flutter_bouncy 0.0.6

Scrolling with bouncy and natural item movement

example/lib/main.dart

import 'dart:async';
import 'dart:math';

import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';
import 'package:flutter/physics.dart';
import 'package:flutter/rendering.dart';
import 'package:flutter/scheduler.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        // This is the theme of your application.
        //
        // Try running your application with "flutter run". You'll see the
        // application has a blue toolbar. Then, without quitting the app, try
        // changing the primarySwatch below to Colors.green and then invoke
        // "hot reload" (press "r" in the console where you ran "flutter run",
        // or simply save your changes to "hot reload" in a Flutter IDE).
        // Notice that the counter didn't reset back to zero; the application
        // is not restarted.
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  // This widget is the home page of your application. It is stateful, meaning
  // that it has a State object (defined below) that contains fields that affect
  // how it looks.

  // This class is the configuration for the state. It holds the values (in this
  // case the title) provided by the parent (in this case the App widget) and
  // used by the build method of the State. Fields in a Widget subclass are
  // always marked "final".

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage>
    with SingleTickerProviderStateMixin {
  int _counter = 0;
  final tween = Tween<double>(begin: 0.0, end: 0.0);
  ScrollController scrollController;
  Offset globalPosition;
  double lastPixels;

  @override
  void initState() {
    scrollController = ScrollController();
    super.initState();
  }

  void _incrementCounter() {
    setState(() {
      // This call to setState tells the Flutter framework that something has
      // changed in this State, which causes it to rerun the build method below
      // so that the display can reflect the updated values. If we changed
      // _counter without calling setState(), then the build method would not be
      // called again, and so nothing would appear to happen.
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: SafeArea(
        child: NotificationListener<ScrollNotification>(
          onNotification: (notification) {
            // print('Pixels: ${notification.metrics.pixels}');
            if (notification is ScrollUpdateNotification) {
              if (notification.dragDetails != null) {
                setState(() {
                  globalPosition = notification.dragDetails.globalPosition;
                });
                // tween.begin = scrollDelta;
                // tween.end = notification.scrollDelta;

                // if (controller.isCompleted) {
                //   controller.reset();
                // }

                // if (!controller.isAnimating) {
                //   controller.forward();
                // }
              }
            }

            if (notification is ScrollEndNotification) {
              // controller.reset();

            }
            return false;
          },
          child: Padding(
            padding: EdgeInsets.symmetric(horizontal: 10),
            child: CustomScrollView(
              controller: scrollController,
              slivers: [
                SliverBouncyList(
                  globalPosition: globalPosition,
                  delegate: SliverChildListDelegate(
                    [
                      _buildChatMessage(true),
                      _buildChatMessage(false),
                      _buildChatMessage(true),
                      _buildChatMessage(true),
                      _buildChatMessage(true),
                      _buildChatMessage(false),
                      _buildChatMessage(false),
                      _buildChatMessage(true),
                      _buildChatMessage(false),
                      _buildChatMessage(true),
                      _buildChatMessage(true),
                      _buildChatMessage(false),
                      _buildChatMessage(false),
                      _buildChatMessage(true),
                      _buildChatMessage(true),
                      _buildChatMessage(true),
                      _buildChatMessage(false),
                      _buildChatMessage(true),
                      _buildChatMessage(true),
                      _buildChatMessage(true),
                      _buildChatMessage(false),
                      _buildChatMessage(false),
                      _buildChatMessage(true),
                      _buildChatMessage(false),
                      _buildChatMessage(true),
                      _buildChatMessage(true),
                      _buildChatMessage(false),
                      _buildChatMessage(true),
                      _buildChatMessage(true),
                      _buildChatMessage(false),
                      _buildChatMessage(true),
                      _buildChatMessage(true),
                    ],
                  ),
                ),
              ],
            ),
          ),
        ),
      ),
    );
  }

  Widget _buildChatMessage(bool isSender) {
    return Padding(
      padding: EdgeInsets.symmetric(vertical: 10),
      child: Align(
        alignment: isSender ? Alignment.centerRight : Alignment.centerLeft,
        child: FractionallySizedBox(
          widthFactor: 0.8,
          child: Container(
            height: 25,
            decoration: BoxDecoration(
              color: isSender ? Colors.blue : Colors.grey.withOpacity(0.4),
              borderRadius: BorderRadius.circular(10),
            ),
          ),
        ),
      ),
    );
  }
}

class SliverBouncyList extends SliverMultiBoxAdaptorWidget {
  final Offset globalPosition;

  const SliverBouncyList({
    Key key,
    @required SliverChildDelegate delegate,
    @required this.globalPosition,
  }) : super(key: key, delegate: delegate);

