Shopify/batman — pull request #479 by kristianpd (2012-06-26 14:45:16+00:00)
Title: Add key path to observe callbacks to allow for common handlers

Adds key path to observe callbacks to allow for shared handlers that only need inspect the key.

```coffee
commonHandler = (newVal, oldVal, key) ->
console.log(key, ' changed value from ', oldVal, ' to ', newVal)

obj.observe key, commonHandler for key in ['foo', 'bar', 'baz']
```

/cc @hornairs @nciagra