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// Copyright 2014 The Flutter Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
import 'dart:math' as math;
import 'package:flutter/rendering.dart';
import 'package:flutter/gestures.dart' show DragStartBehavior;
import 'basic.dart';
import 'framework.dart';
import 'primary_scroll_controller.dart';
import 'scroll_controller.dart';
import 'scroll_physics.dart';
import 'scrollable.dart';
/// A box in which a single widget can be scrolled.
///
/// This widget is useful when you have a single box that will normally be
/// entirely visible, for example a clock face in a time picker, but you need to
/// make sure it can be scrolled if the container gets too small in one axis
/// (the scroll direction).
///
/// It is also useful if you need to shrink-wrap in both axes (the main
/// scrolling direction as well as the cross axis), as one might see in a dialog
/// or pop-up menu. In that case, you might pair the [SingleChildScrollView]
/// with a [ListBody] child.
///
/// When you have a list of children and do not require cross-axis
/// shrink-wrapping behavior, for example a scrolling list that is always the
/// width of the screen, consider [ListView], which is vastly more efficient
/// that a [SingleChildScrollView] containing a [ListBody] or [Column] with
/// many children.
///
/// ## Sample code: Using [SingleChildScrollView] with a [Column]
///
/// Sometimes a layout is designed around the flexible properties of a
/// [Column], but there is the concern that in some cases, there might not
/// be enough room to see the entire contents. This could be because some
/// devices have unusually small screens, or because the application can
/// be used in landscape mode where the aspect ratio isn't what was
/// originally envisioned, or because the application is being shown in a
/// small window in split-screen mode. In any case, as a result, it might
/// make sense to wrap the layout in a [SingleChildScrollView].
///
/// Simply doing so, however, usually results in a conflict between the [Column],
/// which typically tries to grow as big as it can, and the [SingleChildScrollView],
/// which provides its children with an infinite amount of space.
///
/// To resolve this apparent conflict, there are a couple of techniques, as
/// discussed below. These techniques should only be used when the content is
/// normally expected to fit on the screen, so that the lazy instantiation of
/// a sliver-based [ListView] or [CustomScrollView] is not expected to provide
/// any performance benefit. If the viewport is expected to usually contain
/// content beyond the dimensions of the screen, then [SingleChildScrollView]
/// would be very expensive.
///
/// ### Centering, spacing, or aligning fixed-height content
///
/// If the content has fixed (or intrinsic) dimensions but needs to be spaced out,
/// centered, or otherwise positioned using the [Flex] layout model of a [Column],
/// the following technique can be used to provide the [Column] with a minimum
/// dimension while allowing it to shrink-wrap the contents when there isn't enough
/// room to apply these spacing or alignment needs.
///
/// A [LayoutBuilder] is used to obtain the size of the viewport (implicitly via
/// the constraints that the [SingleChildScrollView] sees, since viewports
/// typically grow to fit their maximum height constraint). Then, inside the
/// scroll view, a [ConstrainedBox] is used to set the minimum height of the
/// [Column].
///
/// The [Column] has no [Expanded] children, so rather than take on the infinite
/// height from its [BoxConstraints.maxHeight], (the viewport provides no maximum height
/// constraint), it automatically tries to shrink to fit its children. It cannot
/// be smaller than its [BoxConstraints.minHeight], though, and It therefore
/// becomes the bigger of the minimum height provided by the
/// [ConstrainedBox] and the sum of the heights of the children.
///
/// If the children aren't enough to fit that minimum size, the [Column] ends up
/// with some remaining space to allocate as specified by its
/// [Column.mainAxisAlignment] argument.
///
/// {@tool dartpad --template=stateless_widget_material}
/// In this example, the children are spaced out equally, unless there's no more
/// room, in which case they stack vertically and scroll.
///
/// When using this technique, [Expanded] and [Flexible] are not useful, because
/// in both cases the "available space" is infinite (since this is in a viewport).
/// The next section describes a technique for providing a maximum height constraint.
///
/// ```dart
/// Widget build(BuildContext context) {
/// return DefaultTextStyle(
/// style: Theme.of(context).textTheme.bodyText2,
/// child: LayoutBuilder(
/// builder: (BuildContext context, BoxConstraints viewportConstraints) {
/// return SingleChildScrollView(
/// child: ConstrainedBox(
/// constraints: BoxConstraints(
/// minHeight: viewportConstraints.maxHeight,
/// ),
/// child: Column(
/// mainAxisSize: MainAxisSize.min,
/// mainAxisAlignment: MainAxisAlignment.spaceAround,
/// children: <Widget>[
/// Container(
/// // A fixed-height child.