  @override
  SliverMultiBoxAdaptorElement createElement() =>
      SliverMultiBoxAdaptorElement(this, replaceMovedChildren: true);

  @override
  void updateRenderObject(
    BuildContext context,
    covariant RenderSliverBouncyList renderObject,
  ) {
    renderObject..globalPosition = globalPosition;
  }

  @override
  RenderSliverBouncyList createRenderObject(BuildContext context) {
    final SliverMultiBoxAdaptorElement element =
        context as SliverMultiBoxAdaptorElement;
    return RenderSliverBouncyList(
      childManager: element,
    );
  }
}

class RenderSliverBouncyList extends RenderSliverMultiBoxAdaptor {
  double lastScrollOffset = 0.0;
  Offset globalPosition;
  final springMap = <RenderObject, Spring>{};

  /// Creates a sliver that places multiple box children in a linear array along
  /// the main axis.
  ///
  /// The [childManager] argument must not be null.
  RenderSliverBouncyList({
    @required RenderSliverBoxChildManager childManager,
  }) : super(childManager: childManager);

  @override
  void handleEvent(PointerEvent event, covariant HitTestEntry entry) {
    globalPosition = event.position;
    if (event is PointerCancelEvent) {
      for (var spring in springMap.values) {
        spring.setTarget(0);
      }
    }
    markNeedsPaint();
  }

  @override
  void setupParentData(covariant RenderBox child) {
    super.setupParentData(child);
    if (springMap[child] == null) {
      var i = 0;
      springMap[child] = Spring(() {
        final spring = springMap[child];
        Future.microtask(() {
          if (i % 100 == 0) {
            // print('Length: ${spring.length}');
          }
          markNeedsPaint();
          // child.markNeedsLayout();
        });
        if (!child.attached) {
          return false;
        }
      });
    }
  }

  @override
  void performLayout() {
    final SliverConstraints constraints = this.constraints;
    childManager.didStartLayout();
    childManager.setDidUnderflow(false);

    final double scrollOffset =
        constraints.scrollOffset + constraints.cacheOrigin;
    assert(scrollOffset >= 0.0);
    final double remainingExtent = constraints.remainingCacheExtent;
    assert(remainingExtent >= 0.0);
    final double targetEndScrollOffset = scrollOffset + remainingExtent;
    final BoxConstraints childConstraints = constraints.asBoxConstraints();

    int leadingGarbage = 0;
    int trailingGarbage = 0;
    bool reachedEnd = false;

    // This algorithm in principle is straight-forward: find the first child
    // that overlaps the given scrollOffset, creating more children at the top
    // of the list if necessary, then walk down the list updating and laying out
    // each child and adding more at the end if necessary until we have enough
    // children to cover the entire viewport.
    //
    // It is complicated by one minor issue, which is that any time you update
    // or create a child, it's possible that the some of the children that
    // haven't yet been laid out will be removed, leaving the list in an
    // inconsistent state, and requiring that missing nodes be recreated.
    //
    // To keep this mess tractable, this algorithm starts from what is currently
    // the first child, if any, and then walks up and/or down from there, so
    // that the nodes that might get removed are always at the edges of what has
    // already been laid out.

    // Make sure we have at least one child to start from.
    if (firstChild == null) {
      if (!addInitialChild()) {
        // There are no children.
        geometry = SliverGeometry.zero;
        childManager.didFinishLayout();
        return;
      }
    }

    // We have at least one child.

    // These variables track the range of children that we have laid out. Within
    // this range, the children have consecutive indices. Outside this range,
    // it's possible for a child to get removed without notice.
    RenderBox leadingChildWithLayout, trailingChildWithLayout;

    RenderBox earliestUsefulChild = firstChild;

    // A firstChild with null layout offset is likely a result of children
    // reordering.
    //
    // We rely on firstChild to have accurate layout offset. In the case of null
    // layout offset, we have to find the first child that has valid layout
    // offset.
    if (childScrollOffset(firstChild) == null) {
      int leadingChildrenWithoutLayoutOffset = 0;
      while (childScrollOffset(earliestUsefulChild) == null) {
        earliestUsefulChild = childAfter(firstChild);
        leadingChildrenWithoutLayoutOffset += 1;
      }
      // We should be able to destroy children with null layout offset safely,
      // because they are likely outside of viewport
      collectGarbage(leadingChildrenWithoutLayoutOffset, 0);
      assert(firstChild != null);
    }

