In our Xcode project we have multiple targets which share some common code. Each target includes only sources which are actually used by it. So when we use some category methods inside classes which are shared between targets we need to make sure that this category implementation is also included in all targets. Xcode doesn't show any warnings during compile time or link time if we forget to include category implementation to some of the targets. And it is troublesome to do it by hand.
Is there any automated way to ensure that category implementations are included to the targets which use them?
Categories are not automatically linked to the final binary.
They are linked if the linker finds the file where they are defined is used (which was a source of constant bug some times ago).
What you can do is use a special flag on the linker: '-all_load' and '-ObjC' in Build Settings/Linking/Other Linker flags
-ObjC Loads all members of static archive libraries that implement an Objective-C class or category.
And from this discussion:
-all_load and -force_load tell the linker to link the entire static archive in the final executable, even if the linker thinks that parts
of the archive are unused.
Another way I use to force link the module is to put a C function in the file:
void _linkWithNBLogClass(void)
{
NSLog(#"%s", __FUNCTION__);
}
and call it at the start of my application:
linkWithNBLogClass();
This way, by the console feedback, I'm sure my module is loaded and ready to be used.
The described behavior is as intended and much existing code would break, if it is changed.
Prior to formal protocols there was a need to declare methods without defining them. This was for optional methods, i. e. for declaring a delegate API. The usual technique was to declare a so-called informal protocol, consisting of a category on NSObject that is never implemented.
But if you have a category implementation, of course the completeness of it is checked against the category interface. (Otherwise you get a "Method definition for X is not found" error.) So you do not have a missing method in the category implementation, but a missing category implementation.
I do not think that this is a big deal. You will get a runtime error instead of a compile time error and simply add the category implementation to the target.
Related
I'm working on few plugins for Quartz Composer, that all link to the same custom static library copied for each of them in the bundles frameworks folder. The plugins could be used separately, so I have to distribute the library in each plugin.
Everything goes well, apart from the isMemberOfClass and isKindOfClass methods. I read here that importing twice the same classes could be the origin of the problem.
I have no error at compilation.
Let's say that I have 2 plugins (NSBundles) that contains the lib XCode project and compile it before linking to it.
They both copy the lib in their resources folder.
Then, they both instantiate a custom hOzPolygon2D class from that library.
The first plugin return true to the test of the hOzPolygon2D object with isMemberOfClass method.
The second return false.
isKindOfCLass method returns the same "error".
I can't imagine a solution in my case. I'm really not a compilation professional and would really appreciate some help.
You should distribute the static library separately (possibly as its own framework). From the question title I assume you're seeing duplicate symbol errors from the linker. If you statically link the same static library into multiple other libraries and then try to link an application to more than one of those libraries you're bound to see these duplicate symbol issues. I haven't actually tried this with frameworks, but I know of this issue from linking iOS apps against interdependent static libraries.
You shouldn't worry about the fact that the modules can be used separately. Just make sure your users can also get the base library. This is a normal situation. For example AppKit and UIKit depend on Foundation, but neither of them actually contains a copy of Foundation.
I'm trying to find the best way to package a static library(lets call it Lib1) that includes an optional class(say, ClassA), which itself requires a second static library(Lib2). In other words, Lib2 is only needed if ClassA is referenced in the project's code. Things seem to work fine, unless Lib1 is used in a project that doesn't use ClassA(and hence does not include Lib2), but requires the -ObjC linker flag(because of other project dependencies, not mine).
I'm trying to come up with a an easy solution for the following three scenarios:
1) project includes my static lib, does NOT use the optional class, does not specify the -ObjC flag
2) project includes my static lib, does NOT use the optional class, but requires -ObjC flag
3) project includes my static lib + second static library, and DOES use the optional class (we don't care about the -ObjC flag at this point)
Is there a linker flag out there to strip my optional class out of the final project app so that it doesn't require the second static lib? I guess my other alternatives are to release multiple versions of my static lib, one that includes the option class(the standard choice), one that does not(the alternate, for projects with -ObjC requirements), or maybe supply a stub file, that supplies empty implementations of all the classes needed from the second static library? This seems like it could be a common problem in the static library world... is there a best practice for this scenario?