/// color: const Color(0xffeeee00), // Yellow
/// height: 120.0,
/// alignment: Alignment.center,
/// child: const Text('Fixed Height Content'),
/// ),
/// Container(
/// // Another fixed-height child.
/// color: const Color(0xff008000), // Green
/// height: 120.0,
/// alignment: Alignment.center,
/// child: const Text('Fixed Height Content'),
/// ),
/// ],
/// ),
/// ),
/// );
/// },
/// ),
/// );
/// }
/// ```
/// {@end-tool}
///
/// ### Expanding content to fit the viewport
///
/// The following example builds on the previous one. In addition to providing a
/// minimum dimension for the child [Column], an [IntrinsicHeight] widget is used
/// to force the column to be exactly as big as its contents. This constraint
/// combines with the [ConstrainedBox] constraints discussed previously to ensure
/// that the column becomes either as big as viewport, or as big as the contents,
/// whichever is biggest.
///
/// Both constraints must be used to get the desired effect. If only the
/// [IntrinsicHeight] was specified, then the column would not grow to fit the
/// entire viewport when its children were smaller than the whole screen. If only
/// the size of the viewport was used, then the [Column] would overflow if the
/// children were bigger than the viewport.
///
/// The widget that is to grow to fit the remaining space so provided is wrapped
/// in an [Expanded] widget.
///
/// This technique is quite expensive, as it more or less requires that the contents
/// of the viewport be laid out twice (once to find their intrinsic dimensions, and
/// once to actually lay them out). The number of widgets within the column should
/// therefore be kept small. Alternatively, subsets of the children that have known
/// dimensions can be wrapped in a [SizedBox] that has tight vertical constraints,
/// so that the intrinsic sizing algorithm can short-circuit the computation when it
/// reaches those parts of the subtree.
///
/// {@tool dartpad --template=stateless_widget_material}
/// In this example, the column becomes either as big as viewport, or as big as
/// the contents, whichever is biggest.
///
/// ```dart
/// Widget build(BuildContext context) {
/// return DefaultTextStyle(
/// style: Theme.of(context).textTheme.bodyText2,
/// child: LayoutBuilder(
/// builder: (BuildContext context, BoxConstraints viewportConstraints) {
/// return SingleChildScrollView(
/// child: ConstrainedBox(
/// constraints: BoxConstraints(
/// minHeight: viewportConstraints.maxHeight,
/// ),
/// child: IntrinsicHeight(
/// child: Column(
/// children: <Widget>[
/// Container(
/// // A fixed-height child.
/// color: const Color(0xffeeee00), // Yellow
/// height: 120.0,
/// alignment: Alignment.center,
/// child: const Text('Fixed Height Content'),
/// ),
/// Expanded(
/// // A flexible child that will grow to fit the viewport but
/// // still be at least as big as necessary to fit its contents.
/// child: Container(
/// color: const Color(0xffee0000), // Red
/// height: 120.0,
/// alignment: Alignment.center,
/// child: const Text('Flexible Content'),
/// ),
/// ),
/// ],
/// ),
/// ),
/// ),
/// );
/// },
/// ),
/// );
/// }
/// ```
/// {@end-tool}
///
/// See also:
///
/// * [ListView], which handles multiple children in a scrolling list.
/// * [GridView], which handles multiple children in a scrolling grid.
/// * [PageView], for a scrollable that works page by page.
/// * [Scrollable], which handles arbitrary scrolling effects.
class SingleChildScrollView extends StatelessWidget {
/// Creates a box in which a single widget can be scrolled.
const SingleChildScrollView({
Key? key,
this.scrollDirection = Axis.vertical,
this.reverse = false,
this.padding,
bool? primary,
this.physics,
this.controller,
this.child,
this.dragStartBehavior = DragStartBehavior.start,
this.clipBehavior = Clip.hardEdge,
this.restorationId,
}) : assert(scrollDirection != null),
assert(dragStartBehavior != null),
assert(clipBehavior != null),
assert(!(controller != null && primary == true),
'Primary ScrollViews obtain their ScrollController via inheritance from a PrimaryScrollController widget. '
'You cannot both set primary to true and pass an explicit controller.'
),
primary = primary ?? controller == null && identical(scrollDirection, Axis.vertical),
super(key: key);
/// The axis along which the scroll view scrolls.
