sky/framework/animation/scroll_behavior.dart

// Copyright 2015 The Chromium 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 'mechanics.dart';
import 'generators.dart';

const double _kSlope = 0.02;
const double _kFriction = 0.004;

abstract class ScrollBehavior {
  Simulation release(Particle particle) => null;

  // Returns the new scroll offset.
  double applyCurve(double scrollOffset, double scrollDelta);
}

class BoundedScrollBehavior extends ScrollBehavior {
  double minOffset;
  double maxOffset;

  BoundedScrollBehavior({this.minOffset: 0.0, this.maxOffset});

  double applyCurve(double scrollOffset, double scrollDelta) {
    double newScrollOffset = scrollOffset + scrollDelta;
    if (minOffset != null)
      newScrollOffset = math.max(minOffset, newScrollOffset);
    if (maxOffset != null)
      newScrollOffset = math.min(maxOffset, newScrollOffset);
    return newScrollOffset;
  }
}

class OverscrollBehavior extends ScrollBehavior {
  Simulation release(Particle particle) {
    if (particle.position >= 0.0) {
      if (particle.velocity == 0.0)
        return null;
      System system = new ParticleInBoxWithFriction(
        particle: particle,
        box: new Box(min: 0.0),
        friction: _kFriction);
      return new Simulation(system,
        terminationCondition: () => particle.velocity == 0.0);
    }
    System system = new ParticleClimbingRamp(
        particle: particle,
        box: new Box(max: 0.0),
        slope: _kSlope,
        targetPosition: 0.0);
    return new Simulation(system,
        terminationCondition: () => particle.position == 0.0);
  }

  double applyCurve(double scrollOffset, double scrollDelta) {
    double newScrollOffset = scrollOffset + scrollDelta;
    if (newScrollOffset < 0.0) {
      // If we're overscrolling, we want move the scroll offset 2x slower than
      // we would otherwise. Therefore, we "rewind" the newScrollOffset by half
      // the amount that we moved it above. Notice that we clap the "old" value
      // to 0.0 so that we only reduce the portion of scrollDelta that's applied
      // beyond 0.0.
      newScrollOffset -= (newScrollOffset - math.min(0.0, scrollOffset)) / 2.0;
    }
    return newScrollOffset;
  }
}