Thumbtack Swift Dependency Injector

Injector.swift

import Foundation

/// Thread-safe Swift dependency injector
///
/// **Using Injector**
/// 1. Call `Injector.setup()` early in the application lifecycle.
/// 2. In dependency classes, conform to one of `Injectable`, `Singleton`, `WeakSingleton`, `EagerSingleton`.
/// 3. In application code, construct dependencies using property injection like:
///     ```
///     private let logger: TTLogger = inject()
///     private let graphQLService: TTGraphQLService = inject()
///     ...
///     ```
///
/// **Testing with Injector**
/// 1. Define a mocked dependency as a subclass of the real dependency, overriding any functions you would like to mock.
/// 2. At the beginning of the unit test (e.g., in `XCTestCase.setUp()`), construct an instance of the mocked dependency
///     and set it as the stub for injection of the dependency type:
///     ```
///     override func setUp() {
///         super.setUp()
///
///         let logger = TTLoggerMock()
///         stub(TTLogger.self, with: logger)
///     }
///
///     class TTLoggerMock: TTLogger {
///         // Override TTLogger functionality as needed.
///     }
///     ```
///     Any code that `inject`s `TTLogger` after `stub(_:with:)` is called will be given the mock instance.
/// 3. At the end of the test (e.g., in `XCTestCase.tearDown()`), call `removeAllStubs()` to reset the state.
///
/// **Injectable protocols**
/// *Injectable*: `inject()` calls `init()` to construct a new instance every time.
///     If `inject()` is called N times, then N instances of dependency will be created.
/// *Singleton*: `inject()` calls `init()` the first time and holds a strong reference to this instance to return on subsequent calls to `inject()`.
///     1 instance of dependency exists at all times (after `inject()` is called at least once).
/// *WeakSingleton*: `inject()` calls `init()` the first time and holds a *weak* reference to this instance to return on subsequent calls to `inject()`.
///     ≤1 instance of dependency exists at any given time.
public enum Injector {
    private static let injectorQueueSpecificKey = DispatchSpecificKey<Void>()
    private static let injectorQueue = DispatchQueue.main
    private static var singletons: [ObjectIdentifier: Singleton] = [:]
    private static var eagerSingletons: [ObjectIdentifier: EagerSingleton] = [:]
    private static var weakSingletons: NSMapTable<NSString, WeakSingleton> = NSMapTable(keyOptions: .strongMemory, valueOptions: .weakMemory)
    private static var boundClassTypes: [ObjectIdentifier: Injectable.Type] = [:]
    private static var boundSingletonClassTypes: [ObjectIdentifier: Singleton.Type] = [:]
    private static var isSetup = false

    #if IS_TEST_BUILD
        fileprivate static var instanceStubs: [ObjectIdentifier: Stub] = [:]
        fileprivate static var singletonStubs: [ObjectIdentifier: Stub] = [:]
        fileprivate static var eagerSingletonStubs: [ObjectIdentifier: Stub] = [:]
        fileprivate static var weakSingletonStubs: [ObjectIdentifier: Stub] = [:]
        private static var usedStubIdentifiers: Set<ObjectIdentifier> = []
    #endif

    public static func setup() {
        injectorQueue.setSpecific(key: injectorQueueSpecificKey, value: ())
        isSetup = true
    }

    public static func bind<T: EagerSingleton>(instance: T, to type: T.Type) {
        enforceIsSetup()

        let identifier = ObjectIdentifier(type)
        eagerSingletons[identifier] = instance
    }

    public static func bind<T: Injectable, U: Injectable>(type: T.Type, to superclassType: U.Type) {
        enforceIsSetup()

        let identifier = ObjectIdentifier(superclassType)
        boundClassTypes[identifier] = type
    }

    public static func bind<T: Singleton, U: Singleton>(type: T.Type, to superclassType: U.Type) {
        enforceIsSetup()

        let identifier = ObjectIdentifier(superclassType)
        boundSingletonClassTypes[identifier] = type
    }

    /// Constructs and retains the singleton yet does not return the instance.
    /// Intended for use by singletons that are not directly interacted with, but instead receive
    /// their input via other means, e.g NotificationCenter.
    public static func internalize<T: Singleton>(_ type: T.Type) {
        _ = get(T.self)
    }

    public static func existingWeakSingleton<T: WeakSingleton>(forKey key: String) -> T? {
        guard let stored = weakSingletons.object(forKey: key as NSString) else { return nil }

        guard let instance = stored as? T else {
            preconditionFailure("A weak singleton instance with a different type exists for this key. " +
                "This should never happen. Expected: \(type(of: T.self)), actual: \(type(of: stored))")
        }

        return instance
    }

    /// Hold a weak reference to the singleton for that key.
    /// - Precondition: there must be no singleton instance for that key. use existingWeakSingleton(forKey:) to check
    public static func setWeakSingleton<T: WeakSingleton>(_ repository: T, forKey key: String) {
        precondition(weakSingletons.object(forKey: key as NSString) == nil, "Weak singletons cannot be replaced while in use.")

        weakSingletons.setObject(repository, forKey: key as NSString)
    }

    // MARK: - Private

    @usableFromInline
    internal static func get<T: Singleton>(_ type: T.Type) -> T {
        enforceIsSetup()

        return sync {
            var singletonType = type
            var identifier = ObjectIdentifier(type)

            var superclassIdentifier: ObjectIdentifier?
            var superclassSingletonType: T.Type?
            if let superclassType = boundSingletonClassTypes[identifier] {
                superclassIdentifier = ObjectIdentifier(superclassType)
                superclassSingletonType = (superclassType as! T.Type)
            }

            #if IS_TEST_BUILD
                let instance: T? = sync { // singletonStubs may be mutated during testing.
                    // First check if there is a stub for T.
                    if let stub = singletonStubs[identifier] {
                        usedStubIdentifiers.insert(identifier)
                        return (stub.instance as! T)
                    }

                    // If not, then check if there is a stub for T's bound class type.
                    if let superclassIdentifier = superclassIdentifier, let stub = singletonStubs[superclassIdentifier] {
                        usedStubIdentifiers.insert(superclassIdentifier)
                        return (stub.instance as! T)
                    }

                    return nil
                }

                if let instance = instance {
                    return instance
                }
            #endif

            singletonType = superclassSingletonType ?? singletonType
            identifier = superclassIdentifier ?? identifier

            if let instance = singletons[identifier] {
                return instance as! T
            }

            let singleton = singletonType.init()
            singletons[identifier] = singleton
            return singleton
        }
    }

    @usableFromInline
    internal static func get<T: WeakSingleton>(_ type: T.Type, identifier: String) -> T {
        enforceIsSetup()

        return sync {
            #if IS_TEST_BUILD
                let objectIdentifier = ObjectIdentifier(type)
                if let stub = weakSingletonStubs[objectIdentifier] {
                    usedStubIdentifiers.insert(objectIdentifier)
                    return (stub.instance as! T)
                }
            #endif

            if let existing: T = existingWeakSingleton(forKey: identifier) {
                return existing
            }

            let repository = type.init()
            setWeakSingleton(repository, forKey: identifier)
            return repository
        }
    }

    @usableFromInline
    internal static func get<T: Injectable>(_ type: T.Type) -> T {
        enforceIsSetup()

        let identifier = ObjectIdentifier(type)

        #if IS_TEST_BUILD
            let instance: T? = sync { // instanceStubs may be mutated during testing.
                if let stub = instanceStubs[identifier] {
                    usedStubIdentifiers.insert(identifier)
                    return (stub.instance as! T)
                }

                return nil
            }

            if let instance = instance {
                return instance
            }
        #endif

        if let boundClassType = boundClassTypes[identifier] {
            return boundClassType.init() as! T
        }

        return type.init()
    }

    @usableFromInline
    internal static func get<T: EagerSingleton>(_ type: T.Type) -> T {
        enforceIsSetup()

        let identifier = ObjectIdentifier(type)

        #if IS_TEST_BUILD
            let instance: T? = sync { // eagerSingletonStubs may be mutated during testing.
                if let stub = eagerSingletonStubs[identifier] {
                    usedStubIdentifiers.insert(identifier)
                    return (stub.instance as! T)
                }

                return nil
            }

            if let instance = instance {
                return instance
            }
        #endif

        if let instance = eagerSingletons[identifier] {
            return instance as! T
        }

        fatalError("Bound singleton for type \(type) does not exist.")
    }

    #if IS_TEST_BUILD
        fileprivate static func stub<T>(_ type: T.Type, with object: T, isGlobal: Bool, stubs: inout [ObjectIdentifier: Stub], file: StaticString = #file, line: UInt = #line) {
            enforceIsSetup()

            sync {
                let identifier = ObjectIdentifier(type)
                let stub = Stub(instance: object, location: (file: file, line: line), isGlobal: isGlobal)
                stubs[identifier] = stub
            }
        }

        public static var unusedStubLocations: [StubLocation] {
            enforceIsSetup()

            return sync {
                let stubCollections = [instanceStubs, singletonStubs, eagerSingletonStubs, weakSingletonStubs]

                return stubCollections.reduce(into: [StubLocation]()) { result, collection in
                    let unusedIdentifiers = Set(collection.keys).subtracting(usedStubIdentifiers)
                    let locations = unusedIdentifiers
                        .compactMap { collection[$0] }
                        .filter { !$0.isGlobal }
                        .map { $0.location }
                    result.append(contentsOf: locations)
                }
            }
        }

        public static func removeAllStubs() {
            enforceIsSetup()

            sync {
                instanceStubs.removeAll()
                singletonStubs.removeAll()
                eagerSingletonStubs.removeAll()
                weakSingletonStubs.removeAll()
            }
        }
    #endif

    // MARK: - Private

    #if IS_TEST_BUILD
        fileprivate struct Stub {
            let instance: Any
            let location: StubLocation
            let isGlobal: Bool
        }
    #endif

    private static func sync<T>(block: () -> T) -> T {
        if DispatchQueue.getSpecific(key: injectorQueueSpecificKey) == nil {
            // We're not on the main queue but may still be on the main thread, in which case using synchonrous dispatch
            // can cause a deadlock.
            if Thread.isMainThread {
                return block()
            } else {
                return injectorQueue.sync(execute: block)
            }
        } else {
            return block()
        }
    }

    private static func enforceIsSetup() {
        guard !isSetup else { return }
        assertionFailure("Injector cannot be used until after it has been setup.")
    }
}

public protocol Injectable: AnyObject {
    init()
}

/// Only one instance should exist at any given time. The instance is created upon first use, and then
/// is persisted in memory for the lifetime of the app.
public protocol Singleton: Injectable {
    // Intentionally empty, used for specialization.
}

/// A singleton that cannot be constructed using `init()` and therefore must be manually constructed
/// via some arbitrary meams before being bound to its injectable type.
///
/// For example:
///    let instance = SomeClass(someArgument: value)
///    Injector.bind(eager: instance, to: SomeClass.self)
///
/// Once bound, the instance is persisted in memory for the lifetime of the app.
public protocol EagerSingleton: AnyObject {
    // Intentionally empty, used for specialization.
}

/// Only one instance should exist at any given time, or no instance if it is not being used.
@objc public protocol WeakSingleton: AnyObject {
    init()
}

/// Re: @inlinable
///
/// The expected behavior of a generic function is effectively that of a compile-time
/// template, in that it should inline itself and be statically dispatched from any call
/// site.  That said, this behavior is not enabled by default when the generic function is
/// invoked across module boundaries.  Instead, runtime machinery is setup for these
/// cross-module generic methods and calls to them are dispatched dynamically on the Swift
/// runtime.
///
/// The @inlinable annotation (+ its accompanying @usableFromInline annotation) add additional
/// metadata to the target that enable cross-module calls of generic functions to be inlined
/// and statically dispatched.  Standard Library methods like map() and reduce() leverage it
/// for that very purpose.
///
/// https://github.com/apple/swift-evolution/blob/master/proposals/0193-cross-module-inlining-and-specialization.md

@inlinable
public func inject<T: Singleton>(_ type: T.Type? = nil) -> T {
    Injector.get(T.self)
}