///
/// Defaults to [Axis.vertical].
final Axis scrollDirection;
/// Whether the scroll view scrolls in the reading direction.
///
/// For example, if the reading direction is left-to-right and
/// [scrollDirection] is [Axis.horizontal], then the scroll view scrolls from
/// left to right when [reverse] is false and from right to left when
/// [reverse] is true.
///
/// Similarly, if [scrollDirection] is [Axis.vertical], then the scroll view
/// scrolls from top to bottom when [reverse] is false and from bottom to top
/// when [reverse] is true.
///
/// Defaults to false.
final bool reverse;
/// The amount of space by which to inset the child.
final EdgeInsetsGeometry? padding;
/// An object that can be used to control the position to which this scroll
/// view is scrolled.
///
/// Must be null if [primary] is true.
///
/// A [ScrollController] serves several purposes. It can be used to control
/// the initial scroll position (see [ScrollController.initialScrollOffset]).
/// It can be used to control whether the scroll view should automatically
/// save and restore its scroll position in the [PageStorage] (see
/// [ScrollController.keepScrollOffset]). It can be used to read the current
/// scroll position (see [ScrollController.offset]), or change it (see
/// [ScrollController.animateTo]).
final ScrollController? controller;
/// Whether this is the primary scroll view associated with the parent
/// [PrimaryScrollController].
///
/// On iOS, this identifies the scroll view that will scroll to top in
/// response to a tap in the status bar.
///
/// Defaults to true when [scrollDirection] is vertical and [controller] is
/// not specified.
final bool primary;
/// How the scroll view should respond to user input.
///
/// For example, determines how the scroll view continues to animate after the
/// user stops dragging the scroll view.
///
/// Defaults to matching platform conventions.
final ScrollPhysics? physics;
/// The widget that scrolls.
///
/// {@macro flutter.widgets.child}
final Widget? child;
/// {@macro flutter.widgets.scrollable.dragStartBehavior}
final DragStartBehavior dragStartBehavior;
/// {@macro flutter.widgets.Clip}
///
/// Defaults to [Clip.hardEdge].
final Clip clipBehavior;
/// {@macro flutter.widgets.scrollable.restorationId}
final String? restorationId;
AxisDirection _getDirection(BuildContext context) {
return getAxisDirectionFromAxisReverseAndDirectionality(context, scrollDirection, reverse);
}
@override
Widget build(BuildContext context) {
final AxisDirection axisDirection = _getDirection(context);
Widget? contents = child;
if (padding != null)
contents = Padding(padding: padding!, child: contents);
final ScrollController? scrollController = primary
? PrimaryScrollController.of(context)
: controller;
final Scrollable scrollable = Scrollable(
dragStartBehavior: dragStartBehavior,
axisDirection: axisDirection,
controller: scrollController,
physics: physics,
restorationId: restorationId,
viewportBuilder: (BuildContext context, ViewportOffset offset) {
return _SingleChildViewport(
axisDirection: axisDirection,
offset: offset,
child: contents,
clipBehavior: clipBehavior,
);
},
);
return primary && scrollController != null
? PrimaryScrollController.none(child: scrollable)
: scrollable;
}
}
class _SingleChildViewport extends SingleChildRenderObjectWidget {
const _SingleChildViewport({
Key? key,
this.axisDirection = AxisDirection.down,
required this.offset,
Widget? child,
required this.clipBehavior,
}) : assert(axisDirection != null),
assert(clipBehavior != null),
super(key: key, child: child);
final AxisDirection axisDirection;
final ViewportOffset offset;
final Clip clipBehavior;
@override
_RenderSingleChildViewport createRenderObject(BuildContext context) {
return _RenderSingleChildViewport(
axisDirection: axisDirection,
offset: offset,
clipBehavior: clipBehavior,
);
}
@override
void updateRenderObject(BuildContext context, _RenderSingleChildViewport renderObject) {
// Order dependency: The offset setter reads the axis direction.
