Another way of phrasing this question: is it possible for a subclass to be a delegate of its super class? I'm trying to make my code reusable within my app and have a situation where the subsclass needs to implement two methods for it to be functional. How can I ensure this occurs? Or what is the proper way of defining these methods?
Update
I didn't mean to imply that I want the compiler to generate flags. I just want a clean way of organizing my code. Currently I override methods of the superclass. Using that approach the superclass can call [super methodToOverride] and it works. However this doesn't feel very clean to me as there's no way to specify "these are the methods you should override" aside from putting a comment somewhere.
In obj-c, it is not possible to force subclasses to overwrite methods of its superclass. But you can raise an exception in the superclass, should it ever be called because the subclass did not implement a certain method.
But a subclass can be a delegate of its superclass, if the superclass does not implement certain methods, and you can enforce that the delegate implements these methods, if the superclass specifies the protocol, i.e. required methods, and the subclass adopts it.
If you want to force your subclass to implement methods from super class, you can do this as below:
//In super class
- (id)someMethod:(SomeObject*)bla
{
[self doesNotRecognizeSelector:_cmd];
return nil;
}
Your app will crash if subclass will not implement this method and you don't need to call
[super someMethod:bla];
There is no way to do this in compile time. However you can raise an exception in the base class.
Something like this:
#throw [NSException exceptionWithName:NSInternalInconsistencyException
reason:[NSString stringWithFormat:#"You must override %# in a subclass", NSStringFromSelector(_cmd)]
userInfo:nil];
If your question is "how can I get the compiler to flag that a certain class doesn't implement a certain function" then I would say
Define a protocol with non-optional methods -- "By default, all methods declared in a protocol are required methods. This means that any class that conforms to the protocol must implement those methods."
Define a class ("stub") that declares it implements the protocol
Now when a subclass of your stub class is written, the compiler will flag it as an error if the mandatory method(s) aren't implemented
I know that it's awful, but supposed that you need to do this since your 3rdParty SDK requires this design pattern, you could use a Factory pattern:
Supposed then to have the base class MyParentAPIClient and two sub classes like MyFacebookAPIClient and MyGooglePlusAPIClient and that you do something like
self.myAPIClient = [MyParentAPIClient alloc] initWithAPIKey:apiKey];
and that you have defined
##interface MyParentAPIClient : NSObject {
}
-(void)callAPI;
#end
and you have override this in the two subclasses
#implementation MyFacebookAPIClient
-(void)callAPI {
[super callAPI];
// do something specific for this api client
}
#end
and
#implementation MyGooglePlusAPIClient
-(void)callAPI {
[super callAPI];
// do something specific for this api client
}
#end
Then you are doing in your controller
[self.myAPIClient callAPI];
but the super class MyParentAPIClient method is being called.
Now you could do a factory in the base class like:
-(void)callAPI {
if([self isKindOfClass:[MyFacebookAPIClient class]]) {
[((MyFacebookAPIClient*)self) callAPI];
} else if([self isKindOfClass:[MyGooglePlusAPIClient class]]) {
[((MyGooglePlusAPIClient*)self) callAPI];
}
}
Of course this have a downside that is to do not call the super in the sub classes that now become:
#implementation MyFacebookAPIClient
-(void)callAPI {
// [super callAPI]; the factory method called that
// do something specific for this api client
}
#end
and
#implementation MyGooglePlusAPIClient
-(void)callAPI {
// [super callAPI]; being called in the factory
// do something specific for this api client
}
#end
The good news is that there is no change in the methods calls since as soon as you call from the controller:
[self.myAPIClient callAPI];
You will have the calls
[MyParentAPIClient callAPI]; // parent class
[MyFacebookAPIClient callAPI]; // sub class
The other downside is that the parent class must known the subclass instances.
