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.
Related
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.
Baby new to Xcode, Cocoa touch and iOS development in general. And am taking the Stanford walkthrough for their iPhone class. I am a little confused at a couple of places and need to shoot my doubts to you guys:
I have two classes that I have created for my model, essentially CalculatorBrain.m and CalculatorBrain.h.
From what I gather, in Objective C, creating a class essentially consists of two functions, one is to declare the class which contains the method/messages and other variables while the other is the actual implementation for the same. From this stems two questions:
Why must I declare a class without implementing it's methods at first? (the concept seems to be borrowed from interfaces) and only then move on to implementing it .
From the above question, as I go through the walkthrough, I notice that the class declaration took place in CalculatorBrain.h whereas the methods were actually implemented in CalculatorBrain.m. I am unable to grasp the nuances of why this was done so if anybody is willing to shed some light on this, it would really help
Thanks again,
Parijat Kalia!!!
These are traditions from the C world, and they're just common practice to avoid some problems. They aren't two classes, they are the definition (in the .h file) and the implementation (in the .c or .cc file).
If you defined the class in the .c file, you couldn't refer to it elsewhere because it wouldn't be defined. You could include your .c file, but then you'd have two copies of the code. You could also use the "extern" keyword, but at this point it's kind of odd.
If you put code in the .h file, then when it's included the code gets included. This means you can get compiler errors that you have three "getMyThing" functions.
This means you can give out your headers to others without giving away your top-secret implementation (useful for making libraries), include your header without worrying about the possible multiple definitions, etc. You can also add variables and functions in the .c file which people using the header (like your other code) can't see or use, so you don't have to worry about changing it later and having compilation break.
My XCode project has grown somewhat, and I know that there are class files in there which are no longer being used. Is there an easy way to find all of these and remove them?
If the class files just sit in your project without being part of a target, just click on the project itself in the tree view, so you see all files in the table. Make sure you see the "Target" column in the table view, iterate through your targets and find the files that don't have a check anywhere -> they are no longer compiled.
But if you still compile the classes and they are no longer used, that case is a bit more difficult. Check out this project
http://www.karppinen.fi/analysistool/#dependency-graphs
You could create a dependency graph and try to find orphaned classes that way.
Edit: Link went dead, but there still seem to be projects of Objective-C dependency graphs around, for example https://github.com/nst/objc_dep
if they are C or C++ symbols, then you can just let the linker do the work for you.
if you're looking to remove objc symbols, then try to refactor the class name (e.g. to rename the class), and preview the dependencies that it turns up. if you reference classes/selectors/etc. by strings then... it may not be so effective. unfortunately, you often have to also test manually, to verify that removing a class does not break anything. remember that resources (like xibs) may reference/load objc classes as well.
This is a tricky question due to how dynamic objective-c is as you can never guarantee that a class is not going to be used.
Consider if you generate a class name and a selector at run time and then look up that class, instantiate that class and then call a method on that newly created object using that newly created selector. No where in your code do you explicitly name and instantiate that object but you are able to use it anyways. You could get that class name and selector name from anywhere outside of your code, even from some data from a server some where. How would you ever know which class is not going to be used? Because of this there are no tools that are able to perform what you are requesting.
Searching the project with the class name might be an option, thought it may not be the best solution. Specially it might be time consuming when you have many classes.
I've not found anything that addresses my specific name space question as yet.
I am working on some AudioUnit plug-ins featuring Cocoa based GUIs. The plug-ins use a common library of user interface classes (sliders, buttons etc) which are simply added to each Xcode project.
When I recompile and distribute updates it is pretty much guaranteed that at least one user interface class will have been updated since the last release. If the user launches an older plug-in before an updated plug-in then the old Cocoa classes are already loaded into the run time and the plug-in attempts to use the older implementations - often resulting in a failure one way or another.
I know frameworks are the intended solution but the overhead and backwards compatibility issues are not ideal. I prefix all class names where possible but what options do I have to ensure that each plug-in contains unique class names for the shared user interface classes?
Update:
The solution I seem to be arriving at is as follows:
Set a preprocessor compiler flag e.g. OBJC_PREFIX=1.
