DeferredWithUpdate.js

by Ben Nadel www.bennadel.com

DeferredWithUpdate.js is an AngularJS class that decorates the core $q class with two additional promise methods:

• promise.update()
• promise.mistake()

By default, a Deferred object can only be resolved or rejected once in its life cycle. The same is (mostly) true for the DeferredWithUpdate.js. The difference is that if the resolve or reject methods are called after a Deferred instance has been taken out of its pending states, the update() and mistake() handlers will be invoked (rather than ignored).

Right now, this only decorates deferred objects that are create with the defer() method. The other $q methods (all(), reject(), when()) are simply passed-through to the underlying $q instance.

Intended To Be Used With Cached Responses

At first, this might sound like a completely crazy idea; but, it does have a very specific use case: using cached data to temporarily resolve a deferred response from the server.

When the request is initially made, the service layer could instantly resolve the deferred response with cached data (if its available). Then, once the actual data is returned, the deferred object could be "updated" with the fresh data. Similarly, if an already-resolved response returns a 404 (or some other failed HTTP status code), the deferred object could trigger the "mistake" callbacks.

The beautiful thing about this is that the API for the Deferred object is still exactly the same:

• resolve()
• reject()

The code that instantiates the DeferredWithUpdate.js doesn't have to change its behavior at all. The invoking of the update() and mistake() methods are handled completely internally to the DeferredWithUpdate.js class.