Now if we take a look at the factory:
if([self isKindOfClass:[MyFacebookAPIClient class]]) {
[((MyFacebookAPIClient*)self) callAPI];
} else if([self isKindOfClass:[MyGooglePlusAPIClient class]]) {
[((MyGooglePlusAPIClient*)self) callAPI];
}
}
we could make it better like in several way. Take a look at Dynamic type cast from id to class in objective c and Is there an equivalent to C++'s dynamic cast in Objective-C? or Objective-C dynamic_cast?
Good luck!
The UIGestureRecognizerSubclass.h pattern from UIKit is worth a look, that has all the protected methods that should be overridden and that header is not in the framework include, it is only included in subclasss' .m files. Also, nowadays you can tag methods with NS_REQUIRES_SUPER to require overrides to call super, however it can only be used in interfaces, not protocols so that might influence your design.
For super advanced code, NSAccessibilityProtocols.h in AppKit uses a protocol tag to require subclasses to re-implement methods, even if already implemented by a superclass. Here is an example of that you can paste right into in header in your currently open Xcode project:
NS_PROTOCOL_REQUIRES_EXPLICIT_IMPLEMENTATION
#protocol Protocol
#property (readonly) id theWorstOfTimes;
// -(void)testMethod; // uncomment to test problem
#end
// In this example, ClassA adopts the protocol.
#interface ClassA : NSObject <Protocol>
#property (readonly) id theWorstOfTimes;
#end
#implementation ClassA
- (id)theWorstOfTimes{
return nil; // default implementation does nothing
}
-(void)testMethod{}
#end
// This class subclasses ClassA (which also adopts 'Protocol').
#interface ClassB : ClassA <Protocol>
#end
#implementation ClassB // expected-warning {{property 'theWorstOfTimes' requires method 'theWorstOfTimes' to be defined - use #synthesize, #dynamic or provide a method implementation in this class implementation}}
#end
In Xcode you'll see a yellow line at ClassB's expected-warning that the property method is missing. NS_PROTOCOL_REQUIRES_EXPLICIT_IMPLEMENTATION is just a macro for __attribute__((objc_protocol_requires_explicit_implementation)) and this code sample is modified from the test harness of that feature here.
Although this looks great there is a slight problem. Currently this only works for methods that implement protocols, it used to work also for methods but a bug has been introduced in 2014 via a misunderstanding on the purpose of this feature and thus now it is limited to property methods. I have emailed the author to make them aware so hopefully it changed back to its original and proper behavior. To test the bug you can uncomment the method in the protocol and you will see there is no warning in ClassB. Hopefully you can change some of your methods to read-only properties to at least get some use out of it. On the plus side when Xcode offers to "Fix" the issue it does add stubs for the missing methods.
Here is some documentation on NS_PROTOCOL_REQUIRES_EXPLICIT_IMPLEMENTATION:
ImplementingAccessibilityforCustomControls
nsaccessibilitybutton
If you used this then pat yourself on the back for becoming an ObjC expert if you weren't already!
In a subclass, I'm overriding a method that is not exposed in the super class. I know that I have the correct signature as it is successfully overriding the superclass implementation. However, as part of the the new implementation, I need to call the superclass's implementation from the subclass's implementation.
Because it's not exposed I have to invoke the method via a call to performSelector:
SEL superClassSelector = NSSelectorFromString(#"methodToInvoke");
[super performSelector:superClassSelector];
However, in my application this results in an infinite recursive loop where the subclass's implementation is invoked every time I try to invoke the superclass's implementation.
Any thoughts?
I realize this is an atypical situation but unfortunately there's no way to get around what I'm trying to do.
The way I've dealt with this is to re-declare your super class' interface in your subclass implementation file with the method you want to call from the subclass
#interface MySuperclass()
- (void)superMethodIWantToCall;
#end
#implementation MySubclass
- (void)whateverFunction {
//now call super method here
[super superMethodIWantToCall];
}
#end
I'm not sure if this is the best way to do things but it is simple and works for me!
This doesn't work because you're only sending performSelector:, not the selector you pass to that, to the superclass. performSelector: still looks up the method in the current class's method list. Thus, you end up with the same subclass implementation.
