PropertySpy property wrapper

Sources/Fakes/PropertySpy/PropertySpy.swift

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+/// An immutable property spy.
+@propertyWrapper public struct PropertySpy<T, U> {
+    public var wrappedValue: U {
+        mapping(projectedValue())
+    }
+
+    /// the ``Spy`` recording the getter calls.
+    public let projectedValue: Spy<Void, T>
+
+    public let mapping: @Sendable (T) -> U
+
+    /// Creates an immutable PropertySpy stubbed with the given value, which lets you map from one type to another
+    ///
+    /// - parameter value: The initial value to be stubbed
+    /// - parameter mapping: A closure to map from the initial value to the property's return type
+    ///
+    /// - Note: This initializer is particularly useful when the property is returning a protocol of some value, but you want to stub it with a particular instance of the protocol.
+    public init(_ value: T, as mapping: @escaping @Sendable (T) -> U) {
+        projectedValue = Spy(value)
+        self.mapping = mapping
+    }
+
+    /// Creates an immutable PropertySpy stubbed with the given value
+    ///
+    /// - parameter value: The initial value to be stubbed
+    public init(_ value: T) where T == U {
+        projectedValue = Spy(value)
+        self.mapping = { $0 }
+    }
+}
+
+/// A mutable property spy.
+@propertyWrapper public struct SettablePropertySpy<T, U> {
+    public var wrappedValue: U {
+        get {
+            getMapping(projectedValue.getter())
+        }
+        set {
+            projectedValue.setter(newValue)
+            projectedValue.getter.stub(setMapping(newValue))
+        }
+    }
+
+    public struct ProjectedValue {
+        /// A ``Spy`` recording every time the property has been set, with whatever the new value is, prior to mapping
+        public let setter: Spy<U, Void>
+        /// A ``Spy`` recording every time the property has been called. It is re-stubbed whenever the property's setter is called.
+        public let getter: Spy<Void, T>
+    }
+
+    /// The spies recording the setter and getter calls.
+    public let projectedValue: ProjectedValue
+
+    public let getMapping: @Sendable (T) -> U
+    public let setMapping: @Sendable (U) -> T
+
+    /// Creates a mutable PropertySpy stubbed with the given value, which lets you map from one type to another and back again
+    ///
+    /// - parameter value: The initial value to be stubbed
+    /// - parameter getMapping: A closure to map from the initial value to the property's return type
+    /// - parameter setMapping: A closure to map from the property's return type back to the initial value's type.
+    ///
+    /// - Note: This initializer is particularly useful when the property is returning a protocol of some value, but you want to stub it with a particular instance of the protocol.
+    public init(_ value: T, getMapping: @escaping @Sendable (T) -> U, setMapping: @escaping @Sendable (U) -> T) {
+        projectedValue = ProjectedValue(setter: Spy(), getter: Spy(value))
+        self.getMapping = getMapping
+        self.setMapping = setMapping
+    }
+
+    /// Creatse a mutable PropertySpy stubbed with the given value
+    ///
+    /// - parameter value: The inital value to be stubbed
+    public init(_ value: T) where T == U {
+        projectedValue = ProjectedValue(setter: Spy(), getter: Spy(value))
+        self.getMapping = { $0 }
+        self.setMapping = { $0 }
+    }
+}

Tests/FakesTests/PropertySpyTests.swift

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+import Fakes
+import Nimble
+import XCTest
+
+final class SettablePropertySpyTests: XCTestCase {
+    func testGettingPropertyWhenTypesMatch() {
+        struct AnObject {
+            @SettablePropertySpy(1)
+            var value: Int
+        }
+
+        let object = AnObject()
+
+        expect(object.value).to(equal(1))
+        // because we called it, we should expect for the getter spy to be called
+        expect(object.$value.getter).to(beCalled())
+
+        // We never interacted with the setter, so it shouldn't have been called.
+        expect(object.$value.setter).toNot(beCalled())
+    }
+
+    func testSettingPropertyWhenTypesMatch() {
+        struct AnObject {
+            @SettablePropertySpy(1)
+            var value: Int
+        }
+
+        var object = AnObject()
+        object.value = 3
+
+        expect(object.$value.getter).toNot(beCalled())
+        expect(object.$value.setter).to(beCalled(3))
+
+        // the returned value should now be updated with the new value
+        expect(object.value).to(equal(3))
+
+        // and because we called the getter, the getter spy should be called.
+        expect(object.$value.getter).to(beCalled())
+    }
+
+    func testGettingPropertyProtocolInheritence() {
+        struct ImplementedProtocol: SomeProtocol {
+            var value: Int = 1
+        }
+
+        struct AnObject {
+            @SettablePropertySpy(ImplementedProtocol(value: 2))
+            var value: SomeProtocol
+        }
+
+        let object = AnObject()
+
+        expect(object.value).to(beAKindOf(ImplementedProtocol.self))
+        // because we called it, we should expect for the getter spy to be called
+        expect(object.$value.getter).to(beCalled())
+
+        // We never interacted with the setter, so it shouldn't have been called.
+        expect(object.$value.setter).toNot(beCalled())
+    }
+
+    func testSettingPropertyProtocolInheritence() {
+        struct ImplementedProtocol: SomeProtocol, Equatable {
+            var value: Int = 1
+        }
+
+        struct AnObject {
+            @SettablePropertySpy(ImplementedProtocol())
+            var value: SomeProtocol
+        }
+
+        var object = AnObject()
+        object.value = ImplementedProtocol(value: 2)
+
+        expect(object.$value.getter).toNot(beCalled())
+        expect(object.$value.setter).to(beCalled(satisfyAllOf(
+            beAKindOf(ImplementedProtocol.self),
+            map(\.value, equal(2))
+        )))
+
+        // the returned value should now be updated with the new value
+        expect(object.value).to(satisfyAllOf(
+            beAKindOf(ImplementedProtocol.self),
+            map(\.value, equal(2))
+        ))
+        // and because we called the getter, the getter spy should be called.
+        expect(object.$value.getter).to(beCalled(times: 1))
+    }
+}
+
+final class PropertySpyTests: XCTestCase {
+    func testGettingPropertyWhenTypesMatch() {
+        struct AnObject {
+            @PropertySpy(1)
+            var value: Int
+        }
+
+        let object = AnObject()
+
+        expect(object.value).to(equal(1))
+        // because we called it, we should expect for the getter spy to be called
+        expect(object.$value).to(beCalled())
+    }
+
+    func testGettingPropertyProtocolInheritence() {
+        struct ImplementedProtocol: SomeProtocol {
+            var value: Int = 1
+        }
+
+        struct ObjectUsingProtocol {
+            @PropertySpy(ImplementedProtocol(value: 2))
+            var value: SomeProtocol
+        }
+
+        struct ObjectUsingDirectInstance {
+            @PropertySpy(ImplementedProtocol(value: 2), as: { $0 })
+            var value: SomeProtocol
+        }
+
+        let object = ObjectUsingProtocol()
+
+        expect(object.value).to(beAnInstanceOf(ImplementedProtocol.self))
+        // because we called it, we should expect for the getter spy to be called
+        expect(object.$value).to(beCalled())
+        expect(object.$value).to(beAnInstanceOf(Spy<Void, SomeProtocol>.self))
+
+        let otherObject = ObjectUsingDirectInstance()
+
+        expect(otherObject.$value).to(beAnInstanceOf(Spy<Void, ImplementedProtocol>.self))
+    }
+}
+
+protocol SomeProtocol {
+    var value: Int { get }
+}