Using different delegates for NSXmlParser

Question

I am trying to figure out the best way to design something. I am writing an iPhone App and for the most part I am using async calls to a web service. This means that I cam setting up a URLConnection, calling start and letting it call me back when the data is available or an exception occurs. This works well and I think is the correct way to handle things.

For example:

I request a list of people from a web service. The resulting list is Xml Person elements which will be translated into an objective-c "Person" object by my XmlDelegate.

When I call the function to get the person, I pass in a "PersonResultDelegate", which is a protocol with a single function called "PersonReceived:(Person *)p". So, each time I get a complete Person object, I call that method and all is well. So, my detail view (or search result view) just receives the elements as they are available.

The problem comes when I need to obtain more then one specific object. In my specific case, I need to get the first and last appointment for a person. So, I need to make two API calls to obtain these two single Appointment objects. Each Appointment object will result in a call to the registered AppointmentResultDelegate, but how will I know which is the first and which is the last? I also need to somehow handle the case when there is no "first" or "last" Appointments and the Delegate will never get called.

What would be the correct way design wise to handle this? Should I add some additional context information to the initial request which is passed back to the handle in the delegate? An opaque piece of data which only makes sense to the person who made the initial call? What are my other options?

Solution

What I actually ended up doing is just passing an opaque piece of data along with the Appointment to the delegate. So, when I request an appointment object I have a method like:

getNextAppointment withDelegate:self withContext:@"next"
getPrevAppointment withDelegate:self withContext:@"prev"

This way when the delegate gets called I know what appointment is being delivered.

Answer 1

"Each Appointment object will result in a call to the registered AppointmentResultDelegate, but how will I know which is the first and which is the last?"

By looking at the order in which you receive these callbacks. Or by looking at some value in that xml data. Like a sequence or data. I don't know your data of course.