    // Find the last child that is at or before the scrollOffset.
    earliestUsefulChild = firstChild;
    for (double earliestScrollOffset = childScrollOffset(earliestUsefulChild);
        earliestScrollOffset > scrollOffset;
        earliestScrollOffset = childScrollOffset(earliestUsefulChild)) {
      // We have to add children before the earliestUsefulChild.
      earliestUsefulChild =
          insertAndLayoutLeadingChild(childConstraints, parentUsesSize: true);
      if (earliestUsefulChild == null) {
        final SliverMultiBoxAdaptorParentData childParentData =
            firstChild.parentData as SliverMultiBoxAdaptorParentData;
        childParentData.layoutOffset = 0.0;

        if (scrollOffset == 0.0) {
          // insertAndLayoutLeadingChild only lays out the children before
          // firstChild. In this case, nothing has been laid out. We have
          // to lay out firstChild manually.
          firstChild.layout(childConstraints, parentUsesSize: true);
          earliestUsefulChild = firstChild;
          leadingChildWithLayout = earliestUsefulChild;
          trailingChildWithLayout ??= earliestUsefulChild;
          break;
        } else {
          // We ran out of children before reaching the scroll offset.
          // We must inform our parent that this sliver cannot fulfill
          // its contract and that we need a scroll offset correction.
          geometry = SliverGeometry(
            scrollOffsetCorrection: -scrollOffset,
          );
          return;
        }
      }

      final double firstChildScrollOffset =
          earliestScrollOffset - paintExtentOf(firstChild);
      // firstChildScrollOffset may contain double precision error
      if (firstChildScrollOffset < -precisionErrorTolerance) {
        // Let's assume there is no child before the first child. We will
        // correct it on the next layout if it is not.
        geometry = SliverGeometry(
          scrollOffsetCorrection: -firstChildScrollOffset,
        );
        final SliverMultiBoxAdaptorParentData childParentData =
            firstChild.parentData as SliverMultiBoxAdaptorParentData;
        childParentData.layoutOffset = 0.0;
        return;
      }

      final SliverMultiBoxAdaptorParentData childParentData =
          earliestUsefulChild.parentData as SliverMultiBoxAdaptorParentData;
      childParentData.layoutOffset = firstChildScrollOffset;
      assert(earliestUsefulChild == firstChild);
      leadingChildWithLayout = earliestUsefulChild;
      trailingChildWithLayout ??= earliestUsefulChild;
    }

    assert(childScrollOffset(firstChild) > -precisionErrorTolerance);

    // If the scroll offset is at zero, we should make sure we are
    // actually at the beginning of the list.
    if (scrollOffset < precisionErrorTolerance) {
      // We iterate from the firstChild in case the leading child has a 0 paint
      // extent.
      while (indexOf(firstChild) > 0) {
        final double earliestScrollOffset = childScrollOffset(firstChild);
        // We correct one child at a time. If there are more children before
        // the earliestUsefulChild, we will correct it once the scroll offset
        // reaches zero again.
        earliestUsefulChild =
            insertAndLayoutLeadingChild(childConstraints, parentUsesSize: true);
        assert(earliestUsefulChild != null);
        final double firstChildScrollOffset =
            earliestScrollOffset - paintExtentOf(firstChild);
        final SliverMultiBoxAdaptorParentData childParentData =
            firstChild.parentData as SliverMultiBoxAdaptorParentData;
        childParentData.layoutOffset = 0.0;
        // We only need to correct if the leading child actually has a
        // paint extent.
        if (firstChildScrollOffset < -precisionErrorTolerance) {
          geometry = SliverGeometry(
            scrollOffsetCorrection: -firstChildScrollOffset,
          );
          return;
        }
      }
    }

    // At this point, earliestUsefulChild is the first child, and is a child
    // whose scrollOffset is at or before the scrollOffset, and
    // leadingChildWithLayout and trailingChildWithLayout are either null or
    // cover a range of render boxes that we have laid out with the first being
    // the same as earliestUsefulChild and the last being either at or after the
    // scroll offset.

    assert(earliestUsefulChild == firstChild);
    assert(childScrollOffset(earliestUsefulChild) <= scrollOffset);

    // Make sure we've laid out at least one child.
    if (leadingChildWithLayout == null) {
      earliestUsefulChild.layout(childConstraints, parentUsesSize: true);
      leadingChildWithLayout = earliestUsefulChild;
      trailingChildWithLayout = earliestUsefulChild;
    }

