I have common files in several projects and I need to stop from compiling some lines and functions for project A, meanwhile it should be compiled for project B.
I know that I can use preprocessor. But it's not convenient for me. Is there any way to stop lines of code from compiling with condition like below?
#if PhotosModuleSettings.type == .documents
... do not commpile
#endif
What's not convenient about using the preprocessor? You can specify the preprocessor macros in build settings of each target, or you can use .xcconfig files to specify them.
There's another simple way to do it, however. Separate the lines and functions that you want to conditionally compile into separate files. Maybe by using Swift extensions or subclassing or just separate global functions, etc..whatever. Then just choose which target(s) and/or project(s) you want those files added as membership.
Depending on your desire to refactor your code to make such a file separation, the preprocessor macros may be the better way to go, though.
You will need to make use of pre processor macros.
Add a configuration for your project, and use that in the pre processor macros.
You can set the value for these configuration in the pre processor macros section for your targets based on your build configuration.
Here is a detailed blog related to the same concept
is is possible to use macros in config files? I want to achieve something like:
if iPad
set variable to 1
else
set variable to 0
Is that possible? I would rather not use scripts for this.
You generally should check this at runtime rather than compile time. See iOS - conditional compilation (xcode).
If you don't do it that way, I typically recommend using different targets as hinted at by #Robert Vojta.
That said, I can imagine cases where this would be useful in some piece of shared code. So...
There is an xcconfig variable you can use called TARGETED_DEVICE_FAMILY. It returns 1 for iPhone and iPod Touch, and 2 for iPad. This can be used to create a kind of macro. I don't highly recommend this approach, but here's how you do it. Let's say you were trying to set some value called SETTINGS:
// Family 1 is iPhone/iPod Touch. Family 2 is iPad
SettingsForFamily1 = ...
SettingsForFamily2 = ...
SETTINGS = $(SettingsForFamily$(TARGETED_DEVICE_FAMILY))
I've done this a few times in my projects (for other problems, not for iPad detection). Every time I've done it, a little more thought has allowed me to remove it and do it a simpler way (usually finding another way to structure my project to remove the need). But this is a technique for creating conditionals in xcconfig.
AFAIK it's not possible. But if you want to solve simple task - lot of common settings and just few variables have different values, you can do this:
generic.xcconfig:
settings for both configs
ipad.xcconfig:
#include "generic.xcconfig"
ipad-specific-settings
iphone.xcconfig
#include "generic.xcconfig"
iphone-specific-settings
This can solve your condition need. I do use this schema frequently.
That's not possible. Configuration files are not preprocessed (and compiled). What are you trying to do?
This is probably a totally noob question but I have missing links in my mind when thinking about linking libraries in iOS. I usually just add a new library that's been cross compiled and set the build and linker paths without really know what I'm doing. I'm hoping someone can help me fill in some gaps.
Let's take the OpenCV library for instance. I have this totally working btw because of a really well written tutorial( http://niw.at/articles/2009/03/14/using-opencv-on-iphone/en ), but I'm just wanting to know what is exactly going on.
What I'm thinking is happening is that when I build OpenCV for iOS is that your creating object code that gets placed in the .a files. This object code is just the implementation files( .m ) compiled. One reason you would want to do this is to make it hard to see the source code and so that you don't have to compile that source code every time.
The .h files won't be put in the library ( .a ). You include the .h in your source files and these header files communicate with the object code library ( .a ) in some way.
You also have to include the header files for your library in the Build Path and the Library itself in the Linker Path.
So, is the way I view linking libraries correct? If , not can someone correct me on this ?
Basically, you are correct.
Compiling the source code of a library produces one object file for each of the source files (in more than one, if compiled multiply times against different architectures). Then all the object files are archived (or packaged) into one .a file (or .lib on Windows). The code is not yet linked at this stage.
The .h files provide an interface for the functionality exposed by the library. They contain constants, function prototypes, possibly global declarations (e.g. extern int bad_global;), etc. -- basically, everything that is required to compile the code which is using the library.