The simplest way to do this may be to just write in your own call to objc_msgSendSuper():
// Top level (this struct isn't exposed in the runtime header for some reason)
struct objc_super
{
id __unsafe_unretained reciever;
Class __unsafe_unretained superklass;
};
// In the subclass's method
struct objc_super sup = {self, [self superclass]};
objc_msgSendSuper(&sup, _cmd, other, args, go, here);
This can cause problems in the general case, as Rob Napier has pointed out below. I suggested this based on the assumption that the method has no return value.
One way to go is to create a category of your class in a separate file with the method you are trying to expose
#interface MyClass (ProtectedMethods)
- (void)myMethod;
#end
and on the .m
#implementation MyClass (ProtectedMethods)
- (void)myMethod {
}
#end
Then, import this category from your .m files, and you're good to go. It's not the prettiest thing, but it'll do the trick
I have a category on NSObject which supposed to so some stuff. When I call it on an object, I would like to override its dealloc method to do some cleanups.
I wanted to do it using method swizzling, but could not figure out how. The only examples I've found are on how to replace the method implementation for the entire class (in my case, it would override dealloc for ALL NSObjects - which I don't want to).
I want to override the dealloc method of specific instances of NSObject.
#interface NSObject(MyCategory)
-(void)test;
#end
#implementation NSObject(MyCategory)
-(void)newDealloc
{
// do some cleanup here
[self dealloc]; // call actual dealloc method
}
-(void)test
{
IMP orig=[self methodForSelector:#selector(dealloc)];
IMP repl=[self methodForSelector:#selector(newDealloc)];
if (...) // 'test' might be called several times, this replacement should happen only on the first call
{
method_exchangeImplementations(..., ...);
}
}
#end
You can't really do this since objects don't have their own method tables. Only classes have method tables and if you change those it will affect every object of that class. There is a straightforward way around this though: Changing the class of your object at runtime to a dynamically created subclass. This technique, also called isa-swizzling, is used by Apple to implement automatic KVO.
This is a powerful method and it has its uses. But for your case there is an easier method using associated objects. Basically you use objc_setAssociatedObject to associate another object to your first object which does the cleanup in its dealloc. You can find more details in this blog post on Cocoa is my Girlfriend.
Method selection is based on the class of an object instance, so method swizzling affects all instances of the same class - as you discovered.
But you can change the class of an instance, but you must be careful! Here is the outline, assume you have a class:
#instance MyPlainObject : NSObject
- (void) doSomething;
#end
Now if for just some of the instances of MyPlainObject you'd like to alter the behaviour of doSomething you first define a subclass:
#instance MyFancyObject: MyPlainObject
- (void) doSomething;
#end
Now you can clearly make instances of MyFancyObject, but what we need to do is take a pre-existing instance of MyPlainObject and make it into a MyFancyObject so we get the new behaviour. For that we can swizzle the class, add the following to MyFancyObject:
static Class myPlainObjectClass;
static Class myFancyObjectClass;
+ (void)initialize
{
myPlainObjectClass = objc_getClass("MyPlainObject");
myFancyObjectClass = objc_getClass("MyFancyObject");
}
+ (void)changeKind:(MyPlainObject *)control fancy:(BOOL)fancy
{
object_setClass(control, fancy ? myFancyObjectClass : myPlainObjectClass);
}
Now for any original instance of MyPlainClass you can switch to behave as a MyFancyClass, and vice-versa:
MyPlainClass *mpc = [MyPlainClass new];
...
// masquerade as MyFancyClass
[MyFancyClass changeKind:mpc fancy:YES]
... // mpc behaves as a MyFancyClass
// revert to true nature
[MyFancyClass changeKind:mpc: fancy:NO];
(Some) of the caveats:
You can only do this if the subclass overrides or adds methods, and adds static (class) variables.
You also need a sub-class for ever class you wish to change the behaviour of, you can't have a single class which can change the behaviour of many different classes.
