Can OCMock mock a class such that it automatically uses the mocked instance in the code under test without injecting it

Question

I'm new to OCMock and had a question.

Can we stub a method of a class, where calling that method over any instance/object of that class gets mocked

For Example : if Class_A has a non static function_a and Class_B has function_b which internally declares a object_o of Class_A.

    Class_A
    {
        - function_a
    }

    Class_B
    {
        - function_b
        {
            Obj_o of Class A
            [Obj_o a];
        }
    }

Now, I want to write unit test for Class_B and test function_b.

Is there some mechanism where I create a mock of Class_A and stub function_b and then run the test function_b and be sure that function_b got stubbed.

I know that if I change the function definition of function_b to have the object_o sent as a parameter, it works. We create an object mock of class_A, stub for function_b, and send that object mock as a parameter to function_b because it gets the reference for the mocked object. But I want to check regarding the feasibility of testing such functions without changing the function definitions. Does OCMock provide such functionality.

Answer 1

If you don't want to change the existing method definitions but are ok with changes to your existing code, you can use this approach:

To create the Obj_o of Class A in function_b use a class method on Class_A, for example newInstance. This method doesn't need to do anything else than the normal [[Class_A] alloc] init]; (change to your init method if you have a custom one). In your test you can that mock this newInstance function to return an instance of your mocked object of Class_B.

See the documentation of OCMock, 9.3 Stubbing methods that create objects.