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

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

- make the ScrollBehavior instance long-lived, rather than recreating
  it each time we update the list contents.
- have OverscrollBehavior track the total height of the contents and
  the height of the scrollable region, so that it can determine when
  to stop scrolling down.
- teach OverscrollBehavior about how to determine when to stop
  scrolling down, and how to bounce when it's too far down.
- replace the 'energy' concept in Particles with a method that sets
  the energy and direction at the same time, instead of assuming that
  the direction is always positive when setting energy.
- make FixedHeightScrollable lists track the number of items in the
  list and have them update their ScrollBehavior regarding this
  information as it changes.
- track how many items are currently showing in the list stock list.

R=eseidel@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/f0da1d12a46c94aa9d72ce63b0eba5c08a47ef05

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

eseidel@chromium.org changed reviewers:
+ eseidel@chromium.org

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

lgtm

https://codereview.chromium.org/1097373002/diff/20001/sky/framework/animation...
File sky/framework/animation/mechanics.dart (right):

https://codereview.chromium.org/1097373002/diff/20001/sky/framework/animation...
sky/framework/animation/mechanics.dart:28: void setVelocityFromEnergy({double
energy, double direction}) {
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]

https://codereview.chromium.org/1097373002/diff/20001/sky/framework/animation...
File sky/framework/animation/mechanics.dart (right):

https://codereview.chromium.org/1097373002/diff/20001/sky/framework/animation...
sky/framework/animation/mechanics.dart:28: void setVelocityFromEnergy({double
energy, double direction}) {
On 2015/04/21 19:52:33, eseidel wrote:
> Why the {}?  Doesn't that make them optional named args?  Does energy=1.0 not
> work when they're non-optional?

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.)

[Hixie, 2015-04-21 20:20:34]

Committed patchset #2 (id:20001) manually as
f0da1d12a46c94aa9d72ce63b0eba5c08a47ef05 (presubmit successful).

[abarth-chromium, 2015-06-04 16:23:16]

abarth@chromium.org changed reviewers:
+ abarth@chromium.org

[abarth-chromium, 2015-06-04 16:23:17]

https://codereview.chromium.org/1097373002/diff/20001/sky/examples/stocks/lib...
File sky/examples/stocks/lib/stock_list.dart (right):

https://codereview.chromium.org/1097373002/diff/20001/sky/examples/stocks/lib...
sky/examples/stocks/lib/stock_list.dart:26: itemCount = filteredStocks.length;
Won't this |length| query force the filter to be eagerly evaluated on the entire
list?  What if the list of stocks is very, very long?  Previously, this code
only needed to touch enough of the list to get the information visible in the
viewport.

[Hixie, 2015-06-04 16:35:00]

On 2015/06/04 at 16:23:17, abarth wrote:
>
https://codereview.chromium.org/1097373002/diff/20001/sky/examples/stocks/lib...
> File sky/examples/stocks/lib/stock_list.dart (right):
> 
>
https://codereview.chromium.org/1097373002/diff/20001/sky/examples/stocks/lib...
> sky/examples/stocks/lib/stock_list.dart:26: itemCount = filteredStocks.length;
> Won't this |length| query force the filter to be eagerly evaluated on the
entire list?  What if the list of stocks is very, very long?  Previously, this
code only needed to touch enough of the list to get the information visible in
the viewport.

Yes. I don't really see a way around that, though.