Domain Skill

Work with domain layer components - create behavior models, validators, formatters, or understand when to extract logic to the domain layer.

Common Tasks

code

/domain create EmailValidator              # Create validation behavior
/domain add PhoneNumberFormatter           # Create formatting utility
/domain explain when to use domain layer   # Architecture guidance
/domain move validation from screen        # Extract shared logic
/domain test PasswordStrengthChecker       # Write behavior tests

When to Create Domain Layer

Create domain components when:

•Business logic is needed by multiple screens
•Complex validation or transformation logic exists
•Application-level business rules need enforcement

Don't create domain for:

•Logic used by only one screen (keep in screen's Model)
•Simple data transformations
•UI-specific logic

Two Types of Domain Components

Type	Purpose	Pattern
Behavior Model	Validation, formatting, transformation	`@Inject class`
Reactive Source	Shared state across screens	`ViceSource` (see vice-sources)

Quick Reference - Behavior Model

kotlin

@Inject
class EmailValidator {
  @Immutable
  enum class Result { Valid, Invalid, Required }

  fun validate(email: CharSequence): Result =
    when {
      email.isBlank() -> Result.Required
      !email.matches(ValidEmailRegex) -> Result.Invalid
      else -> Result.Valid
    }

  companion object {
    private val ValidEmailRegex = Regex(
      """[a-zA-Z0-9+._%\-]{1,256}@[a-zA-Z0-9][a-zA-Z0-9\-]{0,64}(\.[a-zA-Z0-9][a-zA-Z0-9\-]{0,25})+""",
    )
  }
}

Module Structure

code

domain/<feature>/
├── public/    # Interfaces (if needed) and models
├── impl/      # Implementations with DI annotations
└── fake/      # Test doubles

Simple behavior models don't need interface/impl split - put them directly in public or a shared module.

Use interface/impl pattern when:

•Testing would be difficult without fakes
•Multiple implementations are expected
•Better encapsulation is needed

Related Skills

•vice-sources - Reactive state patterns for domain layer
•repository - Data layer that domain consumes