Fixes #3065: Add dom-repeat.renderedItemCount property

[arthurevans]

I'm not sure whether this is desirable or not, but it seems like the dom-repeat has this information readily available, and extracting it yourself relies on either accessing internal properties (_instances) or using side effects (domRepeat.indexForElement(domRepeat.previousSibling)+1 for example).

Seems like it could either be exposed as a property with notify, or simply as a property on the dom-change event. The event property might be the most efficient, but an element property seemed more Polymeric.

Feel free to close if this isn't the right API or I'm going about it the wrong way.

[dfreedm]

Let's add readOnly: true, because setting it doesn't make any sense.

[arthurevans]

Good point!

[dfreedm]

Can you add a test case for reacting to renderedItemCount changing?

[arthurevans]

I'm not sure I how to test that using WCT. Is there a similar test case somewhere I can look at?

Specifically, I don't see any tests in the dom-repeat tests that are catching an event.

Could it be as simple as something like:

• set items
• render()
• check renderedItemCount
• remove some items
• render()
• check renderedItemCount

Or do I need to do some kind of async thing to catch the event?

[arthurevans]

I don't know if this is what you're looking for, but I added a test showing the renderedItemCount changing when the repeat is filtered (which was the original use case for this property).

[arthurevans]

Ping. Let me know if this is what you intended.

[dfreedm]

LGTM!