    // Here, earliestUsefulChild is still the first child, it's got a
    // scrollOffset that is at or before our actual scrollOffset, and it has
    // been laid out, and is in fact our leadingChildWithLayout. It's possible
    // that some children beyond that one have also been laid out.

    bool inLayoutRange = true;
    RenderBox child = earliestUsefulChild;
    int index = indexOf(child);
    double endScrollOffset = childScrollOffset(child) + paintExtentOf(child);
    bool advance() {
      // returns true if we advanced, false if we have no more children
      // This function is used in two different places below, to avoid code duplication.
      assert(child != null);
      if (child == trailingChildWithLayout) inLayoutRange = false;
      child = childAfter(child);
      if (child == null) inLayoutRange = false;
      index += 1;
      if (!inLayoutRange) {
        if (child == null || indexOf(child) != index) {
          // We are missing a child. Insert it (and lay it out) if possible.
          child = insertAndLayoutChild(
            childConstraints,
            after: trailingChildWithLayout,
            parentUsesSize: true,
          );
          if (child == null) {
            // We have run out of children.
            return false;
          }
        } else {
          // Lay out the child.
          child.layout(childConstraints, parentUsesSize: true);
        }
        trailingChildWithLayout = child;
      }
      assert(child != null);
      final SliverMultiBoxAdaptorParentData childParentData =
          child.parentData as SliverMultiBoxAdaptorParentData;
      childParentData.layoutOffset = endScrollOffset;
      assert(childParentData.index == index);
      endScrollOffset = childScrollOffset(child) + paintExtentOf(child);
      return true;
    }

    // Find the first child that ends after the scroll offset.
    while (endScrollOffset < scrollOffset) {
      leadingGarbage += 1;
      if (!advance()) {
        assert(leadingGarbage == childCount);
        assert(child == null);
        // we want to make sure we keep the last child around so we know the end scroll offset
        collectGarbage(leadingGarbage - 1, 0);
        assert(firstChild == lastChild);
        final double extent =
            childScrollOffset(lastChild) + paintExtentOf(lastChild);
        geometry = SliverGeometry(
          scrollExtent: extent,
          paintExtent: 0.0,
          maxPaintExtent: extent,
        );
        return;
      }
    }

    // Now find the first child that ends after our end.
    while (endScrollOffset < targetEndScrollOffset) {
      if (!advance()) {
        reachedEnd = true;
        break;
      }
    }

    // Finally count up all the remaining children and label them as garbage.
    if (child != null) {
      child = childAfter(child);
      while (child != null) {
        trailingGarbage += 1;
        child = childAfter(child);
      }
    }

    // At this point everything should be good to go, we just have to clean up
    // the garbage and report the geometry.

    collectGarbage(leadingGarbage, trailingGarbage);

    assert(debugAssertChildListIsNonEmptyAndContiguous());
    double estimatedMaxScrollOffset;
    if (reachedEnd) {
      estimatedMaxScrollOffset = endScrollOffset;
    } else {
      estimatedMaxScrollOffset = childManager.estimateMaxScrollOffset(
        constraints,
        firstIndex: indexOf(firstChild),
        lastIndex: indexOf(lastChild),
        leadingScrollOffset: childScrollOffset(firstChild),
        trailingScrollOffset: endScrollOffset,
      );
      assert(estimatedMaxScrollOffset >=
          endScrollOffset - childScrollOffset(firstChild));
    }
    final double paintExtent = calculatePaintOffset(
      constraints,
      from: childScrollOffset(firstChild),
      to: endScrollOffset,
    );
    final double cacheExtent = calculateCacheOffset(
      constraints,
      from: childScrollOffset(firstChild),
      to: endScrollOffset,
    );
    final double targetEndScrollOffsetForPaint =
        constraints.scrollOffset + constraints.remainingPaintExtent;
    geometry = SliverGeometry(
      scrollExtent: estimatedMaxScrollOffset,
      paintExtent: paintExtent,
      cacheExtent: cacheExtent,
      maxPaintExtent: estimatedMaxScrollOffset,
      // Conservative to avoid flickering away the clip during scroll.
      hasVisualOverflow: endScrollOffset > targetEndScrollOffsetForPaint ||
          constraints.scrollOffset > 0.0,
    );

    // We may have started the layout while scrolled to the end, which would not
    // expose a new child.
    if (estimatedMaxScrollOffset == endScrollOffset)
      childManager.setDidUnderflow(true);
    childManager.didFinishLayout();
  }

