Allow setting new data based on old data

[elsangedy]

Now is not possible update the current data inside deferFn, I only can replace with new data.

It would be nice if deferFn get the current state or data as a parameter, this can be able implement features like infinite scroll

const deferFn  = (params, defaultParams, state, abortCtrl) => Promise.resolve([...state.data, ...newDataArray])

...

const { data, run } = useAsync({ deferFn })

[elsangedy]

sample: https://codesandbox.io/s/react-async-optimistic-update-6p11f

[ghengeveld]

Excellent suggestion! We certainly should support this usecase (updating data based on the previous data and the new data). This doesn't just apply to deferFn, but also promiseFn. Let's consider adding a transform option that looks like this:

transform: (newData, oldData, props, args) => newData + oldData // do whatever

What we need to consider:

• What about reload? Do we skip the transform or do we apply it regardless? Probably passing a reload flag to the transform would help.
• What about setData? I'd expect this one to ignore the transform (since it's a setter), but we may want it to accept a transform function as well.

[ghengeveld]

@all-contributors please add @elsangedy for ideas

[allcontributors]

@ghengeveld

I've put up a pull request to add @elsangedy! 🎉

[ghengeveld]

For those who want this feature right now, here's how you can accomplish it: https://codesandbox.io/s/react-async-pagination-og13b