feat: Add Controller.getState()

[ntucker]

Fixes #1468 .

Motivation

Imperative handlers sometimes need context to choose their next behavior. It's critical we don't lock this into the React lifecycle as that is an antipattern (the useEffect-trap) - not just annoying from a coding perspective.

Solution

We already have imperative access with middlewares, so use the same function to provide to controller.

It's important we do not hide getState() inside any accessors, as it is critical to understand where state comes from so it always represents the expected value.

const controller = useController();

const updateHandler = useCallback(async (updatePayload) => {
  const response = controller.fetch(MyResource.update, { id }, updatePayload);
  const denormalized = controller.getResponse(MyResource.update, { id }, updatePayload, controller.getState());
  redirect(denormalized.getterUrl);
}, [id]);

Open questions

controller.getState() won't include updated state til the providers useEffect() is called. This means it could potentially not have the result. We might need another promise to listen to next state update to complete this functionality.

const denormalized = controller.getResponse(MyResource.update, { id }, updatePayload, await controller.getNextState());

Until this is resolved, one can likely just use a useIdleCallback() as the lower priority nature compared to useEffects should guarantee running after the update.

[codecov-commenter]

Codecov Report

Merging #2252 (7ccf073) into feat/getstate (3061187) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@              Coverage Diff               @@
##           feat/getstate    #2252   +/-   ##
==============================================
  Coverage          98.54%   98.55%           
==============================================
  Files                114      114           
  Lines               1860     1866    +6     
  Branches             273      273           
==============================================
+ Hits                1833     1839    +6     
  Misses                10       10           
  Partials              17       17           

Impacted Files	Coverage Δ
packages/rest/src/createResource.ts	91.66% <ø> (ø)
packages/core/src/controller/Controller.ts	95.14% <100.00%> (+0.09%)	⬆️
...core/src/react-integration/provider/CacheStore.tsx	100.00% <100.00%> (ø)
...act-integration/provider/ExternalCacheProvider.tsx	100.00% <100.00%> (ø)

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