feat(runtime-core): app.onUnmount() for registering cleanup functions (close: #4516)

[LinusBorg]

This PR adds a new feature ~~for Plugins~~. Library Authors can now register a callback function using app.onUnmount() which will be called immediately after the app has been unmounted with app.unmount()

Details

• the function doesn't receive any arguments. the app instance should be available from the closure of the install function
• if the install function returns anything but a function with zero arguments, it will be ignored.This this should ensure a reasonable guard against unintentional effects from plugins accidentally returning something before this feature was added.

// ✅ Valid
function install (app: App) {
  function cleanupSomeSideeffect() { /* ...*/ }

  app.onUnmount(cleanupSomeSideffect)
}

Open Questions (from previous iteration)

• [x] is the argument length guard a good idea? => no.
• [x] Can we accept that we might break rare edge cases where plugins currently return a function with zero arguments that is not meant to be called after unmount? => no

• If the answer to the previous question is "no" - Should we instead provide an API on app like app.onUnmount()? => yes

close: #4516

[github-actions[bot]]

Size report

Path	Size
vue.global.prod.js	46.19 KB (+0.08% 🔺)
runtime-dom.global.prod.js	30.55 KB (+0.11% 🔺)
index.js	18.05 KB (+0.17% 🔺)

[unbyte]

Some thoughts:

1. implicit checking function.length can lead to bugs that are difficult to troubleshoot (for plugin authors).
2. Murphy's law. 🤯
3. at present, the general practice of registering unmount hooks is to replace the original app.mount function; I think app.onUnmount is better, since it's more in line with the existing usage(, and more elegant).

[yyx990803]

I also think the length check seems a bit unnecessary/incomplete.

app.onUnmount is safer and also more generic as it can be used even in non-plugin scenarios.

[LinusBorg]

I agree with your comments and will adjust the PR accordingly

[aantipov]

Hey 👋 any news here?

[github-actions[bot]]

Size Report

Bundles

File	Size	Gzip	Brotli
runtime-dom.global.prod.js	94.1 kB (+3.34 kB)	35.5 kB (+1.04 kB)	32 kB (+916 B)
vue.global.prod.js	151 kB (+3.44 kB)	54.8 kB (+1.13 kB)	48.9 kB (+936 B)

Usages

Name	Size	Gzip	Brotli
createApp	54 kB (+3.2 kB)	20.9 kB (+1 kB)	19 kB (+879 B)
createSSRApp	57.3 kB (+3.2 kB)	22.2 kB (+999 B)	20.2 kB (+870 B)
defineCustomElement	56.3 kB (+3.2 kB)	21.6 kB (+1.02 kB)	19.6 kB (+856 B)
overall	67.8 kB (+3.29 kB)	25.9 kB (+995 B)	23.4 kB (+852 B)