherited
computeMinIntrinsicWidth(double height) → double: Computes the value returned by getMinIntrinsicWidth. Do not call this function directly, instead, call getMinIntrinsicWidth.
inherited
debugAdoptSize(Size value) → Size: Claims ownership of the given Size.
inherited
debugAssertDoesMeetConstraints() → void: Verify that the object's constraints are being met. Override this function in a subclass to verify that your state matches the constraints object. This function is only called when asserts are enabled (i.e. in debug mode) and only when needsLayout is false. If the constraints are not met, it should assert or throw an exception.
inherited
debugCannotComputeDryLayout({String? reason, FlutterError? error}) → bool: Called from computeDryLayout or computeDryBaseline within an assert if the given RenderBox subclass does not support calculating a dry layout.
inherited
debugDescribeChildren() → List<DiagnosticsNode>: Returns a list of DiagnosticsNode objects describing this node's children.
inherited
debugFillProperties(DiagnosticPropertiesBuilder properties) → void: Add additional properties associated with the node.
override
debugHandleEvent(PointerEvent event, HitTestEntry<HitTestTarget> entry) → bool: Implements the debugPaintPointersEnabled debugging feature.
inherited
debugPaint(PaintingContext context, Offset offset) → void: Override this method to paint debugging information.
inherited
debugPaintBaselines(PaintingContext context, Offset offset) → void: In debug mode, paints a line for each baseline.
inherited
debugPaintPointers(PaintingContext context, Offset offset) → void: In debug mode, paints a rectangle if this render box has counted more pointer downs than pointer up events.
inherited
debugPaintSize(PaintingContext context, Offset offset) → void: In debug mode, paints a border around this render box.
inherited
debugRegisterRepaintBoundaryPaint({bool includedParent = true, bool includedChild = false}) → void: Called, in debug mode, if isRepaintBoundary is true, when either the this render object or its parent attempt to paint.
inherited
debugResetSize() → void: If a subclass has a "size" (the state controlled by parentUsesSize, whatever it is in the subclass, e.g. the actual size property of RenderBox), and the subclass verifies that in debug mode this "size" property isn't used when debugCanParentUseSize isn't set, then that subclass should override debugResetSize to reapply the current values of debugCanParentUseSize to that state.
inherited
debugThrowIfNotCheckingIntrinsics() → bool: Throws an exception saying that the object does not support returning intrinsic dimensions if, in debug mode, we are not in the RenderObject.debugCheckingIntrinsics mode.
inherited
debugValidateChild(RenderObject child) → bool: Checks whether the given render object has the correct runtimeType to be a child of this render object.
inherited
describeApproximatePaintClip(covariant RenderSliver child) → Rect?: Returns a rect in this object's coordinate system that describes the approximate bounding box of the clip rect that would be applied to the given child during the paint phase, if any.
inherited
describeForError(String name, {DiagnosticsTreeStyle style = DiagnosticsTreeStyle.shallow}) → DiagnosticsNode: Adds a debug representation of a RenderObject optimized for including in error messages.
inherited
describeSemanticsClip(covariant RenderSliver? child) → Rect: Returns a rect in this object's coordinate system that describes which SemanticsNodes produced by the child should be included in the semantics tree. SemanticsNodes from the child that are positioned outside of this rect will be dropped. Child SemanticsNodes that are positioned inside this rect, but outside of describeApproximatePaintClip will be included in the tree marked as hidden. Child SemanticsNodes that are inside of both rect will be included in the tree as regular nodes.
inherited
describeSemanticsConfiguration(SemanticsConfiguration config) → void: Report the semantics of this node, for example for accessibility purposes.
inherited
detach() → void: Mark this render object as detached from its PipelineOwner.
inherited
dispose() → void: Release any resources held by this render object.
inherited
dropChild(RenderObject child) → void: Called by subclasses when they decide a render object is no longer a child.
inherited
getDistanceToActualBaseline(TextBaseline baseline) → double?: Calls computeDistanceToActualBaseline and caches the result.
inherited
getDistanceToBaseline(TextBaseline baseline, {bool onlyReal = false}) → double?: Returns the distance from the y-coordinate of the position of the box to the y-coordinate of the first given baseline in the box's contents.
inherited
getDryBaseline(covariant BoxConstraints constraints, TextBaseline baseline) → double?: Returns the distance from the top of the box to the first baseline of the box's contents for the given constraints, or null if this RenderBox does not have any baselines.
