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Validate business data

FDC validation is dataset-centered, so the same rules apply whether data is changed from a grid, a standalone editor, or application code.

Put required rules in the schema​

FdcStringField(
  name: 'company',
  label: 'Company',
  required: true,
)

Put single-field rules on the field​

FdcStringField(
  name: 'state',
  label: 'State',
  validator: (row, value) {
    final text = value as String?;
    if (text != null && text.length != 2) {
      return 'Use a two-letter state code.';
    }
    return null;
  },
)

Put cross-field rules on the dataset​

final bookings = FdcDataSet(
  fields: fields,
  recordValidator: (row) {
    final start = row['start_date'] as DateTime?;
    final end = row['end_date'] as DateTime?;

    if (start != null && end != null && end.isBefore(start)) {
      return const [
        FdcValidationError(
          message: 'End date must not be before start date.',
          fieldName: 'end_date',
        ),
      ];
    }

    return const [];
  },
);

Surface validation consistently​

Use onValidationError when the screen needs centralized feedback handling, such as a message area or notification service.

Keep field-specific feedback attached to the field whenever possible so a grid or editor can direct the user to the correct input.

Avoid duplicating UI-only rules​

Do not implement one validation rule in a grid callback and another copy in a form validator. Put business validity in the dataset schema and record validator, then let every data-aware UI share it.

See Validation and Editing Lifecycle.