Question

Accessing Web Services inside an iPhone app is a matter for which I did not find a clear, beautiful solution yet. I’m not talking about how to send queries or parse responses here, but about a “big picture” answer.

Disregarding the server-side technology, how do/would you plug your Model objects to your Web Service ? How do you design your proxy objects ? How do you cache your resources ?

Answer 1

If your web service happens to be a Ruby on Rails application, then Objective Resource is a great tool: http://iphoneonrails.com/.

If not, then what I tend to do is use ASIHTTPRequest (http://allseeing-i.com/ASIHTTPRequest/) which provides a nice network layer. Depending on the API, you might use ASI objects directly, or else you can subclass an existing ASI class if you want to add per-request functionality (such as authentication or response parsing).

You usually want to run requests in the background, so that the UI isn’t blocked while waiting for the request to finish. You can always go the background thread route, but a nice “Objective C” style approach that ASIHTTPRequest provides is to instead provide a delegate which is called when the request finishes (see also “Creating an asynchronous request” at http://allseeing-i.com/ASIHTTPRequest/How-to-use). In many cases the request delegate is the view controller that initiates the request.

The model layer depends on the complexity, and also what format the data comes in. Most of the APIs I’ve worked with use JSON, for which you can use SBJSON or yajl-objc to parse. These usually give you the data parsed into base classes like NSString, NSArray and NSDictionary. Sometimes that’s sufficient, or if you want your models to exist as their own classes, then you can have the models inherit from a base class that takes care of turning the NSDictionary/NSArray into properties.

Finally for caching, Core Data provides a good way to persist to disk. For caching in memory, you can have the requests occur in a separate “manager” class that is shared between controllers. These managers use the Singleton design pattern as described here: What should my Objective-C singleton look like?.