Create a header file to contain all the class name redefinitions and conditionally include it in the header of each class you want to 'rename' e.g:
#ifdef OBJC_PREFIX
#include "CocoaPrefixHeader.h"
#endif
#interface MySlider : ... etc
Fill the header file (in this case CocoaPrefixHeader) with something like the following:
#define MySlider Prefix_MySlider
#define MyButton Prefix_MyButton
Using ibtool convert all your class names in an existing nib/xib file to the new names e.g:
ibtool --convert MySlider-Prefix_MySlider nibfile.xib --write nibfile2.xib
ibtool --convert MyButton-Prefix_MyButton nibfile2.xib --write nibfile2.xib
This last step converts all class names and outlets etc in the nib file. Once converted you can edit the nib as normal and IB keeps track of the redefined names.
This process is tedious and laborious but it is working for me. Far better to cater for it at the outset.
In your pre-compiled header (.pch) file for each plug-in, you can #define the classes to have different names, e.g.:
#define ClassNameUsedInYourCode ClassNameCompiledInThisProject
#define WidgetButton WahWahPedalPluginWidgetButton
As long as you're creating your UI programmatically, this will ensure that the class names are unique per-plugin. Unfortunately this won't work if you have class names baked into nib files.
In that case, you'd probably need to have some sort of pre-processing script that runs before compilation and replaces any instances of the shared class names with the project-specific class names in all files in the project, including the .xib files. This could get pretty messy but I can't see too many options.
I had a similar issue. I needed to have more than one version of the same bundle running in the same application space at the same time (I can't even remember why). It was not easy, I discussed my problems and options on the Objective-C mailing list. In the end, I modified the build-environment to:
Scan every header for classes declared with #interface.
Create a new header filled with only preprocessor macros that redefine classnames from MyClass to MyClass_v1_00 (or whatever version was defined by the Info.plist file). This header was called ClassRenamer.h.
As an intermediate build step, parse all xib XML files and replace references of MyClass to MyClass_v1_00. This doesn't modify the original xib files, which is handy.
Modify the command-line build flags to include ClassRenamer.h for all .m files.
Surprisingly, everything works perfectly, both at runtime and even in the debugger. If I put a breakpoint on a particular line, it breaks on any version of the class that is loaded, and Xcode even shows the class's name as MyClass_v1_00. The biggest concern is code that looks up classes by name, i.e. using NSClassFromString.
Whilst the solution I arrived at in the updated part of the question works as the final step in a project I can't recommend it for anything where your classes are in a state of flux. I was unable to add additional outlets to classes and have them show up in IB, for example.
In the end I just duplicated my classes and added unique name prefixes for different projects. Using ibtool --convert to update the xib file made this process a lot faster.
Once things settle down maybe a framework will be a better idea.
What is the Objective-C equivalent of Java packages? How do you group and organize your classes in Objective-C?
Question 1: Objective-C equivalent of Java packages?
Objective-C doesn't have an equivalent to Java packages or C++ namespaces. Part of the reason for this is that Objective-C was originally a very thin runtime layer on top of C, and added objects to C with minimum fuss. Unfortunately for us now, naming conflicts are something we have to deal with when using Objective-C. You win some, you lose some...
One small clarification (although it's not much for consolation) is that Objective-C actually has two flat namespaces — one for classes and one for protocols (like Java's interfaces). This doesn't solve any class naming conflicts, but it does mean you can have a protocol and class with the same name (like <NSObject> and NSObject) where the latter usually adopts ("implements") the former. This feature can prevent "Foo / FooImpl" pattern rampant in Java, but sadly doesn't help with class conflicts.
Question 2: How to [name] and organize Objective-C classes?
Naming
The following rules are subjective, but they are decent guidelines for naming Objective-C classes.
If your code can't be run by other code (it's not a framework, plugin, etc. but an end-user application or tool) you only need to avoid conflicts with code you link against. Often, this means you can get away with no prefix at all, so long as the frameworks/plugins/bundles you use have proper namespaces.
