Learning the iOS 4 SDK for JavaScript Programmers

iPhone App Development Design Patterns

Not only is delegation powerful on its own merits, but the concept is employed widely across numerous iOS SDK frameworks. For example, if your app needs to retrieve updated data from a server, you will use a class called NSURLConnection to initiate the request. Naturally, you don’t want the app to be locked up while it waits for the server to return data (especially if the wireless network is slow), so the NSURLConnection class can send an asynchronous request to the server and wait in the background for a response, whether it’s data or perhaps an error. How is your app supposed to know when data has arrived so the app can work on that data? That’s where the delegate model comes in: while it waits and receives data, the NSURLConnection object sends a series of messages to whichever object has been assigned as the delegate (usually the same object that issued the request). In that object’s class definition are a group of methods that match the NSURLConnection messages to act on the incoming data. One of the messages signals that all data has arrived, and the matching delegate method can then send other messages to begin processing and displaying the data in the app.

Delegation is one of several design patterns associated with iOS app programming. In object-oriented programming, a design pattern suggests the relationships among objects and how objects should work together. Going back to the NSURLConnection class, your app uses that class without ...