I believe it'd better to add attribute option or even custom binder. Something like this

<div rv-each-todo="todos" rv-track-by="todoIndex"></div>

This binder just gets index value and aliases it with another name

Written like this, rv-track-by becomes a binder. One will believe it will be treated like any other binder. But it's not really a classic binder as it's used to configure an an alias name. We cannot use it as a binder because in the binder we would receive the value of todoIndex wich will probably be undefined

For this to work, the Binder rv-each- would have to read the html value of rv-track-by and not the parsed value (ar you see a better way?).

Finally, is it really necessary to offer customization of index alias? If so, we could handle it with a configuration, like:

rivets.configure({
 // Alias for index in rv-each binder
  iterationAlias: function(modelName) {
    return "%" + modelName + "%";
    // or return "modelName" + Index;
  }
});

Too much customization could lead to less simplicity.

I created a fiddle with possible examples how to implement this without touching rivets source:
http://jsfiddle.net/znv28cmj/6/

I suggest the rv-each-index:todo="todos" syntax

@blikblim can you explain your idea more, I don't understand how it would work.

rv-each-[indexName]:[itemName]="[path]"

The index value would be assigned to indexName instead of hardcoded to index. The previous syntax still valid. Although i think is not a really good syntax

The colon might be a problem anyway http://stackoverflow.com/a/16021232/3098651

I don't think we need to add more syntax on the binders for index generation. It would concern only the rv-each.
Using itemNamein the index name is enough and easy to use (and document).

I believe this is good idea and should be merged in.
It is a small and harmless addition to the code base. I can imagine that people will find this very useful.

Questions:
What should we do about the existing index functionality? It works pretty well : https://jsfiddle.net/0qgge2ek/ It does not support accessing the parent. See line #288 of functional.js for the tests. I think we should keep it. Does anyone have any strong feelings?

@jccazeaux Thanks for the PR. I am going to add the ability to configure the '%' syntax. This will ensure no naming conflicts, and people can name it whatever makes sense in the domain of their app.

@stalniy You bring up a very important point about the rv-each that I feel will need to be addressed eventually. We need a way to set a config on the binder level. For example, if we wanted to implement a more powerful each binder, one that would keep DOM elements in order. We need a way to tell rivets how to keep track of items in a collection. Right now, if you move an item from the middle to the end of a collection, rivets will rebind each item in the collection after the index from which you spliced. If we had a way to tell the binder how to keep track of things, by their ID for example, then it would know that you moved the item and move the dom element to correspond. My gut was the same as yours, to use a data attribute to set config values. Something like data-track-by would be useful. Adding the rv- could potentially collide with custom binders.

I have updated and merged the PR. If anyone objects, please do so soon as we will be releasing a new version soon with this, and other updates.

I think we should keep the old index In the interest of keeping the new release as backwards-compatible as possible.

@Leeds-eBooks I didn't remove the old index in my PR ;-)

👍