Problem understanding Delegating pattern & callback functions in Objective C

Question

Can anyone simply explain delegating pattern and callback function in objective C. Or can point to some documents (with Easy and Basic Explanation) which can make these concepts clearer to me. As i am not getting any idea about it from any book or website or (Apple Developer)references.

All the resources i have come across to understand this, makes me more confuse by using the terminology which i find hard to digest.

Any Help Appreciated.

Answer 1

I will try to explain the value of delegation with one example.

Say you have a table; the table will show some rows. Suppose now, you want to customize the way this table react to some event, e.g., the selection of a specific row.

One common way of doing this in OOP is subclassing a table base class and overriding some methods there.

With delegation, you don't need subclassing the table base class; rather you use the base class and tell it to "forward" some messages to some other object. This is the basic idea.

In our example, when the row is clicked, the table base class does not know what do except sending a message to one object that you specify as the delegate to carry through that action.

So, one basic advantage of delegation is that you do not need to subclass. Another advantage you have is that a delegate can act as the delegate for several other objects. Indeed, if you have a look at the generic declaration of a delegate method, you will see that the first parameter is the object that is delegating:

- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath

so when the delegate receives the message, it knows who sent it and with which object it should interact.

The example I gave you about the table shows one kind of delegation, which I would not compare to callbacks. Anyway, delegation can be used as well as a kind of advanced calling-back-scheme.

Take the class NSURLConnection; it can be used to manage aynchronous communication.

Async communication is the typical case where callbacks are used.

With NSURLConnection, the delegate pattern is preferred; so, instead of specifying a callback (a function, which has to be a static function, or a static class method), you specify an object. This object implements the methods that a protocol defines (NSURLConnectionDelegate protocol); you can see those as a whole set of callback functions. When the NSURLConnection has some data ready, it will call a method of the interface, e.g., – connection:didReceiveResponse: to notify that it has received the response.

In such case, the point is not avoiding subclassing, rather it is a more flexible callback mechanisms (allowing for better encapsulation, basically).

I hope this helps clarifying the two concepts...

Answer 2

delegate noun |ˈdeligit|

A person sent or authorized to represent others, in particular an elected representative sent to a conference.

A delegate in the Cocoa Touch frameworks such as Foundation or UIKit is a separate object instance that is entrusted to listen in to, and optionally decide behaviours on behalf of the delegator.

Take the example of UIWebView. The web view can entrust responsibility onto one delegate as long as that instance conforms to the UIWebViewDelegate protocol, that is promises to behave as the web view expects a delegate to behave.

The delegate gets called from time to time. Embedded in the delegate method names is information about what the call is about.

• webView:shouldStartLoadWithRequest:navigationType: - Tells the delegate that the webview wants to load a request, and ask the delegate if it should do so.
• webView:willStartLoad: - Tells the delegate that the web view will start loading a page, and it is too late to do anything (This is a hypothetical method that is not actually available, I use it as a consistent example).
• webView:didFinishLoad: - Tells the delegate that the web view did load a page and it is now finished.

The value of using delegation is that you do not need to sub-class UIWebView in order to tweak how it behaves. One objkect can also serve as delegate to many different webviews, or even serve as delegate to many different kinds of classes.

A normal UIViewController subclass probably conforms to many delegate protocols, for example UITableViewDelegate to respond to selections in a table, UIActionSheetDelegate to respond to selection from an action sheet, etc.