.h files do not 'communicate' with object code in any way. They simply provide clues for the compiler. Consider this header file:
// library.h
extern int bad_global;
int public_func(int, const void*);
By including this file in your own code, you're simply telling the compiler to copy and paste these declarations into your source file. You could have written declarations for OpenCV library and not use the headers provided with it. In other words, you're asking the compiler to not issue errors about undefined symbols, saying "I have those symbols elsewhere, ok? Here are their declarations, now leave me alone!".
The header files need to be included in the search path in order for compiler to find them. You could simply include them via the full path, e.g. #include "path/to/file.h", or supply an -I option for your compiler, telling him where to look for additional headers, and use #include <file.h> instead.
When your code is compiled, the declarations in header files serve as an indication that symbols your code is using are defined somewhere. Note the difference between the words declaration and definition. Header files contain only declarations most of the time.
Now, when your code is compiled, it must be linked in order to produce the final executable. This is where the actual object code stored in the library comes into play. The linker will look at each symbol, function call, etc. in your object code and then try to find the corresponding definition for each such symbol. If it doesn't find one in the object code of your program, it will look the standard library and any other library you've provided it with.
Thus, it is important to understand that compilation and linkage are two separate stages. You could write any function prototypes at all and use them in your code, it will compile cleanly. However, when it comes to the linking stage, you have to provide implementation for symbols used in your code, or you won't get your executable.
Hope that makes sense!
The .a is the compiled version of the code.
The header files provided with a library are its public interface. They show what classes, methods, properties are available. They do not "communicate" with the binary code.
The compiler needs the headers to know that a symbol (a method name for example) is defined somewhere else. They are associated with the right "piece of code" in the library binary later during the "link" step.
What is the importance of .pch file and what is the significance of"#ifdef OBJC"?
Also, where do we define parameters like "#ifdef IS_PRODUCTION" which are checked in .pch file.
The .pch file allows you to avoid importing common files like UIKit.h and Foundation.h. If you have those files imported in the .pch, your own classes don't need to import them.
The significance of #ifdef OBJC is so that you don't import headers containing objective-c code if you don't have the compiler set to build objective c code (hence avoiding lots of compiler errors).
You define parameters such as IS_PRODUCTION inside the target's build settings. I do it usually in "other C flags".
.pch is a Pre-Compile Header.
In the C and C++ programming languages, a header file is a file whose text may be automatically included in another source file by the C preprocessor, usually specified by the use of compiler directives in the source file.
#ifdef OBJC lets the compiler know that the code is Objective-C.
#ifdef IS_PRODUCTION is something you have defined on your own, a directive telling the compiler to do something only if this is defined, most-likely something for a PRODUCTION build.
Can you guide me how to properly link static library to iPhone project. I use static library project added to app project as direct dependency (target -> general -> direct dependencies) and all works OK, but categories. A category defined in static library is not working in app.
So my question is how to add static library with some categories into other project?
And in general, what is best practice to use in app project code from other projects?
Solution: As of Xcode 4.2, you only need to go to the application that is linking against the library (not the library itself) and click the project in the Project Navigator, click your app's target, then build settings, then search for "Other Linker Flags", click the + button, and add '-ObjC'. '-all_load' and '-force_load' are no longer needed.
Details:
I found some answers on various forums, blogs and apple docs. Now I try make short summary of my searches and experiments.
Problem was caused by (citation from apple Technical Q&A QA1490 https://developer.apple.com/library/content/qa/qa1490/_index.html):
Objective-C does not define linker
symbols for each function (or method,
in Objective-C) - instead, linker
symbols are only generated for each
class. If you extend a pre-existing
class with categories, the linker does
not know to associate the object code
of the core class implementation and
the category implementation. This
prevents objects created in the
resulting application from responding
to a selector that is defined in the
category.
