I found a way that caches the fetcher in useSWR using useCallback in the Gitroom source code.

The above image is from platform-analytics/render.analytics.tsx. Let’s try to understand this code snippet.

we’ll explore how to optimize data fetching in your React applications using the useCallback hook in combination with useSWR. We'll break down the provided code snippet, explain why caching your fetcher function is important.

Understanding the Code

Let’s dive into the code step by step:

const load = useCallback(async () => {
  setLoading(true);
  const load = (
    await fetch(\`/analytics/${integration.id}?date=${date}\`)
  ).json();
  setLoading(false);
  return load;
}, \[integration, date\]);

Here, we’re defining an asynchronous function load inside a useCallback hook. This function fetches data from a specified endpoint and handles the loading state. The useCallback hook ensures that this function is memoized and only recreated when the dependencies (integration and date) change.

Next, we use useSWR to manage the data fetching:

const { data } = useSWR(\`/analytics-${integration?.id}-${date}\`, load, {
  refreshInterval: 0,
  refreshWhenHidden: false,
  revalidateOnFocus: false,
  revalidateOnReconnect: false,
  revalidateIfStale: false,
  refreshWhenOffline: false,
  revalidateOnMount: true,
});

Here, useSWR is configured with a key (/analytics-${integration?.id}-${date}) and our memoized load function. The configuration options control the revalidation behavior of the data.

How useCallback Prevents Unnecessary Re-fetches

To understand how useCallback prevents unnecessary re-fetches, we need to delve into how React handles function references and how useSWR works.

Function References in React

In React, every time a component re-renders, all functions defined within it are recreated. This means that without useCallback, a new instance of your load function would be created on every render.

Impact on useSWR

useSWR is a data fetching library for React. It uses a key to identify the data and a fetcher function to fetch it. useSWR relies on the stability of the fetcher function reference. If the reference changes, useSWR might interpret this as a signal that the data needs to be refetched, even if the actual logic of the fetcher hasn't changed.

Here’s a detailed explanation:

1. Without useCallback:

const load = async () => {
  setLoading(true);
  const load = (
    await fetch(\`/analytics/${integration.id}?date=${date}\`)
  ).json();
  setLoading(false);
  return load;
};

const { data } = useSWR(\`/analytics-${integration?.id}-${date}\`, load, {
  refreshInterval: 0,
  refreshWhenHidden: false,
  revalidateOnFocus: false,
  revalidateOnReconnect: false,
  revalidateIfStale: false,
  refreshWhenOffline: false,
  revalidateOnMount: true,
});

In this case, every render creates a new load function. useSWR sees a different function reference each time, which can lead to unnecessary re-fetches even when integration and date haven't changed.

With useCallback:

const load = useCallback(async () => {
  setLoading(true);
  const load = (
    await fetch(\`/analytics/${integration.id}?date=${date}\`)
  ).json();
  setLoading(false);
  return load;
}, \[integration, date\]);

const { data } = useSWR(\`/analytics-${integration?.id}-${date}\`, load, {
  refreshInterval: 0,
  refreshWhenHidden: false,
  revalidateOnFocus: false,
  revalidateOnReconnect: false,
  revalidateIfStale: false,
  refreshWhenOffline: false,
  revalidateOnMount: true,
});

By wrapping the load function in useCallback, we ensure that it is only recreated when its dependencies (integration and date) change. This stability in the function reference tells useSWR that the fetcher function hasn't changed unless integration or date changes, thus preventing unnecessary re-fetches.

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References:

1. https://github.com/gitroomhq/gitroom/blob/c03b96215fa30b267a97d7eafc2281f482a3192f/apps/frontend/src/components/platform-analytics/render.analytics.tsx#L25
2. https://github.com/search?q=repo:gitroomhq/gitroom useSWR&type=code
3. https://swr.vercel.app/docs/getting-started
4. https://react.dev/reference/react/useCallback