@inlinable
public func inject<T: EagerSingleton>(_ type: T.Type? = nil) -> T {
    Injector.get(T.self)
}

@inlinable
public func inject<T: Injectable>(_ type: T.Type? = nil) -> T {
    Injector.get(T.self)
}

@inlinable
public func inject<T: WeakSingleton>(_ type: T.Type? = nil, identifier: String = String(describing: T.self)) -> T {
    Injector.get(T.self, identifier: identifier)
}

#if IS_TEST_BUILD
    public func stub<T: Injectable>(_ type: T.Type, with object: T, isGlobal: Bool = false, file: StaticString = #file, line: UInt = #line) {
        Injector.stub(T.self, with: object, isGlobal: isGlobal, stubs: &Injector.instanceStubs, file: file, line: line)
    }

    public func stub<T: Singleton>(_ type: T.Type, with object: T, isGlobal: Bool = false, file: StaticString = #file, line: UInt = #line) {
        Injector.stub(T.self, with: object, isGlobal: isGlobal, stubs: &Injector.singletonStubs, file: file, line: line)
    }

    public func stub<T: EagerSingleton>(_ type: T.Type, with object: T, isGlobal: Bool = false, file: StaticString = #file, line: UInt = #line) {
        Injector.stub(T.self, with: object, isGlobal: isGlobal, stubs: &Injector.eagerSingletonStubs, file: file, line: line)
    }

    public func stub<T: WeakSingleton>(_ type: T.Type, with object: T, isGlobal: Bool = false, file: StaticString = #file, line: UInt = #line) {
        Injector.stub(T.self, with: object, isGlobal: isGlobal, stubs: &Injector.weakSingletonStubs, file: file, line: line)
    }

    public func removeAllStubs() {
        Injector.removeAllStubs()
    }

    public var unusedStubLocations: [StubLocation] {
        Injector.unusedStubLocations
    }

    public typealias StubLocation = (file: StaticString, line: UInt)
#endif

InjectorTest.swift

import XCTest

/// Unit tests for `Injector`
class InjectorTest: XCTestCase {
    func testSwiftSingleton() {
        let instance1: SwiftSingleton = inject()
        let instance2: SwiftSingleton = inject()
        XCTAssertTrue(instance1 === instance2)
    }

    func testSwiftSingletonIsDistinct() {
        let instance: SwiftSingleton = inject()
        let otherInstance: OtherSwiftSingleton = inject()
        XCTAssertFalse(instance === otherInstance)
    }

    func testSwiftSingletonWithGenericTypeIsDistinct() {
        let stringInstance1: SwiftSingletonGeneric<String> = inject()
        let stringInstance2: SwiftSingletonGeneric<String> = inject()
        XCTAssertTrue(stringInstance1 === stringInstance2)

        let intInstance: SwiftSingletonGeneric<Int> = inject()
        XCTAssertFalse(stringInstance1 === intInstance)
    }

    func testSwiftSingletonSubclass() {
        let instance1: SwiftSingletonSubclass1 = inject()
        let instance2: SwiftSingletonSubclass1 = inject()
        XCTAssertTrue(instance1 === instance2)
    }

    func testSwiftSingletonSubclassIsDistinct() {
        let superclass: SwiftSingleton = inject()
        let instance1: SwiftSingletonSubclass1 = inject()
        let instance2: SwiftSingletonSubclass2 = inject()
        XCTAssertFalse(instance1 === instance2)
        XCTAssertFalse(superclass === instance1)
        XCTAssertFalse(superclass === instance2)
    }

    func testSwiftSingletonOnBackgroundThread() {
        var backgroundInstance: SwiftSingleton?
        let completed = expectation(description: "background singleton")

        DispatchQueue.global().async {
            backgroundInstance = inject()
            completed.fulfill()
        }

        waitForExpectations(timeout: 10) { error in
            if let error = error {
                XCTFail(error.localizedDescription)
            }