And their solution:
To resolve this issue, the static
library should pass the -ObjC option
to the linker. This flag causes the
linker to load every object file in
the library that defines an
Objective-C class or category. While
this option will typically result in a
larger executable (due to additional
object code loaded into the
application), it will allow the
successful creation of effective
Objective-C static libraries that
contain categories on existing
classes.
and there is also recommendation in iPhone Development FAQ:
How do I link all the Objective-C
classes in a static library? Set the
Other Linker Flags build setting to
-ObjC.
and flags descriptions:
-all_load Loads all members of static archive libraries.
-ObjC Loads all members of static archive libraries that implement an
Objective-C class or category.
-force_load (path_to_archive) Loads all members of the specified static
archive library. Note: -all_load
forces all members of all archives to
be loaded. This option allows you to
target a specific archive.
*we can use force_load to reduce app binary size and to avoid conflicts which all_load can cause in some cases.
Yes, it works with *.a files added to the project.
Yet I had troubles with lib project added as direct dependency. But later I found that it was my fault - direct dependency project possibly was not added properly. When I remove it and add again with steps:
Drag&drop lib project file in app project (or add it with Project->Add to project…).
Click on arrow at lib project icon - mylib.a file name shown, drag this mylib.a file and drop it into Target -> Link Binary With Library group.
Open target info in fist page (General) and add my lib to dependencies list
after that all works OK. "-ObjC" flag was enough in my case.
I also was interested with idea from http://iphonedevelopmentexperiences.blogspot.com/2010/03/categories-in-static-library.html blog. Author say he can use category from lib without setting -all_load or -ObjC flag. He just add to category h/m files empty dummy class interface/implementation to force linker use this file. And yes, this trick do the job.
But author also said he even not instantiated dummy object. Mm… As I've found we should explicitly call some "real" code from category file. So at least class function should be called.
And we even need not dummy class. Single c function do the same.
So if we write lib files as:
// mylib.h
void useMyLib();
#interface NSObject (Logger)
-(void)logSelf;
#end
// mylib.m
void useMyLib(){
NSLog(#"do nothing, just for make mylib linked");
}
#implementation NSObject (Logger)
-(void)logSelf{
NSLog(#"self is:%#", [self description]);
}
#end
and if we call useMyLib(); anywhere in App project
then in any class we can use logSelf category method;
[self logSelf];
And more blogs on theme:
http://t-machine.org/index.php/2009/10/13/how-to-make-an-iphone-static-library-part-1/
http://blog.costan.us/2009/12/fat-iphone-static-libraries-device-and.html
The answer from Vladimir is actually pretty good, however, I'd like to give some more background knowledge here. Maybe one day somebody finds my reply and may find it helpful.
The compiler transforms source files (.c, .cc, .cpp, .m) into object files (.o). There is one object file per source file. Object files contain symbols, code and data. Object files are not directly usable by the operating system.
Now when building a dynamic library (.dylib), a framework, a loadable bundle (.bundle) or an executable binary, these object files are linked together by the linker to produce something the operating system considers "usable", e.g. something it can directly load to a specific memory address.
However when building a static library, all these object files are simply added to a big archive file, hence the extension of static libraries (.a for archive). So an .a file is nothing than an archive of object (.o) files. Think of a TAR archive or a ZIP archive without compression. It's just easier to copy a single .a file around than a whole bunch of .o files (similar to Java, where you pack .class files into a .jar archive for easy distribution).
When linking a binary to a static library (= archive), the linker will get a table of all symbols in the archive and check which of these symbols are referenced by the binaries. Only the object files containing referenced symbols are actually loaded by the linker and are considered by the linking process. E.g. if your archive has 50 object files, but only 20 contain symbols used by the binary, only those 20 are loaded by the linker, the other 30 are entirely ignored in the linking process.
