Swift Mocking is a collection of Swift macros used to generate mock dependencies.

• Features
• Example
• Installation
• Usage
• Macros
  • @Mocked
    • Compilation Condition
    • Sendable Conformance
    • Access Levels
    • Actor Conformance
    • Associated Types
    • Members
  • @MockedMembers
    • Static Members
  • @MockableProperty
  • @MockableMethod
  • @Mockable vs. @_Mocked
• Contributing
• License

Features

Swift Mocking is Swift 6 compatible, fully concurrency-safe, and generates conditionally compiled mocks that can handle:

• Any access level
• Associated types, including primary associated types
• Actor conformance
• Generic where clauses
• Initializers
• Static members
• Instance members
• Read-only properties, including those with getters marked with async, throws, mutating, etc.
• Read-write properties
• Mutating methods
• Async methods
• Throwing methods
• Generic methods
• Method overloads
• Attributed types (inout, consuming, sending, etc.)
• Variadic parameters

Example

@Mocked(compilationCondition: .debug)
protocol WeatherService {
    func currentTemperature(latitude Double, longitude: Double) async throws -> Double
}

struct WeatherViewModelTests {
    @Test
    func loadCurrentTemperature() async {
        let weatherServiceMock = WeatherServiceMock()
        let viewModel = WeatherViewModel(weatherService: weatherServiceMock)

        // Set the dependency's implementation.
        weatherServiceMock._currentTemperature.implementation = .returns(75)

        // Invoke the method being tested.
        await viewModel.loadCurrentTemperature(latitude: 37.3349, longitude: 122.0090)

        // Validate the number of times the dependency was called.
        #expect(weatherServiceMock._currentTemperature.callCount == 1)

        // Validate the arguments passed to the dependency.
        #expect(weatherServiceMock._currentTemperature.lastInvocation?.latitude == 37.3349)
        #expect(weatherServiceMock._currentTemperature.lastInvocation?.longitude == 122.0090)

        // Validate the view model's new state.
        #expect(viewModel.state == .loaded(temperature: 75))
    }
}

Installation

To add Swift Mocking to a Swift package manifest file:

• Add the swift-mocking package to your package's dependencies:

  .package(
      url: "https://github.com/fetch-rewards/swift-mocking.git",
      from: "<#latest swift-mocking tag#>"
  )

• Add the Mocking product to your target's dependencies:

  .product(name: "Mocking", package: "swift-mocking")

Usage

Import Mocking:

import Mocking

Attach the @Mocked macro to your protocol:

@Mocked(compilationCondition: .debug)
protocol Dependency {
    var property: Int { get set }

    func method(x: Int, y: Int) async throws -> Int
}

And that's it! You now have a sophisticated mock dependency that will be updated automatically any time you change your protocol.

Note

For mocking protocols that inherit from other protocols, see @MockedMembers.

Important

Using @Mocked without an explicit compilationCondition argument will result in the generated mock being wrapped in an #if compiler directive with a SWIFT_MOCKING_ENABLED condition (i.e. #if SWIFT_MOCKING_ENABLED). To continue using @Mocked without any additional setup, use @Mocked(compilationCondition: .debug) as shown in the examples above. If you would like fine-tuned control over when generated mocks are compiled, see Compilation Condition.

Now let's take a look at the mock we've generated, stripping out some of the implementation details to highlight the mock's API:

#if DEBUG
final class DependencyMock: Dependency {
    var property: Int
    var _property: MockReadWriteProperty<Int>

    func method(x: Int, y: Int) async throws -> Int
    var _method: MockReturningParameterizedAsyncThrowingMethod<...>
}
#endif

Each member of the generated mock is backed by a single, underscored property. These backing properties contain the invocation records and implementation details for each member.

For example, the backing property for property from the above mock would have the following structure and implementation constructors:

// Invocation Records
mock._property.getter.callCount // Int
mock._property.getter.returnedValues // [Int]
mock._property.getter.lastReturnedValue // Int?

mock._property.setter.callCount // Int
mock._property.setter.invocations // [Int]
mock._property.setter.lastInvocation // Int?

