iOS Framework weak link: undefined symbols error

Question

I'm building my own framework which proposed to be distributed to other developers for including to their projects. This framework links optionally certain frameworks (e.g. CoreLocation). The problem is that when I link my framework to real stand-alone project which doesn't contain CoreLocation in Build Phases, I'm getting linker errors like 'Undefined symbols for architecture' when tryin to build this host-project

Undefined symbols for architecture x86_64:
  "_OBJC_CLASS_$_CLLocationManager", referenced from:
      objc-class-ref in MySDK(MyServerConnection.o)
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Is it possible to avoid this because I don't want to force developers to include CoreLocation to their projecgts? Actually, I know it's possible, but what should I do to achieve this?

Answer 1

You could wrap any code and imports that uses the CoreLocation framework in a

#ifdef __CORELOCATION__
... do stuff
#endif

This will negate all CoreLocation code when the framework is not being linked at compile time

Answer 2

From the description of your issue, I assume you already know how to make Xcode weakly link against the framework by setting it as 'Optional'.

You have two problems to solve: Class availability and symbol availability. Apple covers this in the Framework Programming Guide: Frameworks and Weak Linking and the SDK Compatibility Guide: Using SDK Based Development

Class availability

This is pretty straightforward: use NSClassFromString() to see if the class is available in the current environment.

if (NSClassFromString("CLLocationManager") != NULL){

If the class is available it can be instantiated and sent messages, otherwise it cannot.

Symbol availability

What you are specifically interested in is using constants or structs from a weakly linked framework. A C-style function would be similar, but those are not a concern when using CoreLocation. We'll use a CoreLocation constant as an example.

Every time you use it you MUST check to make sure it exists:

if (&kCLErrorDomain != NULL){
   // Use the constant
}

Note that the & takes the address of the constant for the comparison. Also note that you CANNOT do this:

if (kCLErrorDomain){
   // Use the constant
}

Or this:

if (&kCLErrorDomain){
   // Use the constant
}

Constant symbols can also be lookup up at runtime using dlsym. Using the negation operator in this way will not work. You must check the address of the constant against the NULL address.

I have put a very simple example of an application that is using a static library which weak links against Core Location on GitHub. The example application does not link against Core Location:

But the dependency does, as an optional (weak) framework: