Move state update listeners from constructor to componentDidMount

[t3chguy]

Fixes #28306

Also ensures this.unmounted is set appropriately to account for the React 18 strict mode double-render. Without it your inline definition (private unmounted = false;) might only be called once, and you may be unmounted before you are rendered, so none of your data updates end up applying due to this.unmounted = true in the unmount before the remount.

[richvdh]

I'm not really familiar with React's strict mode, so could use some help understanding the changes here. I skimmed https://react.dev/reference/react/StrictMode, but it wasn't that helpful and I could do with some help understanding its specific effects on our application.

For example, why do we need to add this.unmounted = false; to componentDidMount? This is because the same Component object can now be mounted, unmounted, and re-mounted, I guess? (Aside: I thought most of the reasons we used the this.unmounted flag no longer applied? Did you consider ripping it out rather than propagating it further?)

The PR title is "Move state update listeners from constructor to componentDidMount", but (a) the PR doesn't explain why that is a good thing to do; (b) it's not obvious that this.unmounted = false counts as a "state update listener".

A few words in the description of the PR might help understand what's going on here.

[richvdh]

looks sane in general but some clarification would be appreciated

[t3chguy]

This is because the same Component object can now be mounted, unmounted, and re-mounted, I guess?

Indeed, without it your inline definition (private unmounted = false;) might only be called once, and you may be unmounted before you are rendered, so none of your data updates end up applying due to this.unmounted = true in the unmount before the remount.

Did you consider ripping it out rather than propagating it further?

I did but thought it much higher risk when in the medium term everything will be getting replaced with functional components which can deal with that.

[richvdh]

lgtm