Show HN: MobX-Style Observables in Svelte

I love Mobx with React and also Svelte, but Svelte already has reactive and derived values? https://svelte.dev/tutorial/reactive-statements

Not clear what this solves or improves upon.

Reactivity, yes - works the same as MobX's autorun. But not derived values. You can ensure something gets updated by wrapping it in $:

``` $: value = $x + 2 ```

but you can do that somewhere else as well

``` $: { let y = 20 $value += y } ```

Makes it insanely hard to track where changes come from. You can't say "this value is _only_ updated by these changes".

You can do this with derived stores, though. But each store must be it's own, separate value - with MobX's observable you have a central object representing your entire state, each of which is made automatically reactive, and putting in a derived store is just prepending with -get-. It's much cleaner.