Consider adding prevState to Selector get method.

[terminalqo]

I'm using Next.js, However , I get initData from ssr, so I adding initData like this:

<RecoilRoot initializeState={initializeState}>

And I'm using selector to observe some atom state:

export const chartDataState = selector({
  key: "chartDataState",
  get: ({ get }) => {
    const currentState = get(someAtomState);
    // I got currentState first time from ssr, how could I avoid first fetch?
    if (currentState === prevState) {
      // return old data
    } else {
     // return fetched new data.
    }
  },
});

How can I get prevState to prevent unnecessary fetch ?
Thank you for help!

[terminalqo]

May be alternative ?

atom({
  key: "",
  default: {
    market: "",
    group: "",
    inst: "",
  },
  effects: [
    ({ onSet }) => {
      onSet((newVal, oldVal) => {
        // Not call in first, but will call in subsequent state change.
        console.log("[ newVal, oldVal ] >", newVal, oldVal);
      });
    },
  ],
});

[drarmstr]

Selector evaluation functions should be "idempotent", meaning they should always return the same output for a given set of inputs without side-effects (exceptions for caching and such). So, the concept of "previous state" isn't really meaningful for a selector evaluation.