Can a protocol can be inherited privately so that in next inheritance level this should not be accessed ?
Objective-C has no “private inheritance” (or “private conformance”) the way C++ does.
You can conform to a protocol without advertising your conformance in your header file. For example, you can conform to NSCoding “secretly” if you put this above your #implementation statement in your .m file:
#interface MyObject () <NSCoding>
#end
That declares a class extension that adds the NSCoding protocol to the MyObject class.
However, anyone (including a subclass) can ask whether you adopt the protocol:
[[MyObject class] conformsToProtocol:#protocol(NSCoding)]
// returns YES
[[MySubObject class] conformsToProtocol:#protocol(NSCoding)]
// also returns YES, if MySubObject is a subclass of MyObject
and anyone can send an NSCoding message to a MyObject by casting the object first:
[(id<NSCoding>)someObject encodeWithCoder:someCoder]
And if you make a subclass of MyObject, and your subclass also declares that it conforms to NSCoding, then it almost certainly needs to call [super encodeWithCoder:] from its own encodeWithCoder: method.
No, it's not possible because of the language design.
Moreover, as any method could be override, you can't ever be sure that your called method if from the right class.
There may be some hack which permit to directly invoke the method you want, but it's not that easy and may even not be possible.
Protocol or otherwise, there is no way to stop methods being overridden by subclasses
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I'm wondering if it is possible, in a certain subclass, to "recognise" methods declared in it's superclass as implementations of methods declared in a protocol the subclass adheres to, given that they have the same signatures.
It kind of gets hard to even formulate this, that's why any searches I made turned out fruitless so far.
But let me make the case with an example for better understanding.
Protocol:
#protocol TheProtocol <NSObject>
- (void)theProtocolMethod;
#end
Superclass:
// Superclass does not adhere to TheProtocol
#interface TheSuperClass : NSObject
- (void)theProtocolMethod;
#end
#implementation TheSuperClass
- (void)theProtocolMethod
{
// stuff
}
#end
Subclass:
// SubClass adheres to TheProtocol but does not implement it's methods
// since they are implemented in the class it is subclassing. Is this OK?
#interface TheSubClass : TheSuperClass <TheProtocol>
#end
#implementation TheSubClass
#end
Is this anywhere close to being "OK"?
What about the case TheSubClass was in fact a category of TheSuperClass (declaring adherence to TheProtocol) and not a subclass?
A bit more context:
It's a scenario involving Core Data. I need to publish an accessor defined in an Entity Class, in a Protocol that will be used in a separate Framework for developing plugins for my app. The Protocol itself is fully implemented by a Category of the Core Data Entity Class, except for said accessor which is implemented in the Entity Class itself, hence my question.
In absolute terms, this is perfectly legal. Calling -theProtocolMethod on an instance of TheSubClass would indeed invoke TheSuperClass implementation if TheSubClass itself doesn't implement it. You could even call [super theProtocolMethod] in your subclass implementation, if you wanted.
If TheSubClass was a category on TheSuperClass, the superclass implementation would still be called. However, implementing -theProtocolMethod in the category would replace the super class implementation, so you have to be careful here.
Subjectively, in code-design terms, it's maybe a little odd. You essentially have two separate declarations of the same method to manage, which could potentially cause problems if you tried to refactor. I'm guessing the superclass in your case is a library class that you cannot change. Otherwise, I can't see why TheSuperClass shouldn't just conform to the protocol, rather than declare the method separately.
In theory you're saying the superclass is already compliant with the protocol.
If the compiler complains, you can implement wrapper methods in your subclass that simply call super and return any return value from the call to super.
I'm trying to accomplish something like the following:
- (id<SomeProtocol>)instanceFromClass:(Class<SomeProtocol>)cls
{
return [[cls alloc] initUsingSomeConstructorDefinedInProtocolWithValue:_value];
}
However, I'm getting a No Known class method for selector 'alloc' error. How may I specify in my signature that I want to receive a class that conforms to a protocol? Or, if that part is correct, how may I create an instance from that argument using a constructor defined in the specified protocol?
Not sure why the compiler complains but you can fix by casting your parameter back to Class
- (id<SomeProtocol>)instanceFromClass:(Class<SomeProtocol>)cls
{
return [[(Class)cls alloc] initUsingSomeConstructorDefinedInProtocolWithValue:_value];
}
while still getting you the type checking you want for the parameter as hinted at in this SO answer: Declare an ObjC parameter that's a Class conforming to a protocol
Your use of the protocol is 'fine'. The issue is that the cls parameter is tagged as a class which conforms to a protocol that defines instance methods (the init method). This doesn't tell the compiler that the +alloc method is available because that is a class method on NSObject.
You can add the +alloc method to the protocol. Or you can do some casting to tell the compiler to trust you.
+ alloc is a method defined by the top level class NSObject. When you have a class like Class <SomeProtocol>, the compiler only knows that this is some class and it implements SomeProtocol but it cannot know if that is a subclass of NSObject or not, since in Obj-C you can define own top-level classes that don't inherit from NSObject (not that this is generally a good idea but it is possible).
