customValidate function

String? customValidate({ required String value, String? title, bool trimWhitespace = true, bool isRequired = true, int? minLength, int? maxLength, String? pattern, PatternType? patternType, bool allowOnlyNumbers = false, bool allowOnlyLetters = false, String? emptyMessage, String? minLengthMessage, String? maxLengthMessage, String? invalidPatternMessage, String? invalidNumberMessage, String? invalidLettersMessage, String? customValidator(String value)?, List<String>? existingValues, String? alreadyExistsMessage, })

Provides comprehensive custom validation with multiple configurable rules.

Flexible validation allowing combination of multiple rules. Ideal for usernames, product codes, or any field with specific requirements.

Validation order: required check, existing values, length, character restrictions, pattern matching, custom function.

Parameters:

• value: The input value to validate
• title: Field name for error messages (default: "Field")
• trimWhitespace: Whether to trim whitespace (default: true)
• isRequired: Whether the field is required (default: true)
• minLength, maxLength: Length constraints
• pattern: Custom regex pattern
• patternType: Predefined pattern from PatternType enum
• allowOnlyNumbers, allowOnlyLetters: Character type restrictions
• customValidator: Custom validation function
• existingValues: List of existing values to check against
• Various message parameters for customizing error messages

Implementation

String? customValidate({
  required String value,
  String? title,
  bool trimWhitespace = true,
  bool isRequired = true,
  int? minLength,
  int? maxLength,
  String? pattern,
  PatternType? patternType,
  bool allowOnlyNumbers = false,
  bool allowOnlyLetters = false,
  String? emptyMessage,
  String? minLengthMessage,
  String? maxLengthMessage,
  String? invalidPatternMessage,
  String? invalidNumberMessage,
  String? invalidLettersMessage,
  String? Function(String value)? customValidator,
  List<String>? existingValues,
  String? alreadyExistsMessage,
}) {
  final String fieldName = title ?? "Field";
  final String input = trimWhitespace ? value.trim() : value;

  if (isRequired && input.isEmpty) {
    return emptyMessage ?? "Please enter $fieldName.";
  }

  if (!isRequired && input.isEmpty) {
    return null;
  }

  if (existingValues != null && existingValues.contains(input)) {
    return alreadyExistsMessage ?? "$fieldName already exists.";
  }

  if (minLength != null && input.length < minLength) {
    return minLengthMessage ??
        "$fieldName must be at least $minLength characters long.";
  }

  if (maxLength != null && input.length > maxLength) {
    return maxLengthMessage ??
        "$fieldName must be at most $maxLength characters long.";
  }

  if (allowOnlyNumbers && !RegExp(r'^\d+$').hasMatch(input)) {
    return invalidNumberMessage ?? "$fieldName must contain only numbers.";
  }

  if (allowOnlyLetters && !RegExp(r'^[a-zA-Z]+$').hasMatch(input)) {
    return invalidLettersMessage ?? "$fieldName must contain only letters.";
  }

  if (patternType != null && !patternType.isValid(input)) {
    return invalidPatternMessage ?? "$fieldName format is invalid.";
  }

  if (pattern != null && !RegExp(pattern).hasMatch(input)) {
    return invalidPatternMessage ?? "$fieldName format is invalid.";
  }

  if (customValidator != null) {
    return customValidator(input);
  }

  return null;
}