Thanks!
Solution:
1) Suggest to my -ObjC users that they use -force_load instead. (thanks Rob!)
2) For users that can't do 1, I'll have a alternate build that does not include ClassA
The best practice is always to have the final binary link all the static libs required. You should never bundle one static library into another. You should absolutely never bundle a well-known (i.e. open-source) static library into a static library you ship. This can create incredible headaches for the final consumer because they can wind up with multiple versions of the same code. Tracking down the bugs that can come from this is insanely difficult. If they're lucky, they'll just get confusing compiler errors. If they're unlucky, their code will behave in unpredictable ways and randomly crash.
Ship all the static libraries separately. Tell your clients which ones they need to link for various configurations. Trying to avoid this just makes their lives difficult.
Some other discussions that may be useful:
Duplicate Symbol Error: SBJsonParser.o? (Example of a customer who ran into a vendor doing this to him)
Linking static libraries, that share another static library
Why don't iOS framework dependencies need to be explicitly linked to a static library project or framework project when they do for an app project?
The -ObjC flag should be preventing the automatic stripping of ClassA entirely, whether its used or not (see TN1490 for more details).
If ClassA is never used except in certain circumstances and you want to save space, you should probably move ClassA into its own static library. Or use #ifdef to conditionally compile it.
Alternately, you can remove the -ObjC flag and use -force_load to individually load any category-only compile units (which is the problem -ObjC is used to address).
I left the original, so people can understand the context for the comments. Hopefully, this example will better help explain what I am after.
Can I create a class in Obj-C that has file-scope visibility?
For example, I have written a method-sqizzling category on NSNotificationCenter which will automatically remove any observer when it deallocs.
I use a helper class in the implementation, and to prevent name collision, I have devised a naming scheme. The category is NSNotificationCenter (WJHAutoRemoval), so the private helper class that is used in this code is named...
WJH_NSNotification_WJHAutoRemoval__Private__BlockObserver
That's a mouthful, and currently I just do this...
#define BlockObserver WJH_NSNotification_WJHAutoRemoval__Private__BlockObserver
and just use BlockObserver in the code.
However, I don't like that solution.
I want to tell the compiler, "Hey, this class is named Bar. My code will access it as Bar, but I'm really the only one that needs to know. Generate a funky name yourself, or better yet, don't even export the symbol since I'm the only one who should care."
For plain C, I would is "static" and for C++ "namespace { }"
What is the preferred/best/only way to do this in Obj-C?
Original Question
I want to use a helper class inside the implementation of another. However, I do not want external linkage. Right now, I'm just making the helper class name painfully unique so I will not get duplicate linker symbols.
I can use static C functions, but I want to write a helper class, with linker visibility only inside the compilation unit.
For example, I'd like to have something like the following in multiple .m files, with each "Helper" unique to that file, and no other compilation unit having linker access. If I had this in 10 different files, I'd have 10 separate classes.
#interface Helper : NSObject
...
#end
#implementation Helper : NSObject
...
#end
I have been unable to find even a hint of this anywhere, and my feeble attempts at prepending "static" to the interface/implementation were wrought with errors.
Thanks!
I don't believe you will be able to do what you want because of the Objective-C Runtime. All of your classes are loaded into the runtime and multiple classes with the same name will conflict with each other.
Objective-C is a dynamic language. Unlike other languages which bind method calls at compile time, Objective-C does method resolution at invocation (every invocation). The runtime finds the class in the runtime and then finds the method in the class. The runtime can't support distinct classes with the same name and Objective-C doesn't support namespaces to seperate your classes.
If your Helper classes are different in each case they will need distinct class names (multiple classes with the same name sounds like a bad idea to me, in any language). If they are the same then why do you want to declare them separately.
I think you need to rethink your strategy as what you are trying to do doesn't sound very Objective-C or Cocoa.
There's no way to make a class "hidden." As mttrb notes, classes are accessible by name through the runtime. This isn't like C and C++ where class are just symbols that are resolved to addresses by the linker. Every class is injected into the class hierarchy.