There is a special "hack" in the compiler that in case the type is just Class, the compiler will always assume that it is a subclass of NSObject and would only fail at runtime in case it isn't. But this hack only works for the exact type Class and not for Class <SomeProtocol> which is a distinct type.
So what you can do is to either cast to Class, so the hack works again:
[[(Class)cls alloc] ...]
or you can also do that
[[cls.class alloc] ...]
in case that cls will be a subclass of NSObject at runtime because then it will have a + class method.
Note that if I call instanceFromClass: with a class, that does implement SomeProtocol but is no subclass of NSObject and also does not implement a + alloc method, both methods above will fail at runtime and the app will crash.
I just want to ask you in Objective-C if we conform to a protocol in our class do we have to implement all the methods declared in the protocol?
#interface MyClass:NSObject<NSCopying, NSCoding>
{
}
#end
in this example do I have to implement all the methods inside NSCopying and NSCoding protocols?
You have to implement all the methods that are marked #required, yes. That is the whole point of a protocol; it defines a set of optional and required methods that must be implemented to conform to the protocol.
If the superclass implements the protocol, then you'll inherit that implementation, but it is unlikely to be sufficient for either NSCopying or NSCoding. Thus, you'll override the methods (and call super's implementation in yours).
In Objective-C / Cocoa how do I discover the messages I can implement in a delegate class?
From what I have read in the documentation a class will only allow certain chosen messages to be handled via a delegate but so far have found difficulty finding a list of messages for a class.
To ask the question another way, if I created a delegate for NSApplication, which messages are available for me to handle?
The documentation for NSApplication states a delegate
responds to certain messages on behalf of NSApp.
The documentation then goes onto say
NSApplication defines over twenty delegate methods that offer opportunities for modifying specific aspects of application behavior.
but as far as I can see it fails to list these methods / messages so how do I know which ones will be called from my delegate?
The delegate is usually supposed to conform to a protocol declared for that purpose, e.g. NSApplicationDelegate. If it's not clearly spelled out already, you can often find the name of the appropriate protocol by looking at the type of the delegate property (in this case, id<NSApplicationDelegate>). You can check the documentation for details on the methods, or the #protocol declaration in the appropriate header file for specifics on which methods are available and which are required or optional.
There are some cases where the object does not define a protocol for its delegate, for example NSURLConnection. In this case, you just have to follow the documentation of the class with respect to what it expects of its delegate.
In either case, it is completely up to the class when it sends a message to the delegate and what the semantics of any message are.
These methods are described in the NSApplicationDelegate documentation:
http://developer.apple.com/library/mac/#documentation/cocoa/reference/NSApplicationDelegate_Protocol/Reference/Reference.html
One quick note that is that it is useful to understand the difference between formal and informal protocols. Also formal protocols have optional methods, which needn't be called.
When you write your own protocol you will want to check -respondsToSelector: before sending optional/informal methods.
formal protocols are defined like...
#protocol SomeProtocol <NSObject>
-(void)someMethod:(id)sender;
-(void)someOtherMethod:(NSString*)aString;
#property(nonatomic,retain) someProperty;
#optional
-(BOOL)someOptionalMethod;
#end
and are adopted like...
#interface SomeClass : NSObject <SomeProtocol> {
}
#end
this tells the compiler that you are adopting SomeProtocol, and you wont be required to declare you methods for the protocol in the .h file, it also tell the compiler to warn you if you haven't implemented all of the required methods.
for informal protocols you don't adopt them syntactically, but you will need to declare your methods in the .h file.
In Objective-C, I can add methods to existing classes with a category, e.g.
#interface NSString (MyCategory)
- (BOOL) startsWith: (NSString*) prefix;
#end
Is it also possible to do this with protocols, i.e. if there was a NSString protocol, something like:
#interface <NSString> (MyCategory)
- (BOOL) startsWith: (NSString*) prefix;
#end
I want to do this since I have several extensions to NSObject (the class), using only public NSObject methods, and I want those extensions also to work with objects implementing the protocol .
To give a further example, what if I want to write a method logDescription that prints an object's description to the log:
- (void) logDescription {
NSLog(#"%#", [self description]);
}
I can of course add this method to NSObject, but there are other classes that do not inherit from NSObject, where I'd also like to have this method, e.g. NSProxy. Since the method only uses public members of protocol , it would be best to add it to the protocol.
Edit: Java 8 now has this with "virtual extension methods" in interfaces: http://cr.openjdk.java.net/~briangoetz/lambda/Defender%20Methods%20v4.pdf. This is exactly what I would like to do in Objective-C. I did not see this question earning this much attention...
Regards,
Jochen
Short answer: No.
Long answer: how would this work? Imagine you could add methods to existing protocols? How would this work? Imagine we wanted to add another method to NSCoding, say -(NSArray *) codingKeys; This method is a required method that returns an array of the keys used to encoding the object.
