SingleChildWithKeepPositionScrollView - single child with keep position scroll view in Flutter

single_child_with_keep_position_scroll_view.dart

import 'dart:math' as math;

import 'package:flutter/gestures.dart' show DragStartBehavior;
import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';

class SingleChildWithKeepPositionScrollView extends StatelessWidget {
  /// Creates a box in which a single widget can be scrolled.
  const SingleChildWithKeepPositionScrollView({
    super.key,
    this.scrollDirection = Axis.vertical,
    this.reverse = false,
    this.padding,
    this.primary,
    this.physics,
    this.controller,
    this.child,
    this.dragStartBehavior = DragStartBehavior.start,
    this.clipBehavior = Clip.hardEdge,
    this.restorationId,
    this.keyboardDismissBehavior = ScrollViewKeyboardDismissBehavior.manual,
  }) : assert(
          !(controller != null && (primary ?? false)),
          'Primary ScrollViews obtain their ScrollController via inheritance '
          'from a PrimaryScrollController widget. You cannot both set primary to '
          'true and pass an explicit controller.',
        );

  /// 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;

  /// {@macro flutter.widgets.scroll_view.primary}
  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.ProxyWidget.child}
  final Widget? child;

  /// {@macro flutter.widgets.scrollable.dragStartBehavior}
  final DragStartBehavior dragStartBehavior;

  /// {@macro flutter.material.Material.clipBehavior}
  ///
  /// Defaults to [Clip.hardEdge].
  final Clip clipBehavior;

  /// {@macro flutter.widgets.scrollable.restorationId}
  final String? restorationId;

  /// {@macro flutter.widgets.scroll_view.keyboardDismissBehavior}
  final ScrollViewKeyboardDismissBehavior keyboardDismissBehavior;

  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 bool effectivePrimary = primary ??
        controller == null &&
            PrimaryScrollController.shouldInherit(context, scrollDirection);

    final ScrollController? scrollController = effectivePrimary
        ? PrimaryScrollController.maybeOf(context)
        : controller;

    Widget scrollable = Scrollable(
      dragStartBehavior: dragStartBehavior,
      axisDirection: axisDirection,
      controller: scrollController,
      physics: physics,
      restorationId: restorationId,
      viewportBuilder: (BuildContext context, ViewportOffset offset) {
        return _SingleChildViewport(
          axisDirection: axisDirection,
          offset: offset,
          clipBehavior: clipBehavior,
          child: contents,
        );
      },
    );

    if (keyboardDismissBehavior == ScrollViewKeyboardDismissBehavior.onDrag) {
      scrollable = NotificationListener<ScrollUpdateNotification>(
        child: scrollable,
        onNotification: (ScrollUpdateNotification notification) {
          final FocusScopeNode focusNode = FocusScope.of(context);
          if (notification.dragDetails != null && focusNode.hasFocus) {
            focusNode.unfocus();
          }
          return false;
        },
      );
    }

    return effectivePrimary && scrollController != null
        // Further descendant ScrollViews will not inherit the same
        // PrimaryScrollController
        ? PrimaryScrollController.none(child: scrollable)
        : scrollable;
  }
}

class _SingleChildViewport extends SingleChildRenderObjectWidget {
  const _SingleChildViewport({
    this.axisDirection = AxisDirection.down,
    required this.offset,
    super.child,
    required this.clipBehavior,
  });

  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;
  }

  @override
  SingleChildRenderObjectElement createElement() {
    return _SingleChildViewportElement(this);
  }
}

class _SingleChildViewportElement extends SingleChildRenderObjectElement
    with NotifiableElementMixin, ViewportElementMixin {
  _SingleChildViewportElement(_SingleChildViewport super.widget);
}

class _RenderSingleChildViewport extends RenderBox
    with RenderObjectWithChildMixin<RenderBox>
    implements RenderAbstractViewport {
  _RenderSingleChildViewport({
    AxisDirection axisDirection = AxisDirection.down,
    required ViewportOffset offset,
    RenderBox? child,
    required Clip clipBehavior,
  })  : _axisDirection = axisDirection,
        _offset = offset,
        _clipBehavior = clipBehavior {
    this.child = child;
  }

  AxisDirection get axisDirection => _axisDirection;
  AxisDirection _axisDirection;
  set axisDirection(AxisDirection value) {
    if (value == _axisDirection) {
      return;
    }
    _axisDirection = value;
    markNeedsLayout();
  }

  Axis get axis => axisDirectionToAxis(axisDirection);

  ViewportOffset get offset => _offset;
  ViewportOffset _offset;
  set offset(ViewportOffset value) {
    if (value == _offset) {
      return;
    }
    if (attached) {
      _offset.removeListener(_hasScrolled);
    }
    _offset = value;
    if (attached) {
      _offset.addListener(_hasScrolled);
    }
    markNeedsLayout();
  }

  /// {@macro flutter.material.Material.clipBehavior}
  ///
  /// Defaults to [Clip.none], and must not be null.
  Clip get clipBehavior => _clipBehavior;
  Clip _clipBehavior = Clip.none;
  set clipBehavior(Clip value) {
    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
  Size computeDryLayout(BoxConstraints constraints) {
    if (child == null) {
      return constraints.smallest;
    }
    final Size childSize =
        child!.getDryLayout(_getInnerConstraints(constraints));
    return constraints.constrain(childSize);
  }

  @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) {
    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);
    switch (clipBehavior) {
      case Clip.none:
        return false;
      case Clip.hardEdge:
      case Clip.antiAlias:
      case Clip.antiAliasWithSaveLayer:
        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)) {
        _clipRectLayer.layer = context.pushClipRect(
          needsCompositing,
          offset,
          Offset.zero & size,
          paintContents,
          clipBehavior: clipBehavior,
          oldLayer: _clipRectLayer.layer,
        );
      } else {
        _clipRectLayer.layer = null;
        paintContents(context, offset);
      }
    }
  }

  final LayerHandle<ClipRectLayer> _clipRectLayer =
      LayerHandle<ClipRectLayer>();

  @override
  void dispose() {
    _clipRectLayer.layer = null;
    super.dispose();
  }

  @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;

    final double leadingScrollOffset;
    final double targetMainAxisExtent;
    final double mainAxisExtent;

    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
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    properties.add(DiagnosticsProperty<Offset>('offset', _paintOffset));
  }

  @override
  Rect describeSemanticsClip(RenderObject child) {
    final double remainingOffset = _maxScrollExtent - offset.pixels;
    switch (axisDirection) {
      case AxisDirection.up:
        return Rect.fromLTRB(
          semanticBounds.left,
          semanticBounds.top - remainingOffset,
          semanticBounds.right,
          semanticBounds.bottom + offset.pixels,
        );
      case AxisDirection.right:
        return Rect.fromLTRB(
          semanticBounds.left - offset.pixels,
          semanticBounds.top,
          semanticBounds.right + remainingOffset,
          semanticBounds.bottom,
        );
      case AxisDirection.down:
        return Rect.fromLTRB(
          semanticBounds.left,
          semanticBounds.top - offset.pixels,
          semanticBounds.right,
          semanticBounds.bottom + remainingOffset,
        );
      case AxisDirection.left:
        return Rect.fromLTRB(
          semanticBounds.left - remainingOffset,
          semanticBounds.top,
          semanticBounds.right + offset.pixels,
          semanticBounds.bottom,
        );
    }
  }
}