SIMBL with Method Swizzling

Question

I have some great troubles overriding some functions in an external App that I use SIMBL to hook in to.

In this app, there is a class - let's call it "AppClass". In this class there is a function,

-(void)doSomething;

I got this from class-dumping the Binary. the whole interface is defined as:

@interface AppClass : NSObject
{
}

I'm trying to override this function with jr_swizzleMethod:withMethod:error:

With the lack of documentation, this is what I have come up with:

    #import "JRSwizzle.h"
    #import "AppClass.h"

@interface AppClass (MyPlugin)
- (void)myPlugin_doSomething;
@end

@implementation AppClass (MyPlugin)

- (void)myPlugin_doSomething {
 NSLog(@"lol?");
}

@end

@implementation MyPlugin

+ (void) load {
 Mylugin* plugin = [MyPlugin sharedInstance];

 NSError *err = nil;
 BOOL result = [NSClassFromString(@"AppClass") jr_swizzleMethod:@selector(doSomething) withMethod:@selector(myPlugin_doSomething) error:&err];

 if(!result)
  NSLog(@"<Plugin> Could not install events filter (error: %@)", err);

 NSLog(@"Plugin installed");
}



+ (MyPlugin *)sharedInstance {
 static MyPlugin* plugin = nil;

 if(plugin == nil)
  plugin = [[MyPlugin alloc] init];

 return plugin;
}

@end

That should be enough right? But I get this error on compile:

Undefined symbols:
  "_OBJC_CLASS_$_AppClass", referenced from:
      l_OBJC_$_CATEGORY_AppClass_$_MyPlugin in MyPlugin.o
      objc-class-ref-to-AppClass in MyPlugin.o
ld: symbol(s) not found
collect2: ld returned 1 exit status

How do I solve this?

Answer 1

You're creating a plugin, which refers to symbols in a binary (the app you're trying to extend). Therefore, you need to tell the linker where to look for those symbols (In your case, that's _OBJC_CLASS_$_AppClass, i.e. AppClass that's defined in the binary. ).

This is done by passing the option -bundle_loader executable_name to the linker. See the man page for ld.