I made a swizzling API that also features instance specific swizzling. I think this is exactly what you're looking for: https://github.com/JonasGessner/JGMethodSwizzler
It works by creating a dynamic subclass for the specific instance that you're swizzling at runtime.
I'm originally a Java programmer who now works with Objective-C. I'd like to create an abstract class, but that doesn't appear to be possible in Objective-C. Is this possible?
If not, how close to an abstract class can I get in Objective-C?
Typically, Objective-C class are abstract by convention only—if the author documents a class as abstract, just don't use it without subclassing it. There is no compile-time enforcement that prevents instantiation of an abstract class, however. In fact, there is nothing to stop a user from providing implementations of abstract methods via a category (i.e. at runtime). You can force a user to at least override certain methods by raising an exception in those methods implementation in your abstract class:
[NSException raise:NSInternalInconsistencyException
format:#"You must override %# in a subclass", NSStringFromSelector(_cmd)];
If your method returns a value, it's a bit easier to use
#throw [NSException exceptionWithName:NSInternalInconsistencyException
reason:[NSString stringWithFormat:#"You must override %# in a subclass", NSStringFromSelector(_cmd)]
userInfo:nil];
as then you don't need to add a return statement from the method.
If the abstract class is really an interface (i.e. has no concrete method implementations), using an Objective-C protocol is the more appropriate option.
No, there is no way to create an abstract class in Objective-C.
You can mock an abstract class - by making the methods/ selectors call doesNotRecognizeSelector: and therefore raise an exception making the class unusable.
For example:
- (id)someMethod:(SomeObject*)blah
{
[self doesNotRecognizeSelector:_cmd];
return nil;
}
You can also do this for init.
Just riffing on #Barry Wark's answer above (and updating for iOS 4.3) and leaving this for my own reference:
#define mustOverride() #throw [NSException exceptionWithName:NSInvalidArgumentException reason:[NSString stringWithFormat:#"%s must be overridden in a subclass/category", __PRETTY_FUNCTION__] userInfo:nil]
#define methodNotImplemented() mustOverride()
then in your methods you can use this
- (void) someMethod {
mustOverride(); // or methodNotImplemented(), same thing
}
Notes: Not sure if making a macro look like a C function is a good idea or not, but I'll keep it until schooled to the contrary. I think it's more correct to use NSInvalidArgumentException (rather than NSInternalInconsistencyException) since that's what the runtime system throws in response to doesNotRecognizeSelector being called (see NSObject docs).
The solution I came up with is:
Create a protocol for everything you want in your "abstract" class
Create a base class (or maybe call it abstract) that implements the protocol. For all the methods you want "abstract" implement them in the .m file, but not the .h file.
Have your child class inherit from the base class AND implement the protocol.
This way the compiler will give you a warning for any method in the protocol that isn't implemented by your child class.
It's not as succinct as in Java, but you do get the desired compiler warning.
From the Omni Group mailing list:
Objective-C doesn't have the abstract compiler construct like Java at
this time.
So all you do is define the abstract class as any other normal class
and implement methods stubs for the abstract methods that either are
empty or report non-support for selector. For example...
- (id)someMethod:(SomeObject*)blah
{
[self doesNotRecognizeSelector:_cmd];
return nil;
}
I also do the following to prevent the initialization of the abstract
class via the default initializer.
- (id)init
{
[self doesNotRecognizeSelector:_cmd];
[self release];
return nil;
}
Instead of trying to create an abstract base class, consider using a protocol (similar to a Java interface). This allows you to define a set of methods, and then accept all objects that conform to the protocol and implement the methods. For example, I can define an Operation protocol, and then have a function like this:
- (void)performOperation:(id<Operation>)op
{
// do something with operation
}
Where op can be any object implementing the Operation protocol.
If you need your abstract base class to do more than simply define methods, you can create a regular Objective-C class and prevent it from being instantiated. Just override the - (id)init function and make it return nil or assert(false). It's not a very clean solution, but since Objective-C is fully dynamic, there's really no direct equivalent to an abstract base class.
This thread is kind of old, and most of what I want to share is already here.
