Provide a previousData property in useQuery/useLazyQuery results

[hwillson]

Alongside their returned data property, useQuery and useLazyQuery now also return a previousData property. Before a new data value is set, its current value is stored in previousData. This allows more fine-grained control over component loading states, where developers might want to leverage previous data until new data has fully loaded.

@benjamn just a heads up that I did look into moving the previousData handling up the chain into the AC core itself. The amount of work needed to get this functionality in place would have been more substantial however (as the lastResult tracking we’re doing in ObservableQuery doesn’t quite line up with what we’re looking for here), so I’ve left it in the React layer for now (where we were already tracking previousData, but just not exposing it). This should address most of the oustanding issues around this functionality, and we can always consider this a stepping stone. Changing how this works internally in the future will be hidden from useQuery / useLazyQuery consumers. Thanks!

Fixes #6603

[benjamn]

@hwillson Thanks for this!

I made a few tweaks, but the main thing was to let result.previousData persist across multiple results. See the test I added for an example where result.data can be undefined twice in a row.

[hwillson]

Great catch @benjamn - thanks for finishing this up!