This works quite well for C and C++ code, as these languages try to do as much as possible at compile time (though C++ also has some runtime-only features). Obj-C, however, is a different kind of language. Obj-C heavily depends on runtime features and many Obj-C features are actually runtime-only features. Obj-C classes actually have symbols comparable to C functions or global C variables (at least in current Obj-C runtime). A linker can see if a class is referenced or not, so it can determine a class being in use or not. If you use a class from an object file in a static library, this object file will be loaded by the linker because the linker sees a symbol being in use. Categories are a runtime-only feature, categories aren't symbols like classes or functions and that also means a linker cannot determine if a category is in use or not.
If the linker loads an object file containing Obj-C code, all Obj-C parts of it are always part of the linking stage. So if an object file containing categories is loaded because any symbol from it is considered "in use" (be it a class, be it a function, be it a global variable), the categories are loaded as well and will be available at runtime. Yet if the object file itself is not loaded, the categories in it will not be available at runtime. An object file containing only categories is never loaded because it contains no symbols the linker would ever consider "in use". And this is the whole problem here.
Several solutions have been proposed and now that you know how all this plays together, let's have another look on the proposed solution:
One solution is to add -all_load to the linker call. What will that linker flag actually do? Actually it tells the linker the following "Load all object files of all archives regardless if you see any symbol in use or not'. Of course, that will work; but it may also produce rather big binaries.
Another solution is to add -force_load to the linker call including the path to the archive. This flag works exactly like -all_load, but only for the specified archive. Of course this will work as well.
The most popular solution is to add -ObjC to the linker call. What will that linker flag actually do? This flag tells the linker "Load all object files from all archives if you see that they contain any Obj-C code". And "any Obj-C code" includes categories. This will work as well and it will not force loading of object files containing no Obj-C code (these are still only loaded on demand).
Another solution is the rather new Xcode build setting Perform Single-Object Prelink. What will this setting do? If enabled, all the object files (remember, there is one per source file) are merged together into a single object file (that is not real linking, hence the name PreLink) and this single object file (sometimes also called a "master object file") is then added to the archive. If now any symbol of the master object file is considered in use, the whole master object file is considered in use and thus all Objective-C parts of it are always loaded. And since classes are normal symbols, it's enough to use a single class from such a static library to also get all the categories.
The final solution is the trick Vladimir added at the very end of his answer. Place a "fake symbol" into any source file declaring only categories. If you want to use any of the categories at runtime, make sure you somehow reference the fake symbol at compile time, as this causes the object file to be loaded by the linker and thus also all Obj-C code in it. E.g. it could be a function with an empty function body (which will do nothing when being called) or it could be a global variable accessed (e.g. a global int once read or once written, this is sufficient). Unlike all other solutions above, this solution shifts control about which categories are available at runtime to the compiled code (if it wants them to be linked and available, it accesses the symbol, otherwise it doesn't access the symbol and the linker will ignore it).
That's all folks.
Oh, wait, there's one more thing:
The linker has an option named -dead_strip. What does this option do? If the linker decided to load an object file, all symbols of the object file become part of the linked binary, whether they are used or not. E.g. an object file contains 100 functions, but only one of them is used by the binary, all 100 functions are still added to the binary because object files are either added as a whole or they are not added at all. Adding an object file partially is usually not supported by linkers.
However, if you tell the linker to "dead strip", the linker will first add all the object files to the binary, resolve all the references and finally scan the binary for symbols not in use (or only in use by other symbols not in use). All the symbols found to be not in use are then removed as part of the optimization stage. In the example above, the 99 unused functions are removed again. This is very useful if you use options like -load_all, -force_load or Perform Single-Object Prelink because these options can easily blow up binary sizes dramatically in some cases and the dead stripping will remove unused code and data again.
Dead stripping works very well for C code (e.g. unused functions, variables and constants are removed as expected) and it also works quite good for C++ (e.g. unused classes are removed). It is not perfect, in some cases some symbols are not removed even though it would be okay to remove them, but in most cases it works quite well for these languages.
