Add API to trigger lifeCycles manually.

Feature Description

Some lifecycles are hard to be test in certain cases. For example, I have no idea on how to test activated in shallowMount mode, since <keep-alive> will be a stub. Howerver shallowMount is a MUST in my complicated applications.

Expected behaviour

provide an API like:

// does exacly what it says
// other lifeCycles should work, too.
wrapper.triggerLifeCycle('activated') 

my solution for now (which works):

export function triggerLifeCycle(vm, lifeCycle) {
  if (vm.$options[lifeCycle]) {
    vm.$options[lifeCycle].forEach(fn => {
      fn.call(vm);
    });
  }
}

if the idea sounds accepatble, I’m glad to make a PR. thanks.

[wxsms]

@bponomarenko Thank you very much for the kind answer and solution. You do solved my problem in a way that I have not consider yet. And I kind of agree your point about testing indeed. Good day.

[bponomarenko]

But I think that white box testing is also useful in some cases.

I have been working with Vue codebases for around 3-4 years, and in my experience such “granular” unit tests usually hard to maintain and they break to0 often without tangible reasons. Fixing those “false positives” takes more time than the actual value they bring.

But let me try to address your questions:

1. This is interesting challenge 🤔 But I think you don’t even need Vue test utils to test that:

import Vue from 'vue';
import TestComponent from 'src/components/test-component';

it('test', async () => {
  // Create component constructor
  const Comp = Vue.extend(TestComponent);
  const fetchDataSpy = jest.spyOn(Comp.options.methods, 'fetchData');
  
  expect(fetchDataSpy).not.toHaveBeenCalled();
  
  // Trigger component creation
  const instance = new Comp();
  
  expect(fetchDataSpy).toHaveBeenCalledTimes(1);
  
  // Explicitly mount component
  instance.$mount();
  
  expect(fetchDataSpy).toHaveBeenCalledTimes(2);
})

1. I guess example from the previous answer with slightly different checks should cover this used case too:

// ...

// Trigger component creation
const instance = new Comp();
  
expect(fetchDataSpy).not.toHaveBeenCalled();
  
// Explicitly mount component
instance.$mount();
  
expect(fetchDataSpy).toHaveBeenCalledTimes(1);