What is a "delegate" in Objective C's iPhone development? [duplicate]
A delegate is a pointer to an object with a set of methods the delegate-holder knows how to call. In other words, it's a mechanism to enable specific callbacks from a later-created object.
A good example is UIAlertView. You create a UIAlertView object to show a short message box to users, possibly giving them a choice with two buttons like "OK" and "Cancel". The UIAlertView needs a way to call you back, but it has no information of which object to call back and what method to call.
To solve this problem, you can send your self pointer to UIAlertView as a delegate object, and in exchange you agree (by declaring the UIAlertViewDelegate in your object's header file) to implement some methods that UIAlertView can call, such as alertView:clickedButtonAtIndex:.
Check out this post for a quick high-level intro to the delegate design pattern and other callback techniques.
References:
- UIAlertView class reference
- UIAlertViewDelegate class reference
- Apple's guide to Delegates and Data sources
See this discussion
A delegate allows one object to send messages to another object when an event happens. For example, if you're downloading data from a web site asynchronously using the NSURLConnection class. NSURLConnection has three common delegates:
- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error
- (void)connectionDidFinishLoading:(NSURLConnection *)connection
- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response
One or more of these delegates will get called when NSURLConnection encounters a failure, finishes successfully, or received a response from the web site, respectively.
Delegates are a design pattern; there is no special syntax or language support.
A delegate is just an object that another object sends messages to when certain things happen, so that the delegate can handle app-specific details the original object wasn't designed for. It's a way of customizing behavior without subclassing.
I think this Wikipedia article describes it best: http://en.wikipedia.org/wiki/Delegation_pattern
It is "just" an implementation of a design pattern and very common in Objective-C