What is spying

Defining

A spy is a partial mock in mocking frameworks meaning a part of the object will be mocked and part will be used in real method invocations.

Comparing mockk with spy

• Mock is actually trying to be an interface.
  • When you specify a mock you are just checking the return method of that mock.
  • Mock sees just the method calls and not the structure inside it.
  • Thus mock runs the method and ignores the body.
• Spy is trying to be the actual class.
  • By using a spy, the entire class is running, the body of the class is running, the entire structure is running.
  • Spy sees not just the calls of method but also the body in it.
  • Thus spy runs the method and the body.