What is the Repository pattern?

Answer: The Repository pattern abstracts the data layer behind a protocol. The rest of the app doesn't know or care whether data comes from a network, database, or cache.

Code Example:

// Protocol — defines what the repo can do
protocol ArticleRepository {
    func fetchAll() async throws -> [Article]
    func fetch(id: String) async throws -> Article
    func save(_ article: Article) async throws
}

// Real implementation — hits network + caches to CoreData
class RemoteArticleRepository: ArticleRepository {
    func fetchAll() async throws -> [Article] {
        let articles = try await api.getArticles()
        cache.save(articles)
        return articles
    }
}

// Test implementation — returns fake data instantly
class MockArticleRepository: ArticleRepository {
    func fetchAll() async throws -> [Article] {
        return [Article.mock()]
    }
}

// ViewModel uses the protocol — doesn't know which implementation it has
class ArticleViewModel: ObservableObject {
    private let repo: ArticleRepository
    init(repo: ArticleRepository) { self.repo = repo }
}