If you're developing "componentized" code (like a framework, plugin, etc.) you should choose a prefix (hopefully one that's unique) and document your use of it someplace visible so others know to avoid potential conflicts. For example, the CocoaDev wiki "registry" is a de facto public forum for calling "dibs" on a prefix. However, if your code is something like a company-internal framework, you may be able to use a prefix that someone else already does, so long as you aren't using anything with that prefix.
Organization
Organizing source files on disk is something that many Cocoa developers unfortunately gloss over. When you create a new file in Xcode, the default location is the project directory, right beside your project file, etc. Personally, I put application source in source/, test code (OCUnit, etc.) in test/, all the resources (NIB/XIB files, Info.plist, images, etc.) in resources/, and so on. If you're developing a complex project, grouping source code in a hierarchy of directories based on functionality can be a good solution, too. In any case, a well-organized project directory makes it easier to find what you need.
Xcode really doesn't care where your files are located. The organization in the project sidebar is completely independent of disk location — it is a logical (not physical) grouping. You can organize however you like in the sidebar without affecting disk location, which is nice when your source is stored in version control. On the other hand, if you move the files around on disk, patching up Xcode references is manual and tedious, but can be done. It's easiest to create your organization from the get-go, and create files in the directory where they belong.
My Opinion
Although it could be nice to have a package/namespace mechanism, don't hold your breath for it to happen. Class conflicts are quite rare in practice, and are generally glaringly obvious when they happen. Namespaces are really a solution for a non-problem in Objective-C. (In addition, adding namespaces would obviate the need for workarounds like prefixes, but could introduce a lot more complexity in method invocation, etc.)
The more subtle and devious bugs come from method conflicts when methods are added and/or overridden, not only by subclasses, but also be categories, which can cause nasty errors, since the load order of categories is undefined (nondeterministic). Implementing categories is one of the sharpest edges of Objective-C, and should only be attempted if you know what you're doing, particularly for third-party code, and especially for Cocoa framework classes.
They use long names...
Article on coding style & naming in Cocoa / Objective-C
Discussion whether Obj-C needs namespaces (deleted, archive here)
See
What is the best way to solve an Objective-C namespace collision?
for a discussion of how Objective-C has no namespaces, and the painful hacks this necessitates.
Unfortuantely objective c doesn't have any equivalent to namespace of C#,c++ and package of java....
The naming collisions could be solved by giving contextual name for example if u gonna give a name to method it should imply the class and module that it comes in so that...these problems could be avoided.
Go through the following url to know more on naming convention as advised by apple
http://developer.apple.com/library/ios/#documentation/cocoa/conceptual/ProgrammingWithObjectiveC/Conventions/Conventions.html
What about something like this (inside a directory)?
#define PruebaPaquete ar_com_oxenstudio_paq1_PruebaPaquete
#interface ar_com_oxenstudio_paq1_PruebaPaquete : NSObject {
and importing it like this:
#import "ar/com/oxenstudio/paq1/PruebaPaquete.h"
PruebaPaquete *p = [[PruebaPaquete alloc] init];
and when you have name collision:
#import "ar/com/oxenstudio/paq1/PruebaPaquete.h"
#import "ar/com/oxenstudio/paq2/PruebaPaquete.h"
ar_com_oxenstudio_paq1_PruebaPaquete *p = [[ar_com_oxenstudio_paq1_PruebaPaquete alloc] init];
ar_com_oxenstudio_paq2_PruebaPaquete *p2 = [[ar_com_oxenstudio_paq2_PruebaPaquete alloc] init];
Well, I think all the other answers here seem to focus on naming collisions, but missed at least one important feature, package private access control that java package provides.
When I design a class, I find it is quite often that I just want some specific class(es) to call its methods, b/c they work together to achieve a task, but I don't want all the other unrelated classes to call those methods. That is where java package access control comes in handy, so I can group the related classes into a packaged and make those methods package private access control. But there is no way to do that in objective c.
Without package private access control I find it is very hard to avoid people writing code like this, [[[[[a m1] m2] m3] m4] m5] or [a.b.c.d m1].
Update: Xcode 4.4 introduced "An Objective-C class extension header", in my opinion, that is in some way to provide "package private access control", so if you include the extension header, you can call my "package private" methods; if you only include my public header, you can only call my public API.