The problem is that there are existing classes (like, say NSString) that already implement NSCoding, but don't implement our codingKeys method. What should happen? How would the pre-compiled framework know what to do when this required message gets sent to a class that does not implement it?
You could say "we can add the definition of this method via a category" or "we could say that any methods added via these protocol categories are explicitly optional". Yes, you could do this and theoretically get around the problem I've described above. But if you're going to do that, you might as well just make it a category in the first place, and then check to make sure the class respondsToSelector: before invoking the method.
While it's true that you can't define categories for protocols (and wouldn't want to, because you don't know anything about the existing object), you can define categories in such a way that the code only applies to an object of the given type that has the desired protocol (sort of like C++'s partial template specialization).
The main use for something like this is when you wish to define a category that depends on a customized version of a class. (Imagine that I have UIViewController subclasses that conform to the Foo protocol, meaning they have the foo property, my category code may have need of the foo property, but I can't apply it to the Foo protocol, and if I simply apply it to UIViewController, the code won't compile by default, and forcing it to compile means someone doing introspection, or just screwing up, might call your code which depends on the protocol. A hybrid approach could work like this:
#protocol Foo
- (void)fooMethod
#property (retain) NSString *foo;
#end
#implementation UIViewController (FooCategory)
- (void)fooMethod {
if (![self conformsToProtocol:#protocol(Foo)]) {
return;
}
UIViewController<Foo> *me = (UIViewController<Foo>*) self;
// For the rest of the method, use "me" instead of "self"
NSLog(#"My foo property is \"%#\"", me.foo);
}
#end
With the hybrid approach, you can write the code only once (per class that is supposed to implement the protocol) and be sure that it won't affect instances of the class that don't conform to the protocol.
The downside is that property synthesis/definition still has to happen in the individual subclasses.
extObjC has the NEATEST stuff you can do with Protocols / Categories... first off is #concreteprotocol...
Defines a "concrete protocol," which can provide default implementations of methods within protocol.
An #protocol block should exist in a header file, and a corresponding #concreteprotocol block in an implementation file.
Any object that declares itself to conform to this protocol will receive its method implementations, but only if no method by the same name already exists.
MyProtocol.h
#protocol MyProtocol
#required - (void)someRequiredMethod;
#optional - (void)someOptionalMethod;
#concrete - (BOOL)isConcrete;
MyProtocol.m
#concreteprotocol(MyProtocol) - (BOOL)isConcrete { return YES; } ...
so declaring an object MyDumbObject : NSObject <MyProtocol> will automatically return YES to isConcrete.
Also, they have pcategoryinterface(PROTOCOL,CATEGORY) which "defines the interface for a category named CATEGORY on a protocol PROTOCOL". Protocol categories contain methods that are automatically applied to any class that declares itself to conform to PROTOCOL." There is an accompanying macro you also have to use in your implementation file. See the docs.
Last, but NOT least / not directly related to #protocols is
synthesizeAssociation(CLASS, PROPERTY), which "synthesizes a property for a class using associated objects. This is primarily useful for adding properties to a class within a category. PROPERTY must have been declared with #property in the interface of the specified class (or a category upon it), and must be of object type."
So many of the tools in this library open (way-up) the things you can do with ObjC... from multiple inheritance... to well, your imagination is the limit.
It isn't really meaningful to do so since a protocol can't actually implement the method. A protocol is a way of declaring that you support some methods. Adding a method to this list outside the protocol means that all "conforming" classes accidentally declare the new method even though they don't implement it. If some class implemented the NSObject protocol but did not descend from NSObject, and then you added a method to the protocol, that would break the class's conformance.
You can, however, create a new protocol that includes the old one with a declaration like #protocol SpecialObject <NSObject>.
I think you may be mixing up terms here and there. Extensions, Categories, Protocols, Interfaces and Classes are all different things in Objective-C. In The Objective-C 2.0 Language Apple describes the differences very well, including the benefits and drawbacks to using categories and extensions.
If you think about it, what is a "Category" or "Extension" in the conceptual sense? It's a way of adding functionality to a Class. In Objective-C, protocols are designed to have no implementation. Therefore, how would you add or extend the implementation of something that doesn't have implementation to begin with?
if you're already writing a category, why not just add in the protocol definition in the header right after the category definition?
i.e.
#interface NSString (MyCategory)
- (BOOL) startsWith: (NSString*) prefix;
#end
#protocol MyExtendedProtocolName <NSString>
//Method declarations go here
#end
this way any class that imports the category header will also get the protocol definition, and you can add it into your class..
#interface MyClass <OriginalProtocol,MyExtendedProtocolName>
also, be careful when subclassing NSString, it's a cluster and you may not always get the behaviour you're expecting.
Adam Sharp posted a solution that worked for me.
It involves 3 steps:
Defining the methods you want to add as #optional on a protocol.
Making the objects you want to extend conform to that protocol.
Copying those methods into those objects at runtime.
Check out the link for the full details.