Delegation in a NSThreaded Design? (iPhone)

Question

Hi

Im using a large amount of very small web services in my app and I have been down a couple of roads that does not scale or work as desired.

The Design approach Im thinking about: The task for the viewController is to ask for a set of data from a general webServicesClass, this task is started in a new NSThread -> this will instantiate an object which solely retrieves the xml and returns it to the webServicesClass -> the webServicesClass now instantiates an object which solely can Parse some XML coming from this particular web service. The Parser then returns a nice Entity object to the webServiceClass. The WebserviceClass now needs to inform the viewController about this data.

The viewController implements a webServiceClassDelegate and some delegate methods to see if the web service request went as planned. e.g. -(void)aWebserviceFailed and -(void)aWebserviceSuccess.

0.5 Since the WebserviceClass is running is a different NSThread, will it be a problem calling delegate methods on the main NSThread in the parent object?

1.0 I think this design is sound as it completely incapsulates the retrieval, parsing and returning of the Entity in different classes. But, I will have to write delegation methods and implement delegation protocols on each step of the way, for each different webservice. i.e. starting from the bottom, the WebserviceClass must implement delegation methods for both the object that retrieves the XML (start, fail, success), then for the object that parses the XML(start, fail, success) and the WebserviceClass must be able to delegate each of these responses to the viewController that again must implement delegation methods from the WebserviceClass(start, fail, success). Is there a much simpler way to do this?

I have some design pattern experience, but not from languages that uses delegation so consistently as Objective C. In AS3 or Java I would have events that could bubble up through the objects and inform whoever was listening about changes. In all the Objective example code I have read I have only seen NSNotifications (which would be the equivalent of the AS3 or Java 'Event') used 0.1% of the times.

The Design I described will give me something that scales perfectly for many web services and gives me complete control over where a potential error/exception happens, but it seems to be a lot of code to obtain this loose coupling.

1.1 Or should I fully embrace the delegation approach and get to work:)

Thanks for any pointers or help given. Im not asking for source code or the likes, more a "this is considered best practice in Objective C in the every day situation you just described" :)

Answer 1

I'd recommend taking a look at ASIHttpRequest(obtainable here) and NSOperation + NSOperationQueue (docs here). I don't think you should run a long-lived thread to talk to your web service all the time, unless you absolutely need a constant connection.

Basically ASIHttpRequest and NSOperation both encapsulate all of the networking and threading stuff. Operations make multi-threading on the iPhone really nice. Essentially you create an operation (through a factory or whatnot for ease of use), pop it in a queue and do something with the result.

As for what you do with the result (this applies to your original scenario too and 0.5 and 1.1) what typically happens is your operation/thread will then call a didSucceedAtGettingWhatever or didFailWithError:(NSError*) method. Delegation is pretty much the defacto way of making requests on the phone. If there are multiple delegates, then you can just use subject-observer, like you would in Java.

As for 1.0, ultimately no. What we typically do is we have an OperationDelegate and OperationTypes. Based upon which OperationType succeeded or completed, we have different logic. It's not the greatest and there are a ton of different ways of doing this, but you will have to have separate logic for separate events regardless of what you do. Whether or not that's in one method or many methods is up to you.