flutter_eyevideo/lib/framework/uikit/refresher/physics/scroll_physics.dart

import 'dart:math' as math;

import 'package:eye_video/framework/uikit/refresher/indicator/core/abstract_footer.dart';
import 'package:eye_video/framework/uikit/refresher/indicator/core/abstract_header.dart';
import 'package:eye_video/framework/uikit/refresher/pretty_refresher.dart';
import 'package:flutter/gestures.dart';
import 'package:flutter/physics.dart';
import 'package:flutter/widgets.dart';


class RefreshPhysics extends ScrollPhysics {
  // 任务状态
  final ValueNotifier<TaskState> taskNotifier;

  // 回弹设置
  final ValueNotifier<BouncingSettings> bouncingNotifier;

  // 指示器越界
  final ValueNotifier<RefreshIndicator> indicatorNotifier;

  // Creates scroll physics that bounce back from the edge.
  const RefreshPhysics({
    ScrollPhysics parent,
    this.taskNotifier,
    this.bouncingNotifier,
    this.indicatorNotifier,
  }) : super(parent: parent);

  @override
  RefreshPhysics applyTo(ScrollPhysics ancestor) {
    return RefreshPhysics(
      parent: buildParent(ancestor),
      taskNotifier: taskNotifier,
      bouncingNotifier: bouncingNotifier,
      indicatorNotifier: indicatorNotifier,
    );
  }

  Header get header => indicatorNotifier.value.header;

  bool get headerOverScroll {
    if (header != null) {
      return !header.enableInfiniteRefresh || header.overScroll;
    }
    return true;
  }

  double get headerExtent => header.extent;

  Footer get footer => indicatorNotifier.value.footer;

  bool get footerOverScroll {
    if (footer != null) {
      return !footer.enableInfiniteLoad || footer.overScroll;
    }
    return true;
  }

  double get footerExtent => footer.extent;

  // The multiple applied to overscroll to make it appear that scrolling past
  // the edge of the scrollable contents is harder than scrolling the list.
  // This is done by reducing the ratio of the scroll effect output vs the
  // scroll gesture input.
  //
  // This factor starts at 0.52 and progressively becomes harder to overscroll
  // as more of the area past the edge is dragged in (represented by an increasing
  // `overscrollFraction` which starts at 0 when there is no overscroll).
  double frictionFactor(double overscrollFraction) =>
      0.52 * math.pow(1 - overscrollFraction, 2);

  @override
  bool shouldAcceptUserOffset(ScrollMetrics position) {
    return true;
  }

  @override
  double applyPhysicsToUserOffset(ScrollMetrics position, double offset) {
    assert(offset != 0.0);
    assert(position.minScrollExtent <= position.maxScrollExtent);

    if (!position.outOfRange) return offset;

    final double overscrollPastStart =
        math.max(position.minScrollExtent - position.pixels, 0.0);
    final double overscrollPastEnd =
        math.max(position.pixels - position.maxScrollExtent, 0.0);
    final double overscrollPast =
        math.max(overscrollPastStart, overscrollPastEnd);
    final bool easing = (overscrollPastStart > 0.0 && offset < 0.0) ||
        (overscrollPastEnd > 0.0 && offset > 0.0);

    final double friction = easing
        // Apply less resistance when easing the overscroll vs tensioning.
        ? frictionFactor(
            (overscrollPast - offset.abs()) / position.viewportDimension)
        : frictionFactor(overscrollPast / position.viewportDimension);
    final double direction = offset.sign;

    return direction * _applyFriction(overscrollPast, offset.abs(), friction);
  }

  static double _applyFriction(
      double extentOutside, double absDelta, double gamma) {
    assert(absDelta > 0);
    double total = 0.0;
    if (extentOutside > 0) {
      final double deltaToLimit = extentOutside / gamma;
      if (absDelta < deltaToLimit) return absDelta * gamma;
      total += extentOutside;
      absDelta -= deltaToLimit;
    }
    return total + absDelta;
  }

  @override
  double applyBoundaryConditions(ScrollMetrics position, double value) {
    if (!bouncingNotifier.value.top ||
        (!headerOverScroll &&
            (taskNotifier.value.refreshing ||
                taskNotifier.value.refreshNoMore))) {
      if (value < position.pixels &&
          position.pixels <= position.minScrollExtent) // underscroll
        return value - position.pixels;
      if (value < position.minScrollExtent &&
          position.minScrollExtent < position.pixels) // hit top edge
        return value - position.minScrollExtent;
    }
    if (!headerOverScroll && value - position.minScrollExtent < 0.0) {
      // 防止越界超过header高度
      return value - position.minScrollExtent;
    }
    if (!bouncingNotifier.value.bottom ||
        (!footerOverScroll &&
            (taskNotifier.value.loading || taskNotifier.value.loadNoMore))) {
      if (position.maxScrollExtent <= position.pixels &&
          position.pixels < value) {
        // overscroll
        return value - position.pixels;
      }
      if (position.pixels < position.maxScrollExtent &&
          position.maxScrollExtent < value) // hit bottom edge
        return value - position.maxScrollExtent;
    }
    if (!footerOverScroll && value - position.maxScrollExtent > 0.0) {
      // 防止越界超过footer高度
      return value - position.maxScrollExtent;
    }
    return 0.0;
  }

  @override
  Simulation createBallisticSimulation(
      ScrollMetrics position, double velocity) {
    final Tolerance tolerance = this.tolerance;
    if (velocity.abs() >= tolerance.velocity || position.outOfRange) {
      return BouncingScrollSimulation(
        spring: spring,
        position: position.pixels,
        velocity: velocity * 0.91,
        // TODO(abarth): We should move this constant closer to the drag end.
        leadingExtent: position.minScrollExtent,
        trailingExtent: position.maxScrollExtent,
        tolerance: tolerance,
      );
    }
    return null;
  }

  // The ballistic simulation here decelerates more slowly than the one for
  // ClampingScrollPhysics so we require a more deliberate input gesture
  // to trigger a fling.
  @override
  double get minFlingVelocity => kMinFlingVelocity * 2.0;

  // Methodology:
  // 1- Use https://github.com/flutter/scroll_overlay to test with Flutter and
  //    platform scroll views superimposed.
  // 2- Record incoming speed and make rapid flings in the test app.
  // 3- If the scrollables stopped overlapping at any moment, adjust the desired
  //    output value of this function at that input speed.
  // 4- Feed input/output set into a power curve fitter. Change function
  //    and repeat from 2.
  // 5- Repeat from 2 with medium and slow flings.
  // Momentum build-up function that mimics iOS's scroll speed increase with repeated flings.
  //
  // The velocity of the last fling is not an important factor. Existing speed
  // and (related) time since last fling are factors for the velocity transfer
  // calculations.
  @override
  double carriedMomentum(double existingVelocity) {
    return existingVelocity.sign *
        math.min(0.000816 * math.pow(existingVelocity.abs(), 1.967).toDouble(),
            40000.0);
  }

  // Eyeballed from observation to counter the effect of an unintended scroll
  // from the natural motion of lifting the finger after a scroll.
  @override
  double get dragStartDistanceMotionThreshold => 3.5;
}

// 回弹设置
class BouncingSettings {
  bool top;
  bool bottom;

  BouncingSettings({this.top = true, this.bottom = true});

  BouncingSettings copy({bool top, bool bottom}) {
    return BouncingSettings(
      top: top ?? this.top,
      bottom: bottom ?? this.bottom,
    );
  }
}