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.
Related
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!
I'm writing a library, which will potentially be used by people that aren't me.
Let's say I write a class:
InterestingClass.h
#interface InterestingClass: NSObject
- (id)initWithIdentifier:(NSString *)Identifier;
#end
InterestingClass.m
#interface InterestingClass()
- (void)interestingMethod;
#end
#implementation InterestingClass
- (id)initWithIdentifier:(NSString *)Identifier {
self = [super init];
if (self) {
[self interestingMethod];
}
return self;
}
- (void)interestingMethod {
//do some interesting stuff
}
#end
What if somebody is using the library later down the line and decides to create a subclass of InterestingClass?:
InterestingSubClass.h
#interface InterestingSubClass: InterestingClass
#end
InterestingSubClass.m
#interface InterestingSubClass()
- (void)interestingMethod;
#end
#implementation InterestingSubClass
- (void)interestingMethod {
//do some equally interesting, but completely unrelated stuff
}
#end
The future library user can see from the public interface that initWithIdentifier is a method of the superclass. If they override this method, they'll probably assume (correctly) that the superclass method should be called in the subclass implementation.
However, what if they define a method (in the subclass private interface) which inadvertently has the same name as an unrelated method in the superclass 'private' interface? Without them reading the superclass private interface, they won't know that instead of just creating a new method, they've also overridden something in the superclass. The subclass implementation may end up getting called unexpectedly, and the work that the superclass is expecting to be done when calling the method will not get done.
All of the SO questions I've read seem to suggest that this is just the way that ObjC works and that there isn't a way of getting around it. Is this the case, or can I do something to protect my 'private' methods from being overridden?
Alternatively, is there any way to scope the calling of methods from my superclass so I can be sure that the superclass implementation will be called instead of a subclass implementation?
AFAIK, the best you can hope for is declaring that overrides must call super. You can do that by defining the method in the superclass as:
- (void)interestingMethod NS_REQUIRES_SUPER;
This will compile-time flag any overrides that don't call super.
For framework code a simple way to deal with this is to just give all of your private methods a private prefix.
You'll often notice in stack traces that the Apple frameworks call private methods often starting with an under bar _.
This would only really be a real concern if you are indeed providing a framework for external use where people can not see your source.
NB
Don't start your methods with an under bar prefix as this convention is already reserved
I have a class that essentially acts as a light weight wrapper class around another class. It holds that other class as an iVar. I want to be able to expose certain properties (quite a few actually) of the iVar, but to do so I have to write out each property accessor like so:
- (void) setProperty:(Class *)value{
_iVar.property = value;
}
- (Class *) property{
return _iVar.property;
}
Of course, I have to do this for every single property, which is a pain (there are about 30 of them). I would love to be able to synthesize this but I haven't been able to figure out how.
Is it possible to synthesize?
Also, I can't subclass....well, I might be able to but it's really not recommended. The iVar class is really quite heavy (it implements CoreText). I'd rather write out the methods by hand.
Ok, so here's the solution I found...ended up being pretty simple once you knew what to do. First overwrite '- (id) forwardingTargetForSelector:(SEL)aSelector' and return the iVar:
- (id) forwardingTargetForSelector:(SEL)aSelector{
return iVar;
}
When the runtime is looking for a method and cannot find one, it will call this method to see if there is another object to forward the message to. Note that this method normally returns nil and if you return nil here, your program will crash (which is the appropriate behavior).
The second part of the problem is to shush the compiler errors/warnings you'll get when you try to send a message that's not declared. This is easily done by declaring a category you don't implement.
#interface Class (iVarClassMethods)
#propoperty (strong) Class *property1;
......more properties
#end
As long as you don't put in an implementation anywhere, aka #implementation Class (category), the compiler won't complain (it'll assume that the implementation is somewhere....).