// Implementation Constructors
mock._property.getter.implementation = .invokes { 5 }
mock._property.getter.implementation = .uncheckedInvokes { 5 }
mock._property.getter.implementation = .returns(5)
mock._property.getter.implementation = .uncheckedReturns(5)

mock._property.setter.implementation = .invokes { _ in }
mock._property.setter.implementation = .uncheckedInvokes { _ in }

And the backing property for method from the above mock would have the following structure and implementation constructors:

// Invocation Records
mock._method.callCount // Int
mock._method.invocations // [(x: Int, y: Int)]
mock._method.lastInvocation // (x: Int, y: Int)?
mock._method.returnedValues // [Result<Int, any Error>]
mock._method.lastReturnedValue // Result<Int, any Error>?

// Implementation Constructors
mock._method.implementation = .invokes { _, _ in 5 }
mock._method.implementation = .uncheckedInvokes { _, _ in 5 }
mock._method.implementation = .throws(URLError(.badServerResponse))
mock._method.implementation = .returns(5)
mock._method.implementation = .uncheckedReturns(5)

Note

Depending on the type of member being mocked, the backing property's structure and implementation constructors may differ slightly from the examples above.

Tip

Only use unchecked implementation constructors when dealing with non-sendable types. For sendable types, use the checked version of each implementation constructor (e.g. invokes instead of uncheckedInvokes and returns instead of uncheckedReturns). These checked constructors require the member's arguments and/or return value to be sendable.

With Strict Concurrency Checking or Swift 6+ enabled, you will get concurrency warnings/errors if you try to use these checked constructors with a non-sendable type, whether that non-sendable type is the member's argument or return value or is a type captured by the closure passed to invokes:

let nonSendableInstance = NonSendableType()

mock._methodReturningNonSendableType.implementation = .invokes { // Type 'NonSendableType' does not conform to the 'Sendable' protocol
    nonSendableInstance // Capture of 'nonSendableInstance' with non-sendable type 'NonSendableType' in a `@Sendable` closure
} 

Macros

Swift Mocking contains several Swift macros: @Mocked, @MockedMembers, @MockableProperty, and @MockableMethod.

It also contains two internal, underscored macros (@_MockedProperty and @_MockedMethod) which are not meant to be used directly.

@Mocked

@Mocked is an attached peer macro that generates a mock class from a protocol declaration:

@Mocked(compilationCondition: .debug)
protocol Dependency {}

// Generates:

#if DEBUG
@MockedMembers
final class DependencyMock: Dependency {}
#endif

Compilation Condition

Using @Mocked without an explicit compilationCondition argument will result in the generated mock being wrapped in an #if compiler directive with a SWIFT_MOCKING_ENABLED condition:

@Mocked
protocol Dependency {}

// Generates:

#if SWIFT_MOCKING_ENABLED
@MockedMembers
final class DependencyMock: Dependency {}
#endif

Because of the nature of Swift macros, @Mocked only has access to the raw syntax of its arguments and the protocol to which it's attached. This limitation precludes us from making compilationCondition globally configurable. So when deciding on an appropriate default value for compilationCondition, we had two goals in mind:

1. One-step install and one-line usage (excluding import statement) for users who want conditionally compiled, generated mocks without any additional setup
2. The simplest setup possible for users with large codebases who want fine-tuned control over when generated mocks are compiled

As such, we decided to make the default compilation condition SWIFT_MOCKING_ENABLED. This allows us to accomplish both of these goals, albeit with the tiny caveat that one-step-install users need to explicitly specify the compilation condition when using @Mocked.

If you would like to make use of SWIFT_MOCKING_ENABLED in an Xcode project, add SWIFT_MOCKING_ENABLED as a compiler flag to the build configurations for which you would like mocks to compile.

To make use of SWIFT_MOCKING_ENABLED in a Swift package, add the following SwiftSetting to your target's swiftSettings array:

.define("SWIFT_MOCKING_ENABLED", .when(configuration: .debug))

Note

The .debug build configuration in a Swift package applies to any Xcode project build configuration with a name that begins with either "Debug" or "Development".

