Question

OK so I’m trying to get started with Xcode and I have some experience with OOP in general but mostly I’m used to scripting. Anyhoo, I’m trying to get a handle on some concepts in objective C and xcode and I’m having some problems putting everything together.

For starters, I’m having trouble understanding what delegates and protocols do. I think it would be useful if someone could explain this with a simple analogy of a postman, or a teacher, or a factory or something. I don’t understand the difference between a method in a delegate and a regular class methods.

Say I have a Class Postman. Now postman has methods sortMail() and deleteMail(). What’s an example of a delegate method. And if a delegate is used, where is the data returned? Inside the delegate? Do I have to instantiate the delegate and then read results from it or does the delegate kinda give the results back to the calling object? Where do protocols come in…

Simple examples please 🙂 Baby steps.

Answer 1

Protocols and Delegates go together frequently. It helps to understand what a protocol is first.

Protocol

A protocol is a way of having a class promise to implement a standard set of methods.

Example: A certified electrician has a certain set of skills that all certified electricians will have. If you need someone to do something that a certified electrician is certified to do, then any certified electrician should be able to do it (in theory at least).

Delegate

Now a delegate is an object that has been given a responsibility to fulfill certain requirements. One object can be given the responsibility of fulfilling a need of another object.

Example: When building a house, the house needs to have wires run etc. This responsibility has been given to a certified electrician, and we know he can do it because he’s certified (i.e. implements a certain protocol).

Putting it all together in a Cocoa context:

A UITableView needs cells supplied so it can display them. To supply the cells, a class will need to be created (or at least specified) which implements the UITableViewDataSource protocol. That guarantees that the class does the needed things to supply the UITableView with the needed cells.

So the UITableView delegates the responsibility of providing the cells to a certain class object which implements the protocol which guarantees that the object knows how to supply the needed cells.

Example