// lib/widgets/magnetic_scroll_physics.dart
import 'package:flutter/material.dart';

class MagneticScrollPhysics extends ClampingScrollPhysics {
  final double snapPoint;
  final double magneticZoneStart;

  const MagneticScrollPhysics({
    required this.snapPoint,
    this.magneticZoneStart = 0.20,
    super.parent,
  });

  @override
  MagneticScrollPhysics applyTo(ScrollPhysics? ancestor) {
    return MagneticScrollPhysics(
      snapPoint: snapPoint,
      magneticZoneStart: magneticZoneStart,
      parent: buildParent(ancestor),
    );
  }

  @override
  Simulation? createBallisticSimulation(
    ScrollMetrics position,
    double velocity,
  ) {
    final offset = position.pixels;

    if (offset <= 0 || offset >= snapPoint) {
      return super.createBallisticSimulation(position, velocity);
    }

    final shouldSnap = _shouldSnap(offset, velocity);

    final target = shouldSnap ? snapPoint : 0.0;

    if ((offset - target).abs() < 1.0) {
      return null;
    }

    return ScrollSpringSimulation(
      SpringDescription(
        mass: 1.0,
        stiffness: 320.0,
        damping: 26.0,
      ),
      offset,
      target,
      velocity,
      tolerance: const Tolerance(
        velocity: 0.01,
        distance: 0.5,
      ),
    );
  }

  bool _shouldSnap(double offset, double velocity) {
    final zoneStart = snapPoint * magneticZoneStart;

    // 向上滑 → 吸附展开
    if (velocity > 20) {
      return true;
    }

    // 向下滑 → 回去
    if (velocity < -20) {
      return false;
    }

    // 松手速度很小，用当前位置决定
    return offset >= zoneStart;
  }
}