WIP: Hot reloading with useElmish

[olivercoad]

I've never been able to get hot reloading to work with useElmish so I usually just use regular global style Elmish.

However I wanted to try out vite and Elmish.HMR doesn't currently work with vite so I decided to have another look at useElmish. This PR makes it work as expected with hot reloading, at least when testing a simple counter with vite+preact (which uses prefresh).

From my testing it seems like state in useState is preserved between hot reloads but useRef is not. The useEffect which resets the state and runs cmds also gets executed when doing a hot reload. This change makes it so that the state ref is initialised to childState so that it is preserved between hot reloads. useEffect checks isFirstRender before reinitialising state; because isFirstRender is a useRef, it will get reset to true during a hot reload so that it doesn't reset the state.

I tried testing it with react as well but had so many other issues with react-refresh and using hooks and fable. I think something to do with having extra export function ... in the same js file as the component might be messing up react-refresh somehow. So if anyone can get it working with react let me know :). So that is to say IDK if the behaviour of useRef and useState is the same with react-refresh as it is with prefresh but hopefully it is.

I've marked it as WIP because I need help on making it handle the initial Cmd correctly. See, the current behaviour on hot reloads is to reset the state and run inital Cmd (from init). This PR currently changes it so that on hot reloads it doesn't reset the state but does still run the initial Cmd which is (probably?) not good.

[Zaid-Ajaj]

Hi @olivercoad, thanks for looking into this. I'll have to look into the current behavior and will let you know of my findings.

Sorry for the delayed response, I was on vacation 🌴

[Zaid-Ajaj]

I'll close this PR as considered stale and wait for the solution devised by @MangelMaxime