@Injected - Modern Dependency Injection

Overview

@Injected is WeaveDI's flagship property wrapper, inspired by TCA's @Dependency but optimized for WeaveDI. It provides type-safe, compile-time checked dependency injection with zero configuration overhead.

Why @Injected?

• ✅ Type-Safe: Compile-time type checking
• ✅ TCA-Style: Familiar API for TCA developers
• ✅ Flexible: Supports both KeyPath and Type-based access
• ✅ Immutable: No mutating get required
• ✅ Testable: Easy to override in tests

Basic Usage

1. Define InjectedKey

struct APIClientKey: InjectedKey {
    static let liveValue: APIClient = APIClientImpl()
    static let testValue: APIClient = MockAPIClient()
}

2. Extend InjectedValues

extension InjectedValues {
    var apiClient: APIClient {
        get { self[APIClientKey.self] }
        set { self[APIClientKey.self] = newValue }
    }
}

3. Use @Injected

struct MyFeature: Reducer {
    @Injected(\.apiClient) var apiClient

    func reduce(into state: inout State, action: Action) -> Effect<Action> {
        .run { send in
            let data = try await apiClient.fetchData()
            await send(.dataLoaded(data))
        }
    }
}

Type-Based Access

You can also use @Injected with types directly:

extension ExchangeUseCaseImpl: InjectedKey {
    public static var liveValue: ExchangeRateInterface {
        let repository = UnifiedDI.register(ExchangeRateInterface.self) {
            ExchangeRepositoryImpl()
        }
        return ExchangeUseCaseImpl(repository: repository)
    }
}

struct CurrencyFeature: Reducer {
    @Injected(ExchangeUseCaseImpl.self) var exchangeUseCase
}

Testing

Override dependencies in tests using withInjectedValues:

func testFetchData() async {
    await withInjectedValues { values in
        values.apiClient = MockAPIClient()
    } operation: {
        let feature = MyFeature()
        // Test with mock
    }
}

Comparison with @Inject

Feature	@Injected (Legacy)	@Injected (New)
Type Safety	❌ Optional-based	✅ Compile-time
TCA Style	❌ Different	✅ Familiar
KeyPath	✅ Supported	✅ Supported
Type Access	❌ No	✅ Supported
Immutable	❌ Needs mutating	✅ Non-mutating
Testing	⚠️ Manual	✅ Built-in

Migration Guide

From @Inject

// ❌ Old
@Injected var repository: UserRepository?

// ✅ New
@Injected(\.repository) var repository

From Manual Resolution

// ❌ Old
let repository = UnifiedDI.requireResolve(UserRepository.self)

// ✅ New
@Injected(\.repository) var repository

Best Practices

1. Prefer KeyPath access for better discoverability
2. Use Type access for quick prototyping
3. Always define testValue for easier testing
4. Keep InjectedKey extensions close to the type definition

Advanced: Protocol-based Dependencies

protocol ExchangeRateInterface: Sendable {
    func getExchangeRates(currency: String) async throws -> ExchangeRates?
}

extension ExchangeUseCaseImpl: InjectedKey {
    public static var liveValue: ExchangeRateInterface {
        // Return protocol implementation
        ExchangeUseCaseImpl(repository: ...)
    }
}

extension InjectedValues {
    var exchangeUseCase: ExchangeRateInterface {
        get { self[ExchangeUseCaseImpl.self] }
        set { self[ExchangeUseCaseImpl.self] = newValue }
    }
}

Real-World Example: Currency Exchange App

// 1. Define InjectedKey
extension ExchangeUseCaseImpl: InjectedKey {
    public static var liveValue: ExchangeRateInterface {
        let repository = UnifiedDI.register(ExchangeRateInterface.self) {
            ExchangeRepositoryImpl()
        }
        return ExchangeUseCaseImpl(repository: repository)
    }
}

// 2. Extend InjectedValues
public extension InjectedValues {
    var exchangeUseCase: ExchangeRateInterface {
        get { self[ExchangeUseCaseImpl.self] }
        set { self[ExchangeUseCaseImpl.self] = newValue }
    }
}

// 3. Use in Reducer
struct CurrencyFeature: Reducer {
    @Injected(\.exchangeUseCase) var exchangeUseCase

    func reduce(into state: inout State, action: Action) -> Effect<Action> {
        case .fetchRates(let currency):
            return .run { send in
                let rates = try await exchangeUseCase.getExchangeRates(currency: currency)
                await send(.ratesLoaded(rates))
            }
    }
}

See Also

• Property Wrappers - Complete property wrapper guide
• Testing Guide - Advanced testing patterns
• TCA Integration - Using @Injected with TCA