Open Radar

 
Partial mocking is very, very useful for writing unit tests. Partial mocking is the ability to replace the implementation of a function of a specific instance with a stub that tracks the parameters passed in and returns a convenient value for testing. You can also add expectations on how many times the function is called for a set of parameters.

In C++, we accomplish this either by subclassing the class to be mocked, or by creating a proxy class. There are problems with both of these approaches. First, subclassing does more than a simple mock, so the subclassed object does not function 100% identically to the original (aside from the mocked object). Proxy objects need to implement forwarding for every non-mocked function of the original object, except for the mocked functions. This means you have to update the proxy class to keep it in perfect sync with the original. In C++, you sometimes need multiple proxy classes, but I don't think that's needed in Swift, since functions are first-class objects.

Neither of these solutions are great, and it would be very helpful if Swift just natively supported partial mocks.