What is Dependency Injection and why is it important?

All three separate concerns between data, UI, and logic — but they differ in how the middle layer connects to the view.
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Dependency Injection (DI) means a type receives its dependencies from outside rather than creating them internally.

Answer: Dependency Injection (DI) means a type receives its dependencies from outside rather than creating them internally. This makes code testable, modular, and easier to change.

Code Example:

// ❌ Without DI — hard to test, tightly coupled
class OrderService {
    private let network = NetworkClient()  // created internally
    private let db = Database()
}

// ✅ With DI — inject dependencies
class OrderService {
    private let network: NetworkClientProtocol
    private let db: DatabaseProtocol

    init(network: NetworkClientProtocol, db: DatabaseProtocol) {
        self.network = network
        self.db = db
    }
}

// Production
let service = OrderService(network: NetworkClient(), db: Database())

// Testing — inject mocks
let service = OrderService(network: MockNetwork(), db: MockDatabase())

Three types:

Constructor injection — via init (preferred)
Property injection — via settable property
Method injection — via function parameter