1FormValidator.kt

data class ValidationResult<E>(
  val isValid: Boolean,
  val errors: E
)

interface FieldValidator<I, O> {
  fun validate(input: I): O
}

interface FormValidator<T, E> {
  fun validate(form: T): ValidationResult<E>
}

2EmailFieldValidator.kt

class EmailFieldValidator(prival val resources: Resources) : FieldValidator<String, ValidationResult<String?>> {
  override fun validate(input: String): ValidationResult<String?> {
    val isValid = android.util.Patterns.EMAIL_ADDRESS.matcher(input).matches()
    val errorMessage = if (!isValid) {
      resources.getString(R.string.invalid_email)
    } else {
      null
    }
    return ValidationResult(isValid, errorMessage)
  }
}

3NameFieldValidator.kt

class NameFieldValidator(prival val resources: Resources) : FieldValidator<String, ValidationResult<String?>> {

  override fun validate(input: String): ValidationResult<String?> {
    val isValid = input.isNotBlank() && input.split(" ")
      .fold(true) { _, slice -> slice.isNotBlank() }

    val errorMessage = if (!isValid) {
      resources.getString(R.string.full_name)
    } else {
      null
    }
    return ValidationResult(isValid, errorMessage)
  }
}

4PasswordFieldValidator.kt

class PasswordFieldValidator(prival val resources: Resources) : FieldValidator<Pair<String, String>, ValidationResult<String?>> {
  override fun validate(input: Pair<String, String>): ValidationResult<String?> {
    val isEmpty = input.first.isBlank() || input.second.isBlank()
    val areDifferent = input.first != input.second

    val errorMessage = when {
      isEmpty -> {
        resources.getString(R.string.empty)
      }
      areDifferent -> {
        resources.getString(R.string.mismtach)
      }
      else -> {
        null
      }
    }
    return ValidationResult(!isEmpty && !areDifferent, errorMessage)
  }
}

5FormModels.kt

data class SignUpForm(
  val name: String,
  val email: String,
  val password: String,
  val passwordConfirmation: String
)

data class SignUpFormErrors(
  val name: String?,
  val email: String?,
  val password: String?
)

6SignUpFormValidator.kt

class SignUpFormValidator(
  @NameValidator private val nameFieldValidator: FieldValidator<String, ValidationResult<String?>>,
  @EmailValidator private val emailFieldValidator: FieldValidator<String, ValidationResult<String?>>,
  @PasswordValidator private val passwordFieldValidator: FieldValidator<Pair<String, String>, ValidationResult<String?>>
) : FormValidator<SignUpForm, SignUpFormErrors> {

  override fun validate(form: SignUpForm): ValidationResult<SignUpFormErrors> {
    val nameValidation = nameFieldValidator.validate(form.name)
    val emailValidation = emailFieldValidator.validate(form.email)
    val passwordValidation = passwordFieldValidator
      .validate(Pair(form.password, form.passwordConfirmation))

    val results = arrayOf(
      nameValidation,
      emailValidation,
      passwordValidation
    )

    val errors = SignUpFormErrors(
      nameValidation.errors,
      emailValidation.errors,
      passwordValidation.errors
    )

    return ValidationResult(results.all { it.isValid }, errors)
  }
}