What about Obj-C? Forget about it! There is no dead stripping for Obj-C. As Obj-C is a runtime-feature language, the compiler cannot say at compile time whether a symbol is really in use or not. E.g. an Obj-C class is not in use if there is no code directly referencing it, correct? Wrong! You can dynamically build a string containing a class name, request a class pointer for that name and dynamically allocate the class. E.g. instead of
MyCoolClass * mcc = [[MyCoolClass alloc] init];
I could also write
NSString * cname = #"CoolClass";
NSString * cnameFull = [NSString stringWithFormat:#"My%#", cname];
Class mmcClass = NSClassFromString(cnameFull);
id mmc = [[mmcClass alloc] init];
In both cases mmc is a reference to an object of the class "MyCoolClass", but there is no direct reference to this class in the second code sample (not even the class name as a static string). Everything happens only at runtime. And that's even though classes are actually real symbols. It's even worse for categories, as they are not even real symbols.
So if you have a static library with hundreds of objects, yet most of your binaries only need a few of them, you may prefer not to use the solutions (1) to (4) above. Otherwise you end up with very big binaries containing all these classes, even though most of them are never used. For classes you usually don't need any special solution at all since classes have real symbols and as long as you reference them directly (not as in the second code sample), the linker will identify their usage pretty well on its own. For categories, though, consider solution (5), as it makes it possible to only include the categories you really need.
E.g. if you want a category for NSData, e.g. adding a compression/decompression method to it, you'd create a header file:
// NSData+Compress.h
#interface NSData (Compression)
- (NSData *)compressedData;
- (NSData *)decompressedData;
#end
void import_NSData_Compression ( );
and an implementation file
// NSData+Compress
#implementation NSData (Compression)
- (NSData *)compressedData
{
// ... magic ...
}
- (NSData *)decompressedData
{
// ... magic ...
}
#end
void import_NSData_Compression ( ) { }
Now just make sure that anywhere in your code import_NSData_Compression() is called. It doesn't matter where it is called or how often it is called. Actually it doesn't really have to be called at all, it's enough if the linker thinks so. E.g. you could put the following code anywhere in your project:
__attribute__((used)) static void importCategories ()
{
import_NSData_Compression();
// add more import calls here
}
You don't have to ever call importCategories() in your code, the attribute will make the compiler and linker believe that it is called, even in case it is not.
And a final tip:
If you add -whyload to the final link call, the linker will print in the build log which object file from which library it did load because of which symbol in use. It will only print the first symbol considered in use, but that is not necessarily the only symbol in use of that object file.
This issue has been fixed in LLVM. The fix ships as part of LLVM 2.9 The first Xcode version to contain the fix is Xcode 4.2 shipping with LLVM 3.0. The usage of -all_load or -force_load is no longer needed when working with XCode 4.2 -ObjC is still needed.
Here's what you need to do to resolve this problem completely when compiling your static library:
Either go to Xcode Build Settings and set Perform Single-Object Prelink to YES or
GENERATE_MASTER_OBJECT_FILE = YES in your build configuration file.
By default,the linker generates an .o file for each .m file. So categories gets different .o files. When the linker looks at a static library .o files, it doesn't create an index of all symbols per class (Runtime will, doesn't matter what).
This directive will ask the linker to pack all objects together into one big .o file and by this it forces the linker that process the static library to get index all class categories.
Hope that clarifies it.
One factor that is rarely mentioned whenever the static library linking discussion comes up is the fact that you must also include the categories themselves in the build phases->copy files and compile sources of the static library itself.
Apple also doesn't emphasize this fact in their recently published Using Static Libraries in iOS either.
I spent a whole day trying all sorts of variations of -objC and -all_load etc.. but nothing came out of it.. this question brought that issue to my attention. (don't get me wrong.. you still have to do the -objC stuff.. but it's more than just that).
also another action that has always helped me is that I always build the included static library first on its own.. then i build the enclosing application..
You probably need to have the category in you're static library's "public" header: #import "MyStaticLib.h"