Question

I am using a Object method inside the @protocol , but protocol only supports method declaration , could someone help me about how to connect that with the method definition of the same method .(where to write the method Definition).

Answer 1

Protocols are just a list of method declarations without any implementation. They can be used to restrict a set of possible (classes) to those that implement a certain protocol.

For instance if you had a GasStation class you might want to restrict the type (class) of objects that can be refueled at the station. Of course you could simply require the objects to be a descendant of the class Vehicle, like this:

@interface GasStation : NSObject
- (void)refuel:(Vehicle *)vehicle;
@end

@implementation GasStation 
- (void)refuel:(Vehicle *)vehicle {
    [vehicle addFuel:100];
}
@end

But this wouldn’t allow a jerrycan to be filled up for instance. However allowing any type of object to be passed is a bad idea either, because if you passed an instance of Cat you would certainly get an error when trying to send it an addFuel: message.

So instead you define a protocol that refuelable classes have to implement:

@protocol RefuelableThing
@required
- (void)addFuel:(float)fuel;
@end

Now you can use that protocol to restrict what types of objects can be refueled:

@interface GasStation : NSObject
- (void)refuel:(NSObject<RefuelableThing> *)thing;
@end

A Vehicle will of course have a method addFuel: and so can JerryCan implement its own separate version, even though it’s not a Vehicle.