renderObject
..axisDirection = axisDirection
..offset = offset
..clipBehavior = clipBehavior;
}
}
class _RenderSingleChildViewport extends RenderBox with RenderObjectWithChildMixin<RenderBox> implements RenderAbstractViewport {
_RenderSingleChildViewport({
AxisDirection axisDirection = AxisDirection.down,
required ViewportOffset offset,
double cacheExtent = RenderAbstractViewport.defaultCacheExtent,
RenderBox? child,
required Clip clipBehavior,
}) : assert(axisDirection != null),
assert(offset != null),
assert(cacheExtent != null),
assert(clipBehavior != null),
_axisDirection = axisDirection,
_offset = offset,
_cacheExtent = cacheExtent,
_clipBehavior = clipBehavior {
this.child = child;
}
AxisDirection get axisDirection => _axisDirection;
AxisDirection _axisDirection;
set axisDirection(AxisDirection value) {
assert(value != null);
if (value == _axisDirection)
return;
_axisDirection = value;
markNeedsLayout();
}
Axis get axis => axisDirectionToAxis(axisDirection);
ViewportOffset get offset => _offset;
ViewportOffset _offset;
set offset(ViewportOffset value) {
assert(value != null);
if (value == _offset)
return;
if (attached)
_offset.removeListener(_hasScrolled);
_offset = value;
if (attached)
_offset.addListener(_hasScrolled);
markNeedsLayout();
}
/// {@macro flutter.rendering.viewport.cacheExtent}
double get cacheExtent => _cacheExtent;
double _cacheExtent;
set cacheExtent(double value) {
assert(value != null);
if (value == _cacheExtent)
return;
_cacheExtent = value;
markNeedsLayout();
}
/// {@macro flutter.widgets.Clip}
///
/// Defaults to [Clip.none], and must not be null.
Clip get clipBehavior => _clipBehavior;
Clip _clipBehavior = Clip.none;
set clipBehavior(Clip value) {
assert(value != null);
if (value != _clipBehavior) {
_clipBehavior = value;
markNeedsPaint();
markNeedsSemanticsUpdate();
}
}
void _hasScrolled() {
markNeedsPaint();
markNeedsSemanticsUpdate();
}
@override
void setupParentData(RenderObject child) {
// We don't actually use the offset argument in BoxParentData, so let's
// avoid allocating it at all.
if (child.parentData is! ParentData)
child.parentData = ParentData();
}
@override
void attach(PipelineOwner owner) {
super.attach(owner);
_offset.addListener(_hasScrolled);
}
@override
void detach() {
_offset.removeListener(_hasScrolled);
super.detach();
}
@override
bool get isRepaintBoundary => true;
double get _viewportExtent {
assert(hasSize);
switch (axis) {
case Axis.horizontal:
return size.width;
case Axis.vertical:
return size.height;
}
}
double get _minScrollExtent {
assert(hasSize);
return 0.0;
}
double get _maxScrollExtent {
assert(hasSize);
if (child == null)
return 0.0;
switch (axis) {
case Axis.horizontal:
return math.max(0.0, child!.size.width - size.width);
case Axis.vertical:
return math.max(0.0, child!.size.height - size.height);
}
}
BoxConstraints _getInnerConstraints(BoxConstraints constraints) {
switch (axis) {
case Axis.horizontal:
return constraints.heightConstraints();
case Axis.vertical:
return constraints.widthConstraints();
}
}
@override
double computeMinIntrinsicWidth(double height) {
if (child != null)
return child!.getMinIntrinsicWidth(height);
return 0.0;
}
@override
double computeMaxIntrinsicWidth(double height) {
if (child != null)
return child!.getMaxIntrinsicWidth(height);
return 0.0;
}
@override
double computeMinIntrinsicHeight(double width) {
if (child != null)
return child!.getMinIntrinsicHeight(width);
return 0.0;
}
@override
double computeMaxIntrinsicHeight(double width) {
if (child != null)
return child!.getMaxIntrinsicHeight(width);
return 0.0;
}
// We don't override computeDistanceToActualBaseline(), because we
// want the default behavior (returning null). Otherwise, as you
// scroll, it would shift in its parent if the parent was baseline-aligned,
// which makes no sense.
