Asyncified validate + submit flow

[radekmie]

Let's start with a short flashback from #644:

[...] Well, the first bridge ever was the SimpleSchema (the Meteor one), and its validator throws an error instead of returning it. It was perfectly fine, up to the point when we wanted to add onSubmitSuccess/onSubmitFailure and async validation. Of course, we managed to do so, but the solution is not very elegant.

This not very elegant solution got implemented in #51. Why it's not elegant? Because it's redundant. There's no need for such hooks if one can simply add it directly to the onSubmit. Another problem (non-elegancy) is the asynchronous validation - invented in #17 and implemented in 4a56596. It's weird and is using callbacks.

This PR introduces a new, clean flow (including #711 (comment)):

• Bridge validators can be asynchronous.
• Bridge validators have to return errors instead of throwing them. More background is in Validation Errors should not rely on throw/catch #644.
• The onValidate is now (model, error?) => error? | Promise<error?>. This is far better and obvious than the callback version before.
• Removed onSubmitSuccess and onSubmitFailure. I'm pretty sure there's no viable case for them at all. Migration is straightforward:

  -onSubmit={onSubmit}
  -onSubmitSuccess={onSubmitSuccess}
  -onSubmitFailure={onSubmitFailure}
  +onSubmit={model => {
  +  const result = onSubmit(model);
  +  result.then(onSubmitSuccess, onSubmitFailure);
  +  return result;
  +}}`

• Separated submitting and validating states. Now validating is true only while the bridge validator or onValidate are currently running. Same for submitting - it's try only while onSubmit is running.

TODO:

• Perform some manual tests to check if there's no visible difference with this change. It might be the case as few things got asyncified.
• Run benchmarks to ensure no performance degradation.
• Add missing tests.
• Update CHANGELOG.md.

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Closes #644.

[codecov]

Codecov Report

Merging #711 into v3 will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##               v3     #711      +/-   ##
==========================================
+ Coverage   97.96%   98.02%   +0.05%     
==========================================
  Files         162      162              
  Lines        1669     1668       -1     
  Branches      604      602       -2     
==========================================
  Hits         1635     1635              
  Misses          8        8              
+ Partials       26       25       -1     

Impacted Files	Coverage Δ
packages/uniforms/src/Bridge.ts	100.00% <ø> (ø)
...-bridge-simple-schema-2/src/SimpleSchema2Bridge.ts	100.00% <100.00%> (ø)
...rms-bridge-simple-schema/src/SimpleSchemaBridge.ts	100.00% <100.00%> (ø)
packages/uniforms/src/BaseForm.tsx	98.14% <100.00%> (-0.07%)	⬇️
packages/uniforms/src/ValidatedForm.tsx	100.00% <100.00%> (+1.61%)	⬆️

---

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[radekmie]

During benchmarks, it turned out that making everything async has one flaw: setState happens before then. It means that we have at least one additional render. Right now I'm working on a solution similar to the one in BaseForm to handle both sync and async validators, onValidate, and onSubmit in any configuration.

Another problem araised while working with rejected promises. The thing is that it's not better than throw as we still cannot type it anyhow. We won't be able to type it correctly anytime soon (I don't have an idea on how to type errors as those are created in bridges, used in forms, and then analyzed by bridges), but having such a possibility would be nice. I'll experiment with actually returning errors instead, as discussed in #644.