  @override
  void paint(PaintingContext context, Offset offset) {
    if (firstChild == null) return;
    // _calculateDelta();
    // offset is to the top-left corner, regardless of our axis direction.
    // originOffset gives us the delta from the real origin to the origin in the axis direction.
    Offset mainAxisUnit, crossAxisUnit, originOffset;
    bool addExtent;
    switch (applyGrowthDirectionToAxisDirection(
        constraints.axisDirection, constraints.growthDirection)) {
      case AxisDirection.up:
        mainAxisUnit = const Offset(0.0, -1.0);
        crossAxisUnit = const Offset(1.0, 0.0);
        originOffset = offset + Offset(0.0, geometry.paintExtent);
        addExtent = true;
        break;
      case AxisDirection.right:
        mainAxisUnit = const Offset(1.0, 0.0);
        crossAxisUnit = const Offset(0.0, 1.0);
        originOffset = offset;
        addExtent = false;
        break;
      case AxisDirection.down:
        mainAxisUnit = const Offset(0.0, 1.0);
        crossAxisUnit = const Offset(1.0, 0.0);
        originOffset = offset;
        addExtent = false;
        break;
      case AxisDirection.left:
        mainAxisUnit = const Offset(-1.0, 0.0);
        crossAxisUnit = const Offset(0.0, 1.0);
        originOffset = offset + Offset(geometry.paintExtent, 0.0);
        addExtent = true;
        break;
    }
    assert(mainAxisUnit != null);
    assert(addExtent != null);

    RenderBox child = firstChild;
    var i = 0;
    while (child != null) {
      final double mainAxisDelta = childMainAxisPosition(child);
      final double crossAxisDelta = childCrossAxisPosition(child);
      Offset childOffset = Offset(
        originOffset.dx +
            mainAxisUnit.dx * mainAxisDelta +
            crossAxisUnit.dx * crossAxisDelta,
        originOffset.dy +
            mainAxisUnit.dy * mainAxisDelta +
            crossAxisUnit.dy * crossAxisDelta,
      );

      if (addExtent) childOffset += mainAxisUnit * paintExtentOf(child);

      if (springMap[child] != null) {
        final spring = springMap[child];
        // print('Spring Current: ${spring.current}');
        childOffset += Offset(0, spring.current);
        // print('Length: ${spring.length}');
      }

      // If the child's visible interval (mainAxisDelta, mainAxisDelta + paintExtentOf(child))
      // does not intersect the paint extent interval (0, constraints.remainingPaintExtent), it's hidden.
      // if (mainAxisDelta < constraints.remainingPaintExtent &&
      //     mainAxisDelta + paintExtentOf(child) > 0)
      //   context.paintChild(child, childOffset);
      context.paintChild(child, childOffset);
      i++;
      child = childAfter(child);
    }
    _calculateDelta();
    lastScrollOffset = constraints.scrollOffset;
  }

  void _calculateDelta() {
    if (lastScrollOffset == null || globalPosition == null) {
      return;
    }

    RenderBox child = firstChild;
    var i = 0;
    while (child != null) {
      var resistance = 0.0;

      double delta = constraints.scrollOffset - lastScrollOffset;

      if (lastScrollOffset != null) {
        resistance =
            (globalPosition.dy - child.localToGlobal(Offset(0, 0)).dy) / 100;
      }

      final calculatedDelta = (delta > 0
              ? max(delta, delta * resistance.abs())
              : min(delta, delta * resistance.abs()))
          .floorToDouble();

      // print('Calculated Delta: ${calculatedDelta}');

      final spring = springMap[child];
      if (spring != null) {
        springMap[child].setTarget(calculatedDelta);
      }

      if (i == 0) {
        // print(calculatedDelta);
      }

      child = childAfter(child);
      i++;
    }
  }
}

class Spring {
  final Function handler;

  SpringSimulation _simulation;
  double current = 0.0;
  double dt = 0;

  Spring(this.handler) {
    final timer = Timer.periodic(
      Duration(milliseconds: 1),
      (_) {
        if (_simulation != null) {
          current = _simulation.x(dt / 100);
          // print('Current: $current');
        }
        if (dt % 1000 == 0) {
          // print('DT: $dt');
        }
        dt++;
        handler();
      },
    );
  }

  void setTarget(double target) {
    _simulation = SpringSimulation(
      _getSpringDescription(),
      current,
      target,
      0.01,
    );

    dt = 0;
  }

  SpringDescription _getSpringDescription() {
    return const SpringDescription(
      mass: 20.0,
      stiffness: 0.6,
      damping: 0.4,
    );
  }
}