However, my favorite method is not mentioned, and AFAIK there’s no native support in the current Clang, so here I go…
First, and foremost (as others have pointed out already) abstract classes are something very uncommon in Objective-C — we usually use composition (sometimes through delegation) instead. This is probably the reason why such a feature doesn’t already exist in the language/compiler — apart from #dynamic properties, which IIRC have been added in ObjC 2.0 accompanying the introduction of CoreData.
But given that (after careful assessment of your situation!) you have come to the conclusion that delegation (or composition in general) isn’t well suited to solving your problem, here’s how I do it:
Implement every abstract method in the base class.
Make that implementation [self doesNotRecognizeSelector:_cmd];…
…followed by __builtin_unreachable(); to silence the warning you’ll get for non-void methods, telling you “control reached end of non-void function without a return”.
Either combine steps 2. and 3. in a macro, or annotate -[NSObject doesNotRecognizeSelector:] using __attribute__((__noreturn__)) in a category without implementation so as not to replace the original implementation of that method, and include the header for that category in your project’s PCH.
I personally prefer the macro version as that allows me to reduce the boilerplate as much as possible.
Here it is:
// Definition:
#define D12_ABSTRACT_METHOD {\
[self doesNotRecognizeSelector:_cmd]; \
__builtin_unreachable(); \
}
// Usage (assuming we were Apple, implementing the abstract base class NSString):
#implementation NSString
#pragma mark - Abstract Primitives
- (unichar)characterAtIndex:(NSUInteger)index D12_ABSTRACT_METHOD
- (NSUInteger)length D12_ABSTRACT_METHOD
- (void)getCharacters:(unichar *)buffer range:(NSRange)aRange D12_ABSTRACT_METHOD
#pragma mark - Concrete Methods
- (NSString *)substringWithRange:(NSRange)aRange
{
if (aRange.location + aRange.length >= [self length])
[NSException raise:NSInvalidArgumentException format:#"Range %# exceeds the length of %# (%lu)", NSStringFromRange(aRange), [super description], (unsigned long)[self length]];
unichar *buffer = (unichar *)malloc(aRange.length * sizeof(unichar));
[self getCharacters:buffer range:aRange];
return [[[NSString alloc] initWithCharactersNoCopy:buffer length:aRange.length freeWhenDone:YES] autorelease];
}
// and so forth…
#end
As you can see, the macro provides the full implementation of the abstract methods, reducing the necessary amount of boilerplate to an absolute minimum.
An even better option would be to lobby the Clang team to providing a compiler attribute for this case, via feature requests. (Better, because this would also enable compile-time diagnostics for those scenarios where you subclass e.g. NSIncrementalStore.)
Why I Choose This Method
It get’s the job done efficiently, and somewhat conveniently.
It’s fairly easy to understand. (Okay, that __builtin_unreachable() may surprise people, but it’s easy enough to understand, too.)
It cannot be stripped in release builds without generating other compiler warnings, or errors — unlike an approach that’s based on one of the assertion macros.
That last point needs some explanation, I guess:
Some (most?) people strip assertions in release builds. (I disagree with that habit, but that’s another story…) Failing to implement a required method — however — is bad, terrible, wrong, and basically the end of the universe for your program. Your program cannot work correctly in this regard because it is undefined, and undefined behavior is the worst thing ever. Hence, being able to strip those diagnostics without generating new diagnostics would be completely unacceptable.
It’s bad enough that you cannot obtain proper compile-time diagnostics for such programmer errors, and have to resort to at-run-time discovery for these, but if you can plaster over it in release builds, why try having an abstract class in the first place?
Using #property and #dynamic could also work. If you declare a dynamic property and don't give a matching method implementation, everything will still compile without warnings, and you'll get an unrecognized selector error at runtime if you try to access it. This essentially the same thing as calling [self doesNotRecognizeSelector:_cmd], but with far less typing.
In Xcode (using clang etc) I like to use __attribute__((unavailable(...))) to tag the abstract classes so you get an error/warning if you try and use it.