@override
void performLayout() {
final BoxConstraints constraints = this.constraints;
if (child == null) {
size = constraints.smallest;
} else {
child!.layout(_getInnerConstraints(constraints), parentUsesSize: true);
size = constraints.constrain(child!.size);
}
offset.applyViewportDimension(_viewportExtent);
offset.applyContentDimensions(_minScrollExtent, _maxScrollExtent);
}
Offset get _paintOffset => _paintOffsetForPosition(offset.pixels);
Offset _paintOffsetForPosition(double position) {
assert(axisDirection != null);
switch (axisDirection) {
case AxisDirection.up:
return Offset(0.0, position - child!.size.height + size.height);
case AxisDirection.down:
return Offset(0.0, -position);
case AxisDirection.left:
return Offset(position - child!.size.width + size.width, 0.0);
case AxisDirection.right:
return Offset(-position, 0.0);
}
}
bool _shouldClipAtPaintOffset(Offset paintOffset) {
assert(child != null);
return paintOffset.dx < 0 ||
paintOffset.dy < 0 ||
paintOffset.dx + child!.size.width > size.width ||
paintOffset.dy + child!.size.height > size.height;
}
@override
void paint(PaintingContext context, Offset offset) {
if (child != null) {
final Offset paintOffset = _paintOffset;
void paintContents(PaintingContext context, Offset offset) {
context.paintChild(child!, offset + paintOffset);
}
if (_shouldClipAtPaintOffset(paintOffset) && clipBehavior != Clip.none) {
context.pushClipRect(needsCompositing, offset, Offset.zero & size, paintContents, clipBehavior: clipBehavior);
} else {
paintContents(context, offset);
}
}
}
@override
void applyPaintTransform(RenderBox child, Matrix4 transform) {
final Offset paintOffset = _paintOffset;
transform.translate(paintOffset.dx, paintOffset.dy);
}
@override
Rect? describeApproximatePaintClip(RenderObject? child) {
if (child != null && _shouldClipAtPaintOffset(_paintOffset))
return Offset.zero & size;
return null;
}
@override
bool hitTestChildren(BoxHitTestResult result, { required Offset position }) {
if (child != null) {
return result.addWithPaintOffset(
offset: _paintOffset,
position: position,
hitTest: (BoxHitTestResult result, Offset? transformed) {
assert(transformed == position + -_paintOffset);
return child!.hitTest(result, position: transformed!);
},
);
}
return false;
}
@override
RevealedOffset getOffsetToReveal(RenderObject target, double alignment, { Rect? rect }) {
rect ??= target.paintBounds;
if (target is! RenderBox)
return RevealedOffset(offset: offset.pixels, rect: rect);
final RenderBox targetBox = target;
final Matrix4 transform = targetBox.getTransformTo(child);
final Rect bounds = MatrixUtils.transformRect(transform, rect);
final Size contentSize = child!.size;
double leadingScrollOffset;
double targetMainAxisExtent;
double mainAxisExtent;
assert(axisDirection != null);
switch (axisDirection) {
case AxisDirection.up:
mainAxisExtent = size.height;
leadingScrollOffset = contentSize.height - bounds.bottom;
targetMainAxisExtent = bounds.height;
break;
case AxisDirection.right:
mainAxisExtent = size.width;
leadingScrollOffset = bounds.left;
targetMainAxisExtent = bounds.width;
break;
case AxisDirection.down:
mainAxisExtent = size.height;
leadingScrollOffset = bounds.top;
targetMainAxisExtent = bounds.height;
break;
case AxisDirection.left:
mainAxisExtent = size.width;
leadingScrollOffset = contentSize.width - bounds.right;
targetMainAxisExtent = bounds.width;
break;
}
final double targetOffset = leadingScrollOffset - (mainAxisExtent - targetMainAxisExtent) * alignment;
final Rect targetRect = bounds.shift(_paintOffsetForPosition(targetOffset));
return RevealedOffset(offset: targetOffset, rect: targetRect);
}
@override
void showOnScreen({
RenderObject? descendant,
Rect? rect,
Duration duration = Duration.zero,
Curve curve = Curves.ease,
}) {
if (!offset.allowImplicitScrolling) {
return super.showOnScreen(
descendant: descendant,
rect: rect,
duration: duration,
curve: curve,
);
}
final Rect? newRect = RenderViewportBase.showInViewport(
descendant: descendant,
viewport: this,
offset: offset,
rect: rect,
duration: duration,
curve: curve,
);
super.showOnScreen(
rect: newRect,
duration: duration,
curve: curve,
);
}
@override
Rect describeSemanticsClip(RenderObject child) {
assert(axis != null);
switch (axis) {
case Axis.vertical:
return Rect.fromLTRB(
semanticBounds.left,
semanticBounds.top - cacheExtent,
semanticBounds.right,
semanticBounds.bottom + cacheExtent,
);
case Axis.horizontal:
return Rect.fromLTRB(
semanticBounds.left - cacheExtent,
semanticBounds.top,
semanticBounds.right + cacheExtent,
semanticBounds.bottom,
);
}
}
}