If you would like to specify a compilation condition other than SWIFT_MOCKING_ENABLED, you can explicitly provide one to the @Mocked macro:

/// The mock will not be wrapped in an `#if` compiler directive.
@Mocked(compilationCondition: .none)
protocol NoneCompilationCondition {}

/// `#if DEBUG`
@Mocked(compilationCondition: .debug)
protocol DebugCompilationCondition {}

/// `#if !RELEASE`
@Mocked(compilationCondition: "!RELEASE")
protocol CustomCompilationCondition {}

Sendable Conformance

The @Mocked macro supports a sendableConformance argument, which can be .checked or .unchecked, allowing you to control the Sendable conformance of the generated mock.

With .checked, the mock's Sendability is inherited from the protocol it is mocking, resulting in checked Sendable conformance if the protocol inherits from Sendable.

@Mocked
protocol Dependency: Sendable {}

// Or

@Mocked(sendableConformance: .checked)
protocol Dependency: Sendable {}

// Both generate:

#if SWIFT_MOCKING_ENABLED
@MockedMembers
final class DependencyMock: Dependency {}
#endif

With .unchecked, the generated mock will explicitly conform to @unchecked Sendable. This is useful when you need your mock to be Sendable but cannot satisfy strict compiler checks and know your usage is concurrency-safe.

@Mocked(sendableConformance: .unchecked)
protocol Dependency: Sendable {}

// Generates:

#if SWIFT_MOCKING_ENABLED
@MockedMembers
final class DependencyMock: @unchecked Sendable, Dependency {}
#endif

Access Levels

The generated mock is marked with the access level required to conform to the protocol: public for public, implicit internal for both implicit and explicit internal, and fileprivate for both fileprivate and private.

Actor Conformance

@Mocked also supports protocols that conform to Actor:

@Mocked(compilationCondition: .debug)
protocol Dependency: Actor {}

// Generates:

#if DEBUG
@MockedMembers
final actor DependencyMock: Dependency {}
#endif

Associated Types

When @Mocked is applied to a protocol that defines associated types, the resulting mock uses those associated types as its generic parameters in order to fulfill the protocol requirements:

@Mocked(compilationCondition: .debug)
protocol Dependency {
    associatedtype Key: Hashable
    associatedtype Value: Equatable
}

// Generates:

#if DEBUG
@MockedMembers
final class DependencyMock<Key: Hashable, Value: Equatable>: Dependency {}
#endif

Members

In addition to the @MockedMembers macro that gets applied to the mock declaration, @Mocked also utilizes the @MockableProperty and @MockableMethod macros when defining the mock's members:

@Mocked(compilationCondition: .debug)
protocol Dependency {
    var readOnlyProperty: Int { get }
    var readOnlyAsyncProperty: Int { get async }
    var readOnlyThrowingProperty: Int { get throws }
    var readOnlyAsyncThrowingProperty: Int { get async throws }
    var readWriteProperty: Int { get set }
}

// Generates:

#if DEBUG
@MockedMembers
final class DependencyMock: Dependency {
    @MockableProperty(.readOnly)
    var readOnlyProperty: Int

    @MockableProperty(.readOnly(.async))
    var readOnlyAsyncProperty: Int

    @MockableProperty(.readOnly(.throws))
    var readOnlyThrowingProperty: Int

    @MockableProperty(.readOnly(.async, .throws))
    var readOnlyAsyncThrowingProperty: Int

    @MockableProperty(.readWrite)
    var readWriteProperty: Int
}
#endif

Because @MockedMembers cannot look outward at the protocol declaration to determine whether, for example, a property is read-only or read-write, @Mocked uses @MockableProperty and @MockableMethod to provide information about each member to @MockedMembers. @MockedMembers then applies the @_MockedProperty and @_MockedMethod macros to those members, which then generate the mock's backing properties.

Note

See @Mockable vs. @_Mocked for more information.