It provides some protection against accidentally using the method.
Example
In the base class #interface tag the "abstract" methods:
- (void)myAbstractMethod:(id)param1 __attribute__((unavailable("You should always override this")));
Taking this one-step further, I create a macro:
#define UnavailableMacro(msg) __attribute__((unavailable(msg)))
This lets you do this:
- (void)myAbstractMethod:(id)param1 UnavailableMacro(#"You should always override this");
Like I said, this is not real compiler protection but it's about as good as your going to get in a language that doesn't support abstract methods.
The answer to the question is scattered around in the comments under the already given answers. So, I am just summarising and simplifying here.
Option1: Protocols
If you want to create an abstract class with no implementation use 'Protocols'. The classes inheriting a protocol are obliged to implement the methods in the protocol.
#protocol ProtocolName
// list of methods and properties
#end
Option2: Template Method Pattern
If you want to create an abstract class with partial implementation like "Template Method Pattern" then this is the solution.
Objective-C - Template methods pattern?
Another alternative
Just check the class in the Abstract class and Assert or Exception, whatever you fancy.
#implementation Orange
- (instancetype)init
{
self = [super init];
NSAssert([self class] != [Orange class], #"This is an abstract class");
if (self) {
}
return self;
}
#end
This removes the necessity to override init
(more of a related suggestion)
I wanted to have a way of letting the programmer know "do not call from child" and to override completely (in my case still offer some default functionality on behalf of the parent when not extended):
typedef void override_void;
typedef id override_id;
#implementation myBaseClass
// some limited default behavior (undesired by subclasses)
- (override_void) doSomething;
- (override_id) makeSomeObject;
// some internally required default behavior
- (void) doesSomethingImportant;
#end
The advantage is that the programmer will SEE the "override" in the declaration and will know they should not be calling [super ..].
Granted, it is ugly having to define individual return types for this, but it serves as a good enough visual hint and you can easily not use the "override_" part in a subclass definition.
Of course a class can still have a default implementation when an extension is optional. But like the other answers say, implement a run-time exception when appropriate, like for abstract (virtual) classes.
It would be nice to have built in compiler hints like this one, even hints for when it is best to pre/post call the super's implement, instead of having to dig through comments/documentation or... assume.
If you are used to the compiler catching abstract instantiation violations in other languages, then the Objective-C behavior is disappointing.
As a late binding language it is clear that Objective-C cannot make static decisions on whether a class truly is abstract or not (you might be adding functions at runtime...), but for typical use cases this seems like a shortcoming. I would prefer the compiler flat-out prevented instantiations of abstract classes instead of throwing an error at runtime.
Here is a pattern we are using to get this type of static checking using a couple of techniques to hide initializers:
//
// Base.h
#define UNAVAILABLE __attribute__((unavailable("Default initializer not available.")));
#protocol MyProtocol <NSObject>
-(void) dependentFunction;
#end
#interface Base : NSObject {
#protected
__weak id<MyProtocol> _protocolHelper; // Weak to prevent retain cycles!
}
- (instancetype) init UNAVAILABLE; // Prevent the user from calling this
- (void) doStuffUsingDependentFunction;
#end
//
// Base.m
#import "Base.h"
// We know that Base has a hidden initializer method.
// Declare it here for readability.
#interface Base (Private)
- (instancetype)initFromDerived;
#end
#implementation Base
- (instancetype)initFromDerived {
// It is unlikely that this becomes incorrect, but assert
// just in case.
NSAssert(![self isMemberOfClass:[Base class]],
#"To be called only from derived classes!");
self = [super init];
return self;
}
- (void) doStuffUsingDependentFunction {
[_protocolHelper dependentFunction]; // Use it
}
#end
//
// Derived.h
#import "Base.h"
#interface Derived : Base
-(instancetype) initDerived; // We cannot use init here :(
#end
//
// Derived.m
#import "Derived.h"
// We know that Base has a hidden initializer method.
// Declare it here.
