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Domain models, value objects, invariants, and use cases

Domain code holds behavior that must remain true regardless of which screen, database, or API is active. Its types use the product’s language and prevent invalid states where practical.

Turn rules into values

struct Tag: Hashable, Sendable {
    let value: String

    init(_ raw: String) throws {
        let normalized = raw.trimmingCharacters(in: .whitespacesAndNewlines)
        guard (1...24).contains(normalized.count) else {
            throw ValidationError.invalidTag
        }
        value = normalized.lowercased()
    }
}

struct Coordinate: Equatable, Sendable {
    let latitude: Double
    let longitude: Double

    init(latitude: Double, longitude: Double) throws {
        guard (-90...90).contains(latitude), (-180...180).contains(longitude) else {
            throw ValidationError.invalidCoordinate
        }
        self.latitude = latitude
        self.longitude = longitude
    }
}

Once created, these values carry their validity into SwiftUI, UIKit, storage adapters, and sync operations.

Put orchestration in use cases

struct CreateNote {
    var makeID: () -> UUID
    var now: () -> Date
    var save: (Note) async throws -> Void

    func callAsFunction(draft: NoteDraft) async throws -> Note {
        let title = try NoteTitle(draft.title)
        let note = Note(id: makeID(), title: title, body: draft.body, createdAt: now())
        try await save(note)
        return note
    }
}

The use case owns the complete action. The UI supplies intent; adapters supply IDs, time, and persistence.

Avoid an anemic mirror

A type that only repeats database columns or JSON fields is a transport shape, not automatically a domain model. Translate external shapes at the boundary, then keep product rules in types whose names and behavior reflect Field Notes.

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Series index: Zero to iOS Hero

References

Structures and classes describes Swift value and reference types.
Initialization covers establishing valid stored state.