inherited
getDryLayout(covariant BoxConstraints constraints) → Size: Returns the Size that this RenderBox would like to be given the provided BoxConstraints.
inherited
getMaxIntrinsicHeight(double width) → double: Returns the smallest height beyond which increasing the height never decreases the preferred width. The preferred width is the value that would be returned by getMinIntrinsicWidth for that height.
inherited
getMaxIntrinsicWidth(double height) → double: Returns the smallest width beyond which increasing the width never decreases the preferred height. The preferred height is the value that would be returned by getMinIntrinsicHeight for that width.
inherited
getMinIntrinsicHeight(double width) → double: Returns the minimum height that this box could be without failing to correctly paint its contents within itself, without clipping.
inherited
getMinIntrinsicWidth(double height) → double: Returns the minimum width that this box could be without failing to correctly paint its contents within itself, without clipping.
inherited
getOffsetToReveal(RenderObject target, double alignment, {Rect? rect, Axis? axis}) → RevealedOffset: Returns the offset that would be needed to reveal the target RenderObject.
inherited
getTransformTo(RenderObject? target) → Matrix4: Applies the paint transform from this RenderObject to the target RenderObject.
inherited
globalToLocal(Offset point, {RenderObject? ancestor}) → Offset: Convert the given point from the global coordinate system in logical pixels to the local coordinate system for this box.
inherited
handleEvent(PointerEvent event, covariant BoxHitTestEntry entry) → void: Override this method to handle pointer events that hit this render object.
inherited
hitTest(BoxHitTestResult result, {required Offset position}) → bool: Determines the set of render objects located at the given position.
inherited
hitTestChildren(BoxHitTestResult result, {required Offset position}) → bool: Override this method to check whether any children are located at the given position.
inherited
hitTestSelf(Offset position) → bool: Override this method if this render object can be hit even if its children were not hit.
inherited
insert(RenderSliver child, {RenderSliver? after}) → void: Insert child into this render object's child list after the given child.
inherited
invokeLayoutCallback<T extends Constraints>(LayoutCallback<T> callback) → void: Allows mutations to be made to this object's child list (and any descendants) as well as to any other dirty nodes in the render tree owned by the same PipelineOwner as this object. The callback argument is invoked synchronously, and the mutations are allowed only during that callback's execution.
inherited
labelForChild(int index) → String: A short string to identify the child with the given index.
override
layout(Constraints constraints, {bool parentUsesSize = false}) → void: Compute the layout for this render object.
inherited
layoutChildSequence({required RenderSliver? child, required double scrollOffset, required double overlap, required double layoutOffset, required double remainingPaintExtent, required double mainAxisExtent, required double crossAxisExtent, required GrowthDirection growthDirection, required RenderSliver? advance(RenderSliver child), required double remainingCacheExtent, required double cacheOrigin}) → double: Determines the size and position of some of the children of the viewport.
inherited
localToGlobal(Offset point, {RenderObject? ancestor}) → Offset: Convert the given point from the local coordinate system for this box to the global coordinate system in logical pixels.
inherited
markNeedsCompositedLayerUpdate() → void: Mark this render object as having changed a property on its composited layer.
inherited
markNeedsCompositingBitsUpdate() → void: Mark the compositing state for this render object as dirty.
inherited
markNeedsLayout() → void: Mark this render object's layout information as dirty, and either register this object with its PipelineOwner, or defer to the parent, depending on whether this object is a relayout boundary or not respectively.
inherited
markNeedsLayoutForSizedByParentChange() → void: Mark this render object's layout information as dirty (like markNeedsLayout), and additionally also handle any necessary work to handle the case where sizedByParent has changed value.
inherited
markNeedsPaint() → void: Mark this render object as having changed its visual appearance.
inherited
markNeedsSemanticsUpdate() → void: Mark this node as needing an update to its semantics description.
inherited
markParentNeedsLayout() → void: Mark this render object's layout information as dirty, and then defer to the parent.
inherited
maxScrollObstructionExtentBefore(RenderSliver child) → double: Returns the total scroll obstruction extent of all slivers in the viewport before child.
override
move(RenderSliver child, {RenderSliver? after}) → void: Move the given child in the child list to be after another child.
inherited
noSuchMethod(Invocation invocation) → dynamic: Invoked when a nonexistent method or property is accessed.
