Package@swift-5.9.swift

@@ -19,7 +19,7 @@ let package = Package(
     .library(
       name: "DependenciesMacros",
       targets: ["DependenciesMacros"]
-    )
+    ),
   ],
   dependencies: [
     .package(url: "https://github.com/apple/swift-syntax", from: "509.0.0"),
@@ -31,6 +31,12 @@ let package = Package(
     .package(url: "https://github.com/pointfreeco/xctest-dynamic-overlay", from: "1.0.0"),
   ],
   targets: [
+    .target(
+      name: "DependenciesTestObserver",
+      dependencies: [
+        .product(name: "XCTestDynamicOverlay", package: "xctest-dynamic-overlay"),
+      ]
+    ),
     .target(
       name: "Dependencies",
       dependencies: [
@@ -85,6 +91,16 @@ let package = Package(
   )
 #endif
 
+#if !os(macOS) && !os(WASI)
+package.products.append(
+  .library(
+    name: "DependenciesTestObserver",
+    type: .dynamic,
+    targets: ["DependenciesTestObserver"]
+  )
+)
+#endif
+
 //for target in package.targets {
 //  target.swiftSettings = target.swiftSettings ?? []
 //  target.swiftSettings?.append(

README.md

@@ -305,6 +305,6 @@ This library is released under the MIT license. See [LICENSE](LICENSE) for detai
 [scrumdinger]: https://developer.apple.com/tutorials/app-dev-training/getting-started-with-scrumdinger
 [syncups-demo]: https://github.com/pointfreeco/syncups
 [swiftui-nav-gh]: http://github.com/pointfreeco/swiftui-navigation
-[dep-values-docs]: https://swiftpackageindex.com/pointfreeco/swift-dependencies/main/documentation/dependencies/dependencyvalues
+[dep-values-docs]: https://swiftpackageindex.com/pointfreeco/swift-dependencies/main/documentation/dependencies/dependencyvalues#dependency-values
 [withdependencies-docs]: https://swiftpackageindex.com/pointfreeco/swift-dependencies/main/documentation/dependencies/withdependencies(_:operation:)-4uz6m
 [immediate-clock-docs]: https://pointfreeco.github.io/swift-clocks/main/documentation/clocks/immediateclock

Sources/Dependencies/Dependency.swift

@@ -97,6 +97,47 @@ public struct Dependency<Value>: @unchecked Sendable, _HasInitialValues {
     self.line = line
   }
 
+  /// Creates a dependency property to read a dependency object.
+  ///
+  /// Don't call this initializer directly. Instead, declare a property with the `Dependency`
+  /// property wrapper, and provide the dependency key of the value that the property should
+  /// reflect.
+  ///
+  /// For example, given a dependency key:
+  ///
+  /// ```swift
+  /// final class Settings: DependencyKey {
+  ///   static let liveValue = Settings()
+  ///
+  ///   // ...
+  /// }
+  /// ```
+  ///
+  /// One can access the dependency using this property wrapper:
+  ///
+  /// ```swift
+  /// final class FeatureModel: ObservableObject {
+  ///   @Dependency(Settings.self) var settings
+  ///
+  ///   // ...
+  /// }
+  /// ```
+  ///
+  /// - Parameter key: A dependency key to a specific resulting value.
+  public init<Key: TestDependencyKey>(
+    _ key: Key.Type,
+    file: StaticString = #file,
+    fileID: StaticString = #fileID,
+    line: UInt = #line
+  ) where Key.Value == Value {
+    self.init(
+      \DependencyValues.[HashableType<Key>(file: file, line: line)],
+      file: file,
+      fileID: fileID,
+      line: line
+    )
+  }
+
   /// The current value of the dependency property.
   public var wrappedValue: Value {
     #if DEBUG
@@ -119,6 +160,24 @@ public struct Dependency<Value>: @unchecked Sendable, _HasInitialValues {
   }
 }
 
+private struct HashableType<T>: Hashable {
+  let file: StaticString
+  let line: UInt
+  static func == (lhs: Self, rhs: Self) -> Bool {
+    true
+  }
+  func hash(into hasher: inout Hasher) {
+    hasher.combine(ObjectIdentifier(T.self))
+  }
+}
+
+extension DependencyValues {
+  fileprivate subscript<Key: TestