Rationalize Dart mojo and sky package structure

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 _kScrollFriction = 0.005;
-const double _kOverscrollFriction = 0.075;
-const double _kBounceSlopeAngle = math.PI / 512.0; // radians
-
-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 {
-
-  double _contentsHeight;
-  double get contentsHeight => _contentsHeight;
-  void set contentsHeight (double value) {
-    if (_contentsHeight != value) {
-      _contentsHeight = value;
-      // TODO(ianh) now what? what if we have a simulation ongoing?
-    }
-  }
-
-  double _containerHeight;
-  double get containerHeight => _containerHeight;
-  void set containerHeight (double value) {
-    if (_containerHeight != value) {
-      _containerHeight = value;
-      // TODO(ianh) now what? what if we have a simulation ongoing?
-    }
-  }
-
-  OverscrollBehavior({double contentsHeight: 0.0, double containerHeight: 0.0})
-    : _contentsHeight = contentsHeight,
-      _containerHeight = containerHeight;
-
-  double get maxScroll => math.max(0.0, _contentsHeight - _containerHeight);
-
-  Simulation release(Particle particle) {
-    System system;
-    if ((particle.position >= 0.0) && (particle.position < maxScroll)) {
-      if (particle.velocity == 0.0)
-        return null;
-      System slowdownSystem = new ParticleInBoxWithFriction(
-        particle: particle,
-        friction: _kScrollFriction,
-        box: new GeofenceBox(min: 0.0, max: maxScroll, onEscape: () {
-          (system as Multisystem).transitionToSystem(new ParticleInBoxWithFriction(
-            particle: particle,
-            friction: _kOverscrollFriction,
-            box: new ClosedBox(),
-            onStop: () => (system as Multisystem).transitionToSystem(getBounceBackSystem(particle))
-          ));
-        }));
-      system = new Multisystem(particle: particle, system: slowdownSystem);
-    } else {
-      system = getBounceBackSystem(particle);
-    }
-    return new Simulation(system, terminationCondition: () => particle.position == 0.0);
-  }
-
-  System getBounceBackSystem(Particle particle) {
-    if (particle.position < 0.0)
-      return new ParticleClimbingRamp(
-        particle: particle,
-        box: new ClosedBox(max: 0.0),
-        theta: _kBounceSlopeAngle,
-        targetPosition: 0.0);
-    return new ParticleClimbingRamp(
-      particle: particle,
-      box: new ClosedBox(min: maxScroll),
-      theta: _kBounceSlopeAngle,
-      targetPosition: maxScroll);
-  }
-
-  double applyCurve(double scrollOffset, double scrollDelta) {
-    double newScrollOffset = scrollOffset + scrollDelta;
-    // 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. We
-    // do similar things for overscroll in the other direction.
-    if (newScrollOffset < 0.0) {
-      newScrollOffset -= (newScrollOffset - math.min(0.0, scrollOffset)) / 2.0;
-    } else if (newScrollOffset > maxScroll) {
-      newScrollOffset -= (newScrollOffset - math.max(maxScroll, scrollOffset)) / 2.0;
-    }
-    return newScrollOffset;
-  }
-}