Simulate internal methods of components

[yxyxy666]

Test beginners, may I ask how to simulate the internal methods of components，I call method a of a component, and then a will call method b of the same component or method c of a sub component. In this case, I need to simulate method b or method c. After simulating, I also assert that the simulation has been successful. However, if I call method a again at this time, it will still go to the original implementation of method b or method c。

For example, if I want to simulate the successful call of foo1 here and only focus on foo2 itself, how should I write it
Example address：https://stackblitz.com/edit/vitest-dev-vitest-haltys?file=package.json

Can you please teach me