added prepareInstancePositionChanged delegate method

added prepareInstancePositionChanged delegate method

R=arv@chromium.org, arv
BUG=

Committed: https://github.com/Polymer/TemplateBinding/commit/786825e

[arv, 2013-10-16 19:12:21]

https://codereview.appspot.com/14743044/diff/1/src/TemplateBinding.js
File src/TemplateBinding.js (right):

https://codereview.appspot.com/14743044/diff/1/src/TemplateBinding.js#newcode...
src/TemplateBinding.js:1006: return false;
seems cleaner to return null than false

https://codereview.appspot.com/14743044/diff/1/src/TemplateBinding.js#newcode...
src/TemplateBinding.js:1083: var previousTerminator = this.getTerminatorAt(index
- 1);
Is getTerminatorAt still O(n)? If so, reportInstancesMoved is going to become
O(n^2) :'(

https://codereview.appspot.com/14743044/diff/1/src/TemplateBinding.js#newcode...
src/TemplateBinding.js:1085: if (previousTerminator === terminator)
I don't think I understand what is going on here. Why does this mean that the
instance wasn't moved?

https://codereview.appspot.com/14743044/diff/1/src/TemplateBinding.js#newcode...
src/TemplateBinding.js:1121: var length = this.terminators.length/2;
ws around binops

[rafaelw, 2013-10-16 19:19:53]

PTAL

https://codereview.appspot.com/14743044/diff/1/src/TemplateBinding.js
File src/TemplateBinding.js (right):

https://codereview.appspot.com/14743044/diff/1/src/TemplateBinding.js#newcode...
src/TemplateBinding.js:1006: return false;
On 2013/10/16 19:12:21, arv wrote:
> seems cleaner to return null than false

Done.

https://codereview.appspot.com/14743044/diff/1/src/TemplateBinding.js#newcode...
src/TemplateBinding.js:1083: var previousTerminator = this.getTerminatorAt(index
- 1);
Nope. O(1). The only caveat is sub-templates, but it's an uncommon case and
worst case is the depth of the sub-template chain.

On 2013/10/16 19:12:21, arv wrote:
> Is getTerminatorAt still O(n)? If so, reportInstancesMoved is going to become
> O(n^2) :'(

https://codereview.appspot.com/14743044/diff/1/src/TemplateBinding.js#newcode...
src/TemplateBinding.js:1085: if (previousTerminator === terminator)
Added a comment. It means there aren't any nodes in the instance. Meaning,
instances with no nodes won't be notified that they were re-ordered.

On 2013/10/16 19:12:21, arv wrote:
> I don't think I understand what is going on here. Why does this mean that the
> instance wasn't moved?

https://codereview.appspot.com/14743044/diff/1/src/TemplateBinding.js#newcode...
src/TemplateBinding.js:1121: var length = this.terminators.length/2;
On 2013/10/16 19:12:21, arv wrote:
> ws around binops

Done.

[arv, 2013-10-16 19:32:58]

LGTM

[rafaelw, 2013-10-16 19:35:51]

Committed patchset #2 manually as r786825e (presubmit successful).