Description

I propose to auto-validate FormField only if its content was changed since its prevents end users to see validation errors even without changing the field's content.

Related Issues

Fixes: #56363
Fixes: #18885
Fixes: #15404
Fixes: #36154
Fixes: #48876

Tests

I added the following test cases:

• Do not auto-validate before value changes if widget.autovalidateMode (new) equals to AutovalidateMode.onUserInteraction;
• auto-validate after value changes if widget.autovalidateMode equals to AutovalidateMode.onUserInteraction;
• Form only validates form fields after one of the form fields changes if widget.autovalidateMode is AutovalidateMode.onUserInteraction;
• auto-validate before value changes if autovalidate is true and widget.autovalidateMode is AutovalidateMode.always;
• Form auto validate form fields even if none of them changes if widget.autovalidateMode is always;
  autovalidate parameter is still used if true;
• DropdownButtonFormField passes autoValidateMode to superclass;
• Form.reset() reset form fields and auto validation will only happen on the next user interaction if autoValidateMode is onUserInteraction;

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]). This will ensure a smooth and quick review process.

• I signed the [CLA].
• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Did any tests fail when you ran them? Please read [Handling breaking changes].

• No, no existing tests failed, so this is not a breaking change.
• Yes, this is a breaking change. If not, delete the remainder of this section.
  • I wrote a design doc: https://docs.google.com/document/d/16HAURcOtYAoHntDOEtdGy9KkKSItulG9Ldyjzdaosqo/edit?usp=sharing
  • I got input from the developer relations team, specifically from: @chunhtai
  • I wrote a migration guide: https://flutter.dev/go/breaking-changes-template Replace this with a link to a pull request that adds the migration guide to https://flutter.dev/docs/release/breaking-changes