Now the only drawback I see is if you change any of the properties in the interface of the iVar Class, you need to make sure you update all other classes that use the method described above, otherwise you'll crash when another class tries to send what is now the wrong method (and the compiler won't warn you beforehand). However, this can be gotten around. You can declare protocols in a category. So instead you create a separate protocol for the iVar class and move the methods/properties you wish out of the iVar class into the protocol.
#protocol iVarClassProtocol
#propoperty (strong) Class *property1;
......more properties
#end
Add that protocol to the iVar subclass so it has those methods declared through the protocol now.
#interface iVarClass <iVarClassProtocol>
....other methods/properties you don't need forwarded
#end
Finally, simply add the protocol to the category. So instead of the aforementioned category with explicit declarations you'll have:
#interface Class (iVarClassMethods) <iVarClassProtocol>
#end
Now, if you need to change any of the to-be-fowarded properties/methods, you change them in the protocol. The compiler will then warn you when you try to send the wrong method to the forwarding class.
I think you can forward the messages to the ivar:
- (void) forwardInvocation: (NSInvocation*) invocation
{
[invocation invokeWithTarget:ivar];
}
- (NSMethodSignature*) methodSignatureForSelector: (SEL) selector
{
NSMethodSignature *our = [super methodSignatureForSelector:selector];
NSMethodSignature *ivars = [ivar methodSignatureForSelector:selector];
return our ? our : ivars;
}
Then you have to hide or fake the type of your object, for example by casting to id, otherwise the compiler will complain that your class does not implement those methods.
Of course it would be best if you could come up with some better design that would do without such tricks.
In objective-c it is possible to add a #dynamic to a property.
Is this also possible for normal instance methods?
EDIT
I think i wasn't clear enough.
I want to do the following:
#interface MyClass
#property (retain) NSObject *somePropertyObject;
- (void) myMethod;
#end
#implementation MyClass
#dynamic somePropertyObject;
//Make myMethod dynamic. I do not want to implement it. Like C++ Virtual
#end
If you mean "How can I declare a method, but not provide a definition which I will subsequently provide at runtime?" Then it's easy, just use a category. Like this:
#interface MyObject : NSObject
// Methods I'll define
- (void)doFoo;
#end
#interface MyObject (DynamicallyProvidedMethods)
// Methods I won't
- (void)myDynamicMethod;
#end
#implementation MyObject
// Methods I'll define
- (void)doFoo
{
}
#end
The compiler will not complain, however if you call -myDynamicMethod at runtime, unless you have provided an implementation for it somehow, it will crash with "unrecognized selector." You can, of course, test for that at runtime by calling respondsToSelector:.
Relatedly, if you're looking to do a near-equivalent of a base class pure virtual method, I would recommend providing an empty implementation that asserts when called if it has not been overridden by a subclass. You can do that like so:
NSAssert((class_getInstanceMethod([self class], _cmd) == class_getInstanceMethod([MyObject class], _cmd)),
#"Subclass of %# must override -%#",
NSStringFromClass([MyObject class]),
NSStringFromSelector(_cmd));
// ...where overridesSelector:ofBaseClass: looks like:
//
// return ;
Of course, that won't alert you to problems at compile time, but it's better than nothing.
HTH
I think you might be asking how to declare a method that will be implemented some time later somewhere else.
The Objective-C way to do that is to use Protocols.