#interface Base (Private)
- (instancetype) initFromDerived;
#end
// Privately inherit protocol
#interface Derived () <MyProtocol>
#end
#implementation Derived
-(instancetype) initDerived {
self= [super initFromDerived];
if (self) {
self->_protocolHelper= self;
}
return self;
}
// Implement the missing function
-(void)dependentFunction {
}
#end
Probably this kind of situations should only happen at development time, so this might work:
- (id)myMethodWithVar:(id)var {
NSAssert(NO, #"You most override myMethodWithVar:");
return nil;
}
You can use a method proposed by #Yar (with some modification):
#define mustOverride() #throw [NSException exceptionWithName:NSInvalidArgumentException reason:[NSString stringWithFormat:#"%s must be overridden in a subclass/category", __PRETTY_FUNCTION__] userInfo:nil]
#define setMustOverride() NSLog(#"%# - method not implemented", NSStringFromClass([self class])); mustOverride()
Here you will get a message like:
<Date> ProjectName[7921:1967092] <Class where method not implemented> - method not implemented
<Date> ProjectName[7921:1967092] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[<Base class (if inherited or same if not> <Method name>] must be overridden in a subclass/category'
Or assertion:
NSAssert(![self respondsToSelector:#selector(<MethodName>)], #"Not implemented");
In this case you will get:
<Date> ProjectName[7926:1967491] *** Assertion failure in -[<Class Name> <Method name>], /Users/kirill/Documents/Projects/root/<ProjectName> Services/Classes/ViewControllers/YourClass:53
Also you can use protocols and other solutions - but this is one of the simplest ones.
Cocoa doesn’t provide anything called abstract. We can create a class abstract which gets checked only at runtime, and at compile time this is not checked.
I usually just disable the init method in a class that I want to abstract:
- (instancetype)__unavailable init; // This is an abstract class.
This will generate an error at compile time whenever you call init on that class. I then use class methods for everything else.
Objective-C has no built-in way for declaring abstract classes.
Changing a little what #redfood suggested by applying #dotToString's comment, you actually have the solution adopted by Instagram's IGListKit.
Create a protocol for all the methods that make no sense to be defined in the base (abstract) class i.e. they need specific implementations in the children.
Create a base (abstract) class that does not implement this protocol. You can add to this class any other methods that make sense to have a common implementation.
Everywhere in your project, if a child from AbstractClass must be input to or output by some method, type it as AbstractClass<Protocol> instead.
Because AbstractClass does not implement Protocol, the only way to have an AbstractClass<Protocol> instance is by subclassing. As AbstractClass alone can't be used anywhere in the project, it becomes abstract.
Of course, this doesn't prevent unadvised developers from adding new methods referring simply to AbstractClass, which would end up allowing an instance of the (not anymore) abstract class.
Real world example: IGListKit has a base class IGListSectionController which doesn't implement the protocol IGListSectionType, however every method that requires an instance of that class, actually asks for the type IGListSectionController<IGListSectionType>. Therefore there's no way to use an object of type IGListSectionController for anything useful in their framework.
In fact, Objective-C doesn't have abstract classes, but you can use Protocols to achieve the same effect. Here is the sample:
CustomProtocol.h
#import <Foundation/Foundation.h>
#protocol CustomProtocol <NSObject>
#required
- (void)methodA;
#optional
- (void)methodB;
#end
TestProtocol.h
#import <Foundation/Foundation.h>
#import "CustomProtocol.h"
#interface TestProtocol : NSObject <CustomProtocol>
#end
TestProtocol.m
#import "TestProtocol.h"
#implementation TestProtocol
- (void)methodA
{
NSLog(#"methodA...");
}
- (void)methodB
{
NSLog(#"methodB...");
}
#end
A simple example of creating an abstract class
// Declare a protocol
#protocol AbcProtocol <NSObject>
-(void)fnOne;
-(void)fnTwo;
#optional
-(void)fnThree;
#end
// Abstract class
#interface AbstractAbc : NSObject<AbcProtocol>
#end
#implementation AbstractAbc
-(id)init{
self = [super init];
if (self) {
}
return self;
}
-(void)fnOne{
// Code
}
-(void)fnTwo{
// Code
}
#end
// Implementation class
#interface ImpAbc : AbstractAbc
#end
#implementation ImpAbc
-(id)init{
self = [super init];
if (self) {
}
return self;
}
// You may override it
-(void)fnOne{
// Code
}
// You may override it
-(void)fnTwo{
// Code
}
-(void)fnThree{
// Code
}
#end
Can't you just create a delegate?