@MockedMembers

Like @MockedMembers, @Mocked also cannot look outward. This presents a problem when the protocol you are trying to mock inherits from another protocol. Because @Mocked cannot see the other protocol's declaration, it is unable to generate conformances to the requirements of that protocol. In this instance, you will need to write the mock declaration yourself, along with the declarations for the properties and methods required by the protocols. Luckily, using Xcode's Fix-It feature to add protocol conformances and @MockedMembers, @MockableProperty, and @MockableMethod to generate backing properties, you can still easily create and maintain these mocks with minimal code:

protocol Dependency: SomeProtocol {
    var propertyFromDependency: Int { get }

    func methodFromDependency()
}

#if DEBUG
@MockedMembers
final class DependencyMock: Dependency {
    @MockableProperty(.readOnly)
    var propertyFromDependency: Int

    @MockableProperty(.readWrite)
    var propertyFromSomeProtocol: Int

    func methodFromDependency()

    func methodFromSomeProtocol()
}
#endif

Static Members

When a mock contains static members, @MockedMembers generates a static method named resetMockedStaticMembers that can be used to reset the backing properties for those static members:

@MockedMembers
public final class DependencyMock: Dependency {
    @MockableProperty(.readOnly)
    public static var staticReadOnlyProperty: Int

    @MockableProperty(.readWrite)
    public static var staticReadWriteProperty: Int

    public static func staticMethod()

    // Generates:

    /// Resets the implementations and invocation records of the mock's 
    /// static properties and methods.
    public static func resetMockedStaticMembers() {
        self.__staticReadOnlyProperty.reset()
        self.__staticReadWriteProperty.reset()
        self.__staticMethod.reset()
    }
}

This method is useful for tearing down static state between test cases.

@MockableProperty

In instances where you are using @MockedMembers directly instead of using @Mocked, @MockableProperty is required for @MockedMembers to be able to generate backing properties for the property conformances within your mock:

protocol Dependency {
    var readOnlyProperty: Int { get }
    var readOnlyAsyncProperty: Int { get }
    var readOnlyThrowingProperty: Int { get }
    var readOnlyAsyncThrowingProperty: Int { get }
    var readWriteProperty: Int { get set }
}

#if DEBUG
@MockedMembers
final class DependencyMock: Dependency {
    @MockableProperty(.readOnly)
    var readOnlyProperty: Int { get }

    @MockableProperty(.readOnly(.async))
    var readOnlyAsyncProperty: Int { get }

    @MockableProperty(.readOnly(.throws))
    var readOnlyThrowingProperty: Int { get }

    @MockableProperty(.readOnly(.async, .throws))
    var readOnlyAsyncThrowingProperty: Int { get }

    @MockableProperty(.readWrite)
    var readWriteProperty: Int { get set }
}
#endif

@MockableMethod

Unlike @MockableProperty, @MockableMethod is not required when using @MockedMembers directly. @MockedMembers can and will generate backing properties for method conformances within your mock whether they are explicitly marked with @MockableMethod or not.

Still, there may be cases where you want or need to use @MockableMethod. While @Mocked and @MockedMembers do an excellent job of dealing with name conflicts caused by method overloads, there's always a possibility that a name conflict may arise between two backing properties. In this case, you can provide an explicit name for the method's backing property using @MockableMethod:

@MockedMembers
final class DependencyMock: Dependency {
    @MockableMethod(mockMethodName: "someUniqueName")
    func methodWithNameConflict()
}

In other cases, you may simply dislike the name that @Mocked or @MockedMembers generates for a method's backing property and wish to give the backing property a different name.

If you believe that @Mocked or @MockedMembers should have been able to resolve a name conflict, or if you think the name conflict resolution logic can be improved in any way, please let us know by opening an issue.

@Mockable vs. @_Mocked

@MockableProperty and @MockableMethod do not produce expansions. They are simply markers that expose information to @MockedMembers. @MockableProperty exposes propertyType (.readOnly, .readWrite, etc.) and @MockableMethod exposes mockMethodName. @MockedMembers then forwards this information to @_MockedProperty and @_MockedMethod along with other parameters that @MockedMembers provides for us. @_MockedProperty and @_MockedMethod then generate the mock's backing properties. @MockableProperty and @MockableMethod exist so that the consumer has to provide as little information as possible when manually applying @MockedMembers. The usage of the prefix Mockable is a deliberate choice to semantically distinguish the macros that serve as markers from those that actually produce mocks.

Contributing

The simplest way to contribute to this project is by opening an issue.

If you would like to contribute code to this project, please read our Contributing Guidelines.

By opening an issue or contributing code to this project, you agree to follow our Code of Conduct.

License

This library is released under the MIT license. See LICENSE for details.