But I'm unclear why you need this anyway. If you have a private class WJHAutoRemovalHelper or whatever, it seems very unlikely to collide with anyone else any more than private Apple classes or private 3rdparty framework classes collide. There's no reason to go to heroic lengths to make it obscure; prefixing with WJHAutoRemoval should be plenty to make it unique. Is there some deeper problem you're trying to fix?
BTW as an aside: How are you implementing the rest of this? Are you ISA-swizzling the observer to override its dealloc? This seems a lot of tricky code to make a very small thing slightly more convenient.
Regarding the question of "private" classes, what you're suggesting is possible if you do it by hand, but there really is no reason for it. You can generate a random, unique classname, call objc_allocateClassPair() and objc_registerClassPair on it, and then assign that to a Class variable at runtime. (And then call class_addMethod and class_addIvar to build it up. You can then always refer to it by that variable when you need it. It's still accessible of course at runtime by calling objc_getClassList, but there won't be a symbol for the classname in the system.
But this is a lot of work and complexity for no benefit. ObjC does not spend much time worrying about protecting the program from itself the way C++ does. It uses naming conventions and compiler warning to tell you when you're doing things wrong, and expects that as a good programmer you're going to avoid doing things wrong.
I am building an iOS 5 app with ARC using clang on Xcode 4.2/Lion. Good practice for me is to try to get rid of as many warnings as possible but I'm lost on this one. The app has quite a few classes, but a limited number of their header files (8 or 9) give linker warnings like this:
warning: no rule to process file '$(PROJECT_DIR)/TKMyClass.h' of type sourcecode.objj.h for architecture arm7`
I don't see any significant commonalities across these headers: a couple are 3rd party OSS code; the rest are all my own. One is a plain-C header file (with no corresponding .c) containing nothing but constants, #defines and enums; a couple are UIView and UIViewController subclasses created with Xcode's templates; the rest are ordinary Obj-C classes, some of which inherit from NSObject and some of which don't. All (of my classes) were created from scratch within the project at various times. Both older and newer classes give no warnings.
My project uses a mix of C++, Objective-C++ and Objective C classes. The warning-generating classes here are mostly Objective C (their implementations are in .m files) but the two 3rd party classes are implemented in Objective C++ (.mm).
The project otherwise builds fine and runs in the simulator and on iDevices.
To my shame I'm not particularly familiar with the project settings pages in Xcode. I expect the solution is contained in there somewhere but I'm not sure where to start without breaking things.
That means that you have accidentally added header files to be compiled. You need to go into your projects Build Phases and remove all header files from the Compile Sources section.
Check your Architecture (Project and Target): Standard (armv7) - $(ARCHS_STANDARD_32_BIT) and go to Build Pharse (Compile Source) and cheek there should not be any .h file added there.
I have two frameworks in my Xcode project that both define a class with the same name (B.framework and C.framework both have a class named MyClass), resulting in a couple warnings like so:
Duplicate symbol _OBJC_METACLASS_$_MyClass originally in B.framework/B(MyClass.o) now lazily loaded from C.framework/C(MyClass.o)
Duplicate symbol _OBJC_CLASS_$_MyClass originally in B.framework/B(MyClass.o) now lazily loaded from C.framework/C(MyClass.o)
Then at run time only one of the implementations is loaded, and trying to use the other one will result in a "unrecognized selector sent to instance" because they are totally different classes (even though they have the same name).
I use one of the MyClass implementations directly in my code, but the other framework only uses its MyClass internally and I have no idea why its even exported (its not even mentioned in the frameworks header files, i used nm to view the symbols).
How can I make both frameworks work?
There's no such thing as "exported" classes in Obj-C. Or rather, there's no such thing as "non-exported" classes. This problem is precisely why the use of 2- or 3-letter prefixes on classes is strongly recommended for all Obj-C code. Your only solution (besides not using these frameworks) is to edit one (or both) of the frameworks to change the class name, or if you don't have access to the source, then you need to contact the vendor and ask them to make that change.