            XCTAssertNotNil(backgroundInstance)

            let instance: SwiftSingleton = inject()
            XCTAssertTrue(backgroundInstance! === instance)
        }
    }

    func testSwiftTransient() {
        let instance1: SwiftTransient = inject()
        let instance2: SwiftTransient = inject()
        XCTAssertFalse(instance1 === instance2)
    }

    func testBoundSingleton() {
        let instance1 = BoundSingleton()
        Injector.bind(instance: instance1, to: BoundSingleton.self)
        let instance2: BoundSingleton = inject()
        XCTAssertTrue(instance1 === instance2)
    }

    func testBoundSingletonSubclass() {
        let instance1 = BoundSingletonSubclass()
        Injector.bind(instance: instance1, to: BoundSingleton.self)
        let instance2: BoundSingleton = inject()
        XCTAssertTrue(instance1 === instance2)
    }

    func testBindSubclassToSuperclass() {
        Injector.bind(type: InjectableSubclass.self, to: InjectableSuperclass.self)
        let instance1: InjectableSuperclass = inject()
        XCTAssertTrue(type(of: instance1) == InjectableSubclass.self)
        let instance2: InjectableSubclass = inject()
        XCTAssertTrue(type(of: instance2) == InjectableSubclass.self)
    }

    func testBindSingletonSubclassToSuperclass() {
        Injector.bind(type: InjectableSingletonSubclass.self, to: InjectableSingletonSuperclass.self)
        let instance1: InjectableSingletonSuperclass = inject()
        XCTAssertTrue(type(of: instance1) == InjectableSingletonSubclass.self)
        let instance2: InjectableSingletonSuperclass = inject()
        XCTAssertTrue(instance1 === instance2)
        let instance3: InjectableSingletonSubclass = inject()
        XCTAssertTrue(instance2 === instance3)
    }

    func testReentrantSingletonInjectionDoesNotDeadlock() {
        _ = inject(ReentrantSingleton1.self)
    }

    func testNotificationCenterCanBeInjected() {
        XCTAssertNoThrow(inject(NotificationCenter.self))
    }

    func testUserDefaultsCanBeInjected() {
        XCTAssertNoThrow(inject(UserDefaults.self))
    }

    func testInjectWeakSingleton() {
        autoreleasepool {
            let s1: SomeWeakSingleton? = inject(SomeWeakSingleton.self)
            let s2: SomeWeakSingleton? = inject(SomeWeakSingleton.self)

            XCTAssertEqual(1, SomeWeakSingleton.instantiationCount)
            XCTAssertTrue(s1 === s2, "WeakSingleton should be a shared instance while in use")
        }

        let nilSingleton: SomeWeakSingleton? = Injector.existingWeakSingleton(forKey: String(describing: SomeWeakSingleton.self))
        XCTAssertNil(nilSingleton, "Shared instance should be released once there are no more references to it")
    }
}

private class SwiftSingleton: Singleton {
    required init() {}
}

private class SwiftSingletonGeneric<T>: Singleton {
    required init() {}
}

private class SwiftSingletonSubclass1: SwiftSingleton {
    required init() {}
}

private class SwiftSingletonSubclass2: SwiftSingleton {
    required init() {}
}

private class OtherSwiftSingleton: Singleton {
    required init() {}
}

private class SomeWeakSingleton: WeakSingleton {
    static var instantiationCount = 0

    required init() {
        Self.instantiationCount += 1
    }
}

private class SwiftTransient: Injectable {
    required init() {}
}

private class BoundSingleton: EagerSingleton {
    required init() {}
}

private class BoundSingletonSubclass: BoundSingleton {
    required init() {}
}

private class InjectableSuperclass: Injectable {
    required init() {}
}

private class InjectableSubclass: InjectableSuperclass {
    required init() {}
}

private class InjectableSingletonSuperclass: Singleton {
    required init() {}
}

private class InjectableSingletonSubclass: InjectableSingletonSuperclass {
    required init() {}
}

private class ReentrantSingleton1: Singleton {
    private let singleton2: ReentrantSingleton2

    required convenience init() {
        self.init(singleton2: inject())
    }

    required init(singleton2: ReentrantSingleton2) {
        self.singleton2 = singleton2
    }
}

private class ReentrantSingleton2: Singleton {
    required init() {}
}

license

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