You declare a protocol like this, usually in a header file
#protocol MyProtocol <NSObject> {
#optional
- (void)optionalMethod;
#required
- (void)requiredMethod;
}
#end
This declares two methods, one which is optional and one is required. To use this protocol you declare the conformance when declaring the class that will implement the protocol
#interface MyConformingClass : NSObject <MyProtocol> {
}
// you don't have to redeclare methods that are declared in the protocol
#end
This new class is checked at compile time for the implementation of requiredMethod so it has to implement it, but it can choose whether or not to implement the optionalMethod
Now, any class that requires instances of objects to conform to the protocol can declare this, for example, in the interface
#interface RequiringClass : NSObject {
MyConformingClass <MyProtocol> *conformingClassObject;
}
…
#end
Again, this is checked at compile time
To make sure that the conforming class implement the #optional methods, we can use this handy structure
if [conformingClassObject respondsToSelector:#selector(optionalMethod)] {
[conformingClassObject optionalMethod];
} else {
// Do something here because the optional method isn't provided
}
Examples of this are all over Cocoa - it's a class can provide a list of actions that it would like to farm out to it's delegate, the delegate adopts the protocol and provides the implementations of those delegate methods. The calling object can then check if this delegate responds to those methods at runtime as I've described above, and call those methods to perform actions, or provide information where ever it needs to.
This is used quite a lot in Objective-C, where classes provide a list of methods that they would like some other class to perform, unlike virtual functions, where a class declares functions it wants subclasses to provide implementations for. Particularly as Composition is favoured over inheritance in the language. Rather than create a subclass to provide an implementation, you just create another class that can do the same thing, and add a reference to that in the class instead.
No.
#dynamic is just an instruction to the compiler that says: "Don't bother generating accessors for this property, I'm going to provide my own."
Using #dynamic with other methods wouldn't be helpful because the compiler doesn't generate any methods other than accessors for you, and of course you're supplying the other methods anyway.
What are you trying to accomplish?
I have three classes which implement the same protocol, and have the same parent class which doesn't implement the protocol. Normally I would have the protocol as pure virtual functions in the parent class but I couldn't find an Objective-C way to do that.
I want to utilize polymorphism on these subclasses by declaring pure virtual functions in the superclass then have the children implement those functions. However the only Objective-C way I've found to do this is to have each child explicitly decide to implement a protocol, when I do it this way the superclass doesn't know the children will implement that protocol so there are compile time warnings all over the place.
Some pseudo-code if that didn't make sense:
#interface superclass: NSObject
{}
#interface child1: superclass<MyProtocol>
{}
#interface child2: superclass<MyProtocol>
{}
The consumer of these classes:
#class child1
#class child2
#class superclass
#interface SomeViewController: UIViewController
{
child1 *oneView;
child2 *otherView;
superclass *currentView;
}
-(void) someMethod
{
[currentView protocolFunction];
}
The only nice way I've found to do pure virtual functions in Objective-C is a hack by putting [self doesNotRecognizeSelector:_cmd]; in the parent class, but it isn't ideal since it will cause runtime errors rather than compile time.
Objective-C developers commonly use dynamic checking rather than compile-time checking in these situations because the language and the frameworks support it so well. So for example, you could write your method like this:
- (void)someMethod
{
// See if the object in currentView conforms to MyProtocol
//
if ([currentView conformsToProtocol:#protocol(MyProtocol)])
{
// Cast currentView to the protocol, since we checked to make
// sure it conforms to it. This keeps the compiler happy.
//
[(SuperClass<MyProtocol> *) currentView protocolMethod];
}
}
I was able to get the compiler to warn me correctly by making the superclass *currentView property look like this:
#property (nonatomic, retain) superclass<MyProtocol> *currentView;
Alternatively you can use the following
if ([unknownObject conformsToProtocol:#protocol(MyProtocol)])
[unknownObject performSelector:#selector(methodInProtocol)];
instead of the following if you just want to suppress the warning.
if ([unknownObject conformsToProtocol:#protocol(MyProtocol)])
[unknownObject methodInProtocol]; // will cause warning
performSelector: will only work if the number of arguments is zero or one. More flexible invocations can be achieved with NSInvocation.
Personally, I would implement the protocol on the super class, but implement the methods like this:
- (id) myProtocolMethod {
NSAssert(NO, [NSString stringWithFormat:#"-[%# %#] must be overridden", NSStringFromClass([self class]), NSStringFromSelector(_cmd)]);
return nil;
}
That way if you ever forget to override a method in a concrete subclass, it should be immediately obvious.