什么时候在Objective-C中不使用协议代表？

Question

A protocol is a list of method statement , anyone who conforms to it must or may implement it .

In most cases,a delegate's implementation relies on protocol,such as below:

@protocol VSActivateCouponDelegate <NSObject>
-(void)activateCouponSuccessWithActivateType:(ActivateType)type;
@end

My question is protocol's usage scenarios except implementing Delegate ?

Answer 1

For example, any interface that you want to keep generic in terms of not knowing the exact class that will be providing a specific role in some relationship. Delegate is one example of that but any relationship between two classes could use the same approach to abstraction.

Answer 2

Check out NSCopying , NSCoding etc. These are also protocols, but they are not used as delegates.

Let's look at NSCopying for example. If a class conforms to NSCopying , you know that you can call copy on any of its instances (and... copy it ), without even looking at the specific class, you can even pass instances as id<NSCopying> - this makes things more decoupled. For instance NSDictionary only accepts key values that are id<NSCopying> .

To summarize, protocols let you declare behavior without coupling it to a specific class. It doesn't always have to be delegates.

Answer 3

an example would be:

say you have an array of arbitrary objects of arbitrary types. now you want to make sure that all the objects at least have a certain function you can call on all of them to make them do something, then you can make all those objects conform to a protocol that has that method, thus making sure all the objects in that array have that certain functionality.

pseudo code of what that may look like:

@protocol MyProtocol {
   - (void) doSomething;
}

//used somewhere
for(MyProtocol object in arrayOfUnknownObjects)
  [object doSomething]; //dont know what object's type is but we know it has [doSomething] method