inherited
paint(PaintingContext context, Offset offset) → void: Paint this render object into the given context at the given offset.
inherited
paintOffsetOf(RenderSliver child) → Offset: The offset at which the given child should be painted.
override
paintsChild(covariant RenderObject child) → bool: Whether the given child would be painted if paint were called.
inherited
performLayout() → void: Do the work of computing the layout for this render object.
override
performResize() → void: Updates the render objects size using only the constraints.
inherited
reassemble() → void: Cause the entire subtree rooted at the given RenderObject to be marked dirty for layout, paint, etc, so that the effects of a hot reload can be seen, or so that the effect of changing a global debug flag (such as debugPaintSizeEnabled) can be applied.
inherited
redepthChild(RenderObject child) → void: Adjust the depth of the given child to be greater than this node's own depth.
inherited
redepthChildren() → void: Adjust the depth of this node's children, if any.
inherited
remove(RenderSliver child) → void: Remove this child from the child list.
inherited
removeAll() → void: Remove all their children from this render object's child list.
inherited
replaceRootLayer(OffsetLayer rootLayer) → void: Replace the layer. This is only valid for the root of a render object subtree (whatever object scheduleInitialPaint was called on).
inherited
scheduleInitialLayout() → void: Bootstrap the rendering pipeline by scheduling the very first layout.
inherited
scheduleInitialPaint(ContainerLayer rootLayer) → void: Bootstrap the rendering pipeline by scheduling the very first paint.
inherited
scheduleInitialSemantics() → void: Bootstrap the semantics reporting mechanism by marking this node as needing a semantics update.
inherited
scrollOffsetOf(RenderSliver child, double scrollOffsetWithinChild) → double: Returns the scroll offset within the viewport for the given scrollOffsetWithinChild within the given child.
override
sendSemanticsEvent(SemanticsEvent semanticsEvent) → void: Sends a SemanticsEvent associated with this render object's SemanticsNode.
inherited
setupParentData(covariant RenderObject child) → void: Override to setup parent data correctly for your children.
override
showOnScreen({RenderObject? descendant, Rect? rect, Duration duration = Duration.zero, Curve curve = Curves.ease}) → void: Attempt to make (a portion of) this or a descendant RenderObject visible on screen.
inherited
toDiagnosticsNode({String? name, DiagnosticsTreeStyle? style}) → DiagnosticsNode: Returns a debug representation of the object that is used by debugging tools and by DiagnosticsNode.toStringDeep.
inherited
toString({DiagnosticLevel minLevel = DiagnosticLevel.info}) → String: A string representation of this object.
inherited
toStringDeep({String prefixLineOne = '', String? prefixOtherLines = '', DiagnosticLevel minLevel = DiagnosticLevel.debug}) → String: Returns a description of the tree rooted at this node. If the prefix argument is provided, then every line in the output will be prefixed by that string.
inherited
toStringShallow({String joiner = ', ', DiagnosticLevel minLevel = DiagnosticLevel.debug}) → String: Returns a one-line detailed description of the render object. This description is often somewhat long.
inherited
toStringShort() → String: Returns a human understandable name.
inherited
updateChildLayoutOffset(RenderSliver child, double layoutOffset, GrowthDirection growthDirection) → void: Called during layoutChildSequence to store the layout offset for the given child.
override
updateCompositedLayer({required covariant OffsetLayer? oldLayer}) → OffsetLayer: Update the composited layer owned by this render object.
inherited
updateOutOfBandData(GrowthDirection growthDirection, SliverGeometry childLayoutGeometry) → void: Called during layoutChildSequence for each child.
override
visitChildren(RenderObjectVisitor visitor) → void: Calls visitor for each immediate child of this render object.
inherited
visitChildrenForSemantics(RenderObjectVisitor visitor) → void: Called when collecting the semantics of this node.
inherited

Operators

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

Constants

excludeFromScrolling → const SemanticsTag: When a top-level SemanticsNode below a RenderAbstractViewport is tagged with excludeFromScrolling it will not be part of the scrolling area for semantic purposes.
useTwoPaneSemantics → const SemanticsTag: If a RenderAbstractViewport overrides RenderObject.describeSemanticsConfiguration to add the SemanticsTag useTwoPaneSemantics to its SemanticsConfiguration, two semantics nodes will be used to represent the viewport with its associated scrolling actions in the semantics tree.