rollbackOnError as function

[huozhi]

Discussed in https://github.com/vercel/swr/discussions/2092

^{Originally posted by RenaudAubert July 29, 2022} Hi 👋 First of all a big thanks for this amazing library.

I've noticed that 2 of the mutate options (populateCache, optimisticData) can accept functions and I was wondering if it'd make sense to also have this possibility for rollbackOnError?

Use Case Let's say I have a mobile app that can be used in areas with a bad network coverage. When user performs an action thanks to optimisticData the cache and view are updated directly and if an error occurs with rollbackOnError data is rolled-back. But errors can occur for many reasons, sometimes the API returns a valid error sometimes the error come from the network for instance TimeoutError.

I thought maybe something like this

const newData = createNewData(...);
mutate(key, updateFn(newData), {
    revalidate: true,
    populateCache: true,
    optimisticData: newData,
    rollbackOnError(error, currentData) {
        if (error.name === "TimeOutError") {
            return true;
        }

        return false;
    },
});

I'm not sure such an implementation could work, please feel free to let me know or suggest alternatives. Maybe I'm going at it completely wrong. What I'm currently implementing is something like the following

const newData = createNewData(...);
try {
    const mutatedData = await mutate(key, data, {
        populateCache: true,
        optimisticData: newData,
        rollbackOnError: false,
    });
} catch (error) {
    // Error might comme from unique constraint in database, unsufficient permissions, etc...
    if (error.name !== "TimeoutError") {
        mutate(key, newData, {
            revalidate: false,
            populateCache: true,
            rollbackOnError: false,
        });
    }
}