[Effen] Prevent scrolling past the bottom of a scrollable list.

sky/framework/animation/mechanics.dart

Index: sky/framework/animation/mechanics.dart
diff --git a/sky/framework/animation/mechanics.dart b/sky/framework/animation/mechanics.dart
index d946917248db46757a25298c97288232f63db5be..6a6cc4cad46f7f8c744afe9cd8ac06757e5ba1a8 100644
--- a/sky/framework/animation/mechanics.dart
+++ b/sky/framework/animation/mechanics.dart
@@ -25,10 +25,10 @@ class Particle extends System {
     position += velocity * deltaT;
   }
 
-  double get energy => 0.5 * mass * velocity * velocity;
-         set energy(double e) { // J
-    assert(e >= 0.0);
-    velocity = math.sqrt(2.0 * e / mass);
+  void setVelocityFromEnergy({double energy, double direction}) {

[eseidel, 2015/04/21 19:52:33]

Why the {}?  Doesn't that make them optional named args?  Does energy=1.0 not
work when they're non-optional?

[Hixie, 2015/04/21 20:18:52]

It makes them named, yes. (They're always "optional" in the sense that even
non-optional arguments can just be passed "null" without anything in Dart
complaining, hence the asserts.)

+    assert(direction == -1.0 || direction == 1.0);
+    assert(energy >= 0.0);
+    velocity = math.sqrt(2.0 * energy / mass) * direction;
   }
 }
 
@@ -174,7 +174,10 @@ class ParticleClimbingRamp extends System {
     // potential energy at the top of the slope, which is g*h*m.
     // If the slope's horizontal component is delta P long, then
     // the height is delta P times tan theta.
-    particle.energy = -kGravity * (deltaPosition * tanTheta) * particle.mass;
+    particle.setVelocityFromEnergy(
+      energy: (kGravity * (deltaPosition * tanTheta) * particle.mass).abs(),
+      direction: deltaPosition > 0.0 ? 1.0 : -1.0
+    );
     box.confine(particle);
   }