A delegate is like an abstract base class in the sense that you say what functions need to be defined, but you don't actually define them.
Then whenever you implement your delegate (i.e abstract class) you are warned by the compiler of what optional and mandatory functions you need to define behavior for.
This sounds like an abstract base class to me.
When using categories, you can override implementation methods with your own like so:
// Base Class
#interface ClassA : NSObject
- (NSString *) myMethod;
#end
#implementation ClassA
- (NSString*) myMethod { return #"A"; }
#end
//Category
#interface ClassA (CategoryB)
- (NSString *) myMethod;
#end
#implementation ClassA (CategoryB)
- (NSString*) myMethod { return #"B"; }
#end
Calling the method "myMethod" after including the category nets the result "B".
What is the easiest way for the Category implementation of myMethod to call the original Class A myMethod? As near as I can figure out, you'd have to use the low level calls to get the original method hook for Class A and call that, but it seemed like there would be a syntactically easier way to do this.
If you want a hackish way to do this that involves mucking with the objective-c runtime you can always use method swizzling (insert standard disclaimers here.) It will allow you to store the different methods as arbitrariliy named selectors, then swap them in at runtime as you need them.
From comp.lang.objective-C FAQ listing: "What if multiple categories implement the same method? Then the fabric of the Universe as we know it ceases to exist. Actually, that's not quite true, but certainly some problems will be caused. When a category implements a method which has already appeared in a class (whether through another category, or the class' primary #implementation), that category's definition overwrites the definition which was previously present. The original definition can no longer be reached by the Objective-C code. Note that if two categories overwrite the same method then whichever was loaded last "wins", which may not be possible to predict before the code is launched."
From developer.apple.com: "When a category overrides an inherited method, the method in the category can, as usual, invoke the inherited implementation via a message to super. However, if a category overrides a method that already existed in the category's class, there is no way to invoke the original implementation"
Check out my article about a solution found on the Mac Developer Library:
http://codeshaker.blogspot.com/2012/01/calling-original-overridden-method-from.html
Basically, it's the same as the above Method Swizzling with a brief example:
#import <objc/runtime.h>
#implementation Test (Logging)
- (NSUInteger)logLength {
NSUInteger length = [self logLength];
NSLog(#"Logging: %d", length);
return length;
}
+ (void)load {
method_exchangeImplementations(class_getInstanceMethod(self, #selector(length)), class_getInstanceMethod(self, #selector(logLength)));
}
#end
With the swizzling "helper" methods included in ConciseKit, you actually call the default implementation… weirdly enough.. by calling your SWIZZLED implementation..
You set it up in + (void) load, calling + (BOOL)swizzleMethod:(SEL)originalSelector with:(SEL)anotherSelector in:(Class)klass;, i.e.
[$ swizzleMethod:#selector(oldTired:)
with:#selector(swizzledHotness:) in:self.class];
and then in the swizzled method.. let's suppose it returns -(id).. you can do your mischief, or whatever reason you are swizzling in the first place… and then, instead of returning an object, or self, or whatnot..
return [self swizzledHotness:yourSwizzledMethodsArgument];
As explained here…
In this method, it looks like we're calling the same method again, causing and endless recursion. But by the time this line is reached the two method have been swapped. So when we call swizzled_synchronize we're actually calling the original method.
It feels and looks odd, but.. it works. This enables you to add endless embellishments to existing methods, and still "call super" (actually self) and reap the benefits of the original method's handiwork… even without access to the original source.