feat: add asyncValidator support to FormBuilderField (#838)

[GhagSagar23]

Summary

Adds asynchronous validation support to FormBuilderField, addressing #838.

New API

API	Location	Description
asyncValidator	FormBuilderField	Future<String?> Function(T?) — runs after sync validation passes
validateAsync()	FormBuilderFieldState	Runs sync → async validation for a single field
validateAsync()	FormBuilderState	Validates all fields (including async) in parallel
saveAndValidateAsync()	FormBuilderState	Saves then validates all fields asynchronously
isValidating	FormBuilderFieldState	true while an async validator is in-flight

Design

• Opt-in only — async validation runs via validateAsync(), never auto-triggered on keystroke (avoids rebuild loops).
• Stale result discarding — monotonic counter drops results from superseded validations.
• Clean state — didChange() clears async errors; reset() cancels in-flight validations.

Usage

FormBuilderTextField(
  name: 'username',
  asyncValidator: (value) async {
    final exists = await api.checkUsername(value);
    return exists ? 'Username taken' : null;
  },
)

final isValid = await formKey.currentState!.validateAsync();

Tests

All 151 tests passed

00:05 +149: FormBuilderField - asyncValidator - Should validate asynchronously and update errorText
00:05 +150: FormBuilderField - asyncValidator - Should discard stale validations when value changes rapidly
00:05 +151: All tests passed!

Closes #838

[codecov]

Codecov Report

❌ Patch coverage is 98.98990% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.45%. Comparing base (f8706f7) to head (ffbf57e).

Files with missing lines	Patch %	Lines
lib/src/form_builder_field.dart	98.78%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1514      +/-   ##
==========================================
+ Coverage   92.35%   93.45%   +1.10%     
==========================================
  Files          21       21              
  Lines         863      948      +85     
==========================================
+ Hits          797      886      +89     
+ Misses         66       62       -4     

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[deandreamatias]

Hi! Thanks for contribution

• Please add some doc on README about this new feature.
• How this interact with AutovalidateMode? I mean, the sync validator takes into account the AutovalidateMode set on FormBuilder and FormFieldBuilder

[GhagSagar23]

Hi! Thanks for contribution

• Please add some doc on README about this new feature.
• How this interact with AutovalidateMode? I mean, the sync validator takes into account the AutovalidateMode set on FormBuilder and FormFieldBuilder

Hey @deandreamatias, both the points have been addressed in the latest commit. Can you please re-review this PR?

[github-actions]

This PR was flagged by our automated quality checks. If you're a genuine
contributor, please reply here and a maintainer will review your PR.

Common reasons for flagging:

• New GitHub account
• Unusually high number of repository forks in a 24-hour window

We appreciate your contribution and apologize if this is a false positive!

[GhagSagar23]

This PR was flagged by our automated quality checks. If you're a genuine contributor, please reply here and a maintainer will review your PR.

Common reasons for flagging:

• New GitHub account
• Unusually high number of repository forks in a 24-hour window

We appreciate your contribution and apologize if this is a false positive!

Genuine contributor here — not agent-generated. I picked up #838 because I needed async validation in a production app and the issue has been open since 2021.

The implementation went through review with @deandreamatias already (added README docs and clarified the AutovalidateMode interaction per his feedback). Happy to answer any further questions.

[deandreamatias]

@GhagSagar23 could be share your production app? A link from Google Play or App Store could be enough

[GhagSagar23]

@GhagSagar23 could be share your production app? A link from Google Play or App Store could be enough

Hi @deandreamatias, my app is not yet deployed on either of the stores. It still in development phase.

[deandreamatias]

So isn't a production app, yet, right?

[GhagSagar23]

So isn't a production app, yet, right?

Not yet @deandreamatias