Info

This is a single header library that exposes a function to hook a method of a specific instance (as opposed to other hooking functions which hook the class itself)

Usage

void hookInstance(id instance, SEL targetSEL, IMP replacementFp, IMP* origFp) 

instance: the instance you want to hook
targetSEL: selector for the method you want to hook
replacementFp: function pointer to the replacement function
origFp: address of a function pointer that will be filled in with a stub which may be used to call the original implementation. This can be NULL if you don't wish to use original implementation

Using with the same instance multiple times is supported, you can do this to hook multiple methods.

There is an example for the usage in "example" branch, as a tip; it is designed to be similiar with MSHookMessageEx function

Update: HOOK_INSTANCE macro

I have added a convenient to use c macro that creates the function pointers for you, see below for usage.

HOOK_INSTANCE(instance, selectorName, retType, argTypes..., block)

instance: the instance you want to hook
selectorName: name of the selector, not a string. mySelector: is correct "mySelector:" is wrong
retType: return type of the method. 
argTypes...: types of arguments, upto 10 arguments are supported
block: c block of the implementation you want to hook with. you can use arguments starting from arg0 going to arg9, you can call the original implementation by calling orig(args);

Some examples here:

// Traditional style
HOOK_INSTANCE(myObject, setValue:forKey:, void, NSString*, NSString*, {
    NSLog(@"Setting value %@ for key %@", arg0, arg1);
    orig(arg0, arg1);
});

// With SIGNATURE macro for cleaner look
HOOK_INSTANCE(myObject, setValue:forKey:, SIGNATURE(void, NSString*, NSString*), {
    NSLog(@"Setting value %@ for key %@", arg0, arg1);
    orig(arg0, arg1);
});

// No arguments
HOOK_INSTANCE(myObject, doSomething, SIGNATURE(void), {
    NSLog(@"doSomething called");
    orig();
});

// One argument
HOOK_INSTANCE(otherIHview.layer, setBackgroundColor:, SIGNATURE(void, CGColorRef), {
    NSLog(@"otherIHview.layer setBackgroundColor: hooked");
    orig(arg0);
});

// Many arguments
HOOK_INSTANCE(myObject, methodWithManyArgs:arg1:arg2:arg3:arg4:arg5:arg6:arg7:arg8:arg9:,
              SIGNATURE(void, int, float, double, char*, NSString*, NSNumber*, NSArray*, NSDictionary*, id, BOOL), {
    NSLog(@"Called with 10 arguments: %d %f %f %s %@ %@ %@ %@ %@ %d",
          arg0, arg1, arg2, arg3, arg4, arg5, arg6, arg7, arg8, arg9);
    orig(arg0, arg1, arg2, arg3, arg4, arg5, arg6, arg7, arg8, arg9);
});

How it works

It is creating a new class everytime you call the function, replaces the target method of it and uses object_setClass on the instance with that class to apply the hook