An Objective-C delegate is an object that has been assigned to the delegate
property another object. To create one, you define a class that implements the delegate methods you're interested in, and mark that class as implementing the delegate protocol.
For example, suppose you have a UIWebView
. If you'd like to implement its delegate's webViewDidStartLoad:
method, you could create a class like this:
@interface MyClass<UIWebViewDelegate>
// ...
@end
@implementation MyClass
- (void)webViewDidStartLoad:(UIWebView *)webView {
// ...
}
@end
Then you could create an instance of MyClass and assign it as the web view's delegate:
MyClass *instanceOfMyClass = [[MyClass alloc] init];
myWebView.delegate = instanceOfMyClass;
On the UIWebView
side, it probably has code similar to this to see if the delegate responds to the webViewDidStartLoad:
message using respondsToSelector:
and send it if appropriate.
if([self.delegate respondsToSelector:@selector(webViewDidStartLoad:)]) {
[self.delegate webViewDidStartLoad:self];
}
The delegate property itself is typically declared weak
(in ARC) or assign
(pre-ARC) to avoid retain loops, since the delegate of an object often holds a strong reference to that object. (For example, a view controller is often the delegate of a view it contains.)
Making Delegates for Your Classes
To define your own delegates, you'll have to declare their methods somewhere, as discussed in the Apple Docs on protocols. You usually declare a formal protocol. The declaration, paraphrased from UIWebView.h, would look like this:
@protocol UIWebViewDelegate <NSObject>
@optional
- (void)webViewDidStartLoad:(UIWebView *)webView;
// ... other methods here
@end
This is analogous to an interface or abstract base class, as it creates a special type for your delegate, UIWebViewDelegate
in this case. Delegate implementors would have to adopt this protocol:
@interface MyClass <UIWebViewDelegate>
// ...
@end
And then implement the methods in the protocol. For methods declared in the protocol as @optional
(like most delegate methods), you need to check with -respondsToSelector:
before calling a particular method on it.
Naming
Delegate methods are typically named starting with the delegating class name, and take the delegating object as the first parameter. They also often use a will-, should-, or did- form. So, webViewDidStartLoad:
(first parameter is the web view) rather than loadStarted
(taking no parameters) for example.
Speed Optimizations
Instead of checking whether a delegate responds to a selector every time we want to message it, you can cache that information when delegates are set. One very clean way to do this is to use a bitfield, as follows:
@protocol SomethingDelegate <NSObject>
@optional
- (void)something:(id)something didFinishLoadingItem:(id)item;
- (void)something:(id)something didFailWithError:(NSError *)error;
@end
@interface Something : NSObject
@property (nonatomic, weak) id <SomethingDelegate> delegate;
@end
@implementation Something {
struct {
unsigned int didFinishLoadingItem:1;
unsigned int didFailWithError:1;
} delegateRespondsTo;
}
@synthesize delegate;
- (void)setDelegate:(id <SomethingDelegate>)aDelegate {
if (delegate != aDelegate) {
delegate = aDelegate;
delegateRespondsTo.didFinishLoadingItem = [delegate respondsToSelector:@selector(something:didFinishLoadingItem:)];
delegateRespondsTo.didFailWithError = [delegate respondsToSelector:@selector(something:didFailWithError:)];
}
}
@end
Then, in the body, we can check that our delegate handles messages by accessing our delegateRespondsTo
struct, rather than by sending -respondsToSelector:
over and over again.
Informal Delegates
Before protocols existed, it was common to use a category on NSObject
to declare the methods a delegate could implement. For example, CALayer
still does this:
@interface NSObject(CALayerDelegate)
- (void)displayLayer:(CALayer *)layer;
// ... other methods here
@end
This tells the compiler that any object might implement displayLayer:
.
You would then use the same -respondsToSelector:
approach as described above to call this method. Delegates implement this method and assign the delegate
property, and that's it (there's no declaring you conform to a protocol). This method is common in Apple's libraries, but new code should use the more modern protocol approach above, since this approach pollutes NSObject
(which makes autocomplete less useful) and makes it hard for the compiler to warn you about typos and similar errors.
Best Answer
People who come from other environments always seem to belive that something complicated is happening with the .c, .m, and .h files used in C and Objective-C programming.
Actually, its very, VERY simple.
For the purpose of buiding a project Integrated Development Environments - like XCode - ignore all the .h files. What they do do is to take each .c and .m file and compile it. If the programmer (thats you) has used any #include, or #import directives, the compiler inserts the entire text of the included/imported .h file where the directive was.
So, if you had a .h file - insert.h - that said:
And a .c file that said:
The compiler would, after processing the #include & #import directives, see this:
It is this very VERY simple file merging behavior that we use to make complicated programs :)
.h is very simply a convention by which programmers can tell each other that the file is suitable to be merged in - potentially multiple times - using #include or #import.
The .c and .m files are not merged like that. Each .c and .m file is compiled seperately - to produce .o files. Each .o file is a collection of compiled functions. The .o files are then merged - or "linked" - to produce the final program. The linking step ensures that each function exists only once, and that all functions that are called do in fact exist somewhere.
C & Objctive-C define one special function that must exist somewhere -
main()
. Again, the language is very relaxed - it doesn't care which .c or .m file themain()
function is in. Merely that it exists in some file somewhere.