test: Add a unit test for "$off event and this event added by $once"

[wujieZ]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe: add a unit test for "$off event and this event added by $once"

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

[Justineo]

Files in packages are automatically generated at build time, please do not include them in the PR. Thanks!

[wujieZ]

Files in packages are automatically generated at build time, please do not include them in the PR. Thanks!

Files in packages are automatically generated at build time, please do not include them in the PR. Thanks!
Thanks, I had revert file what I change in packages

[jkzing]

Thanks for contributing. But I'm afraid this part cannot be removed.

After removing these two lines, if user call vm.$off before the event is triggered, it won't work.

eg.

const fn = () => console.log('foo')
this.$once('foo', fn)
this.$off('foo', fn) // this won't work

Since we don't have unit test to cover this case, if you still willing to contribute, add a unit test for it would be appreciated.

[wujieZ]

Thanks for contributing. But I'm afraid this part cannot be removed.

After removing these two lines, if user call vm.$off before the event is triggered, it won't work.

eg.

const fn = () => console.log('foo')
this.$once('foo', fn)
this.$off('foo', fn) // this won't work

Since we don't have unit test to cover this case, if you still willing to contribute, add a unit test for it would be appreciated.

Thanks, I will revert what I change and add a unit test for this.