API for testing emitted events

Related to TODO in #4

Some ideas

// Creates listener on the component
wrapper.listenTo('event', spy) 

//  returns array of events that have been emitted
wrapper.events

[yyx990803]

Reopen as I am revisiting this feature. After having to manually create spies for all the events I wanted to assert, it felt quite tedious. I am proposing an API that requires minimal setup by simply auto-recording all emitted events in a wrapper.emitted object.

Say you emit the following events:

wrapper.vm.$emit('foo', 1)
wrapper.vm.$emit('foo', 2, 3)
wrapper.vm.$emit('bar')

Then wrapper.emitted would look like this:

{
  foo: [[1], [2, 3]],
  bar: [[]]
}

This simple data structure allows all kinds of assertions:

// assert event emitted
expect(wrapper.emitted.foo).toBeTruthy()

// assert event count
expect(wrapper.emitted.foo.length).toBe(2)

// assert event payload
expect(wrapper.emitted.foo[0]).toEqual([1])

The best part is you don’t need to do any wiring or setup, you can just assert it after you trigger the events.

/cc @eddyerburgh @codebryo

[yyx990803]

What about making it wrapper.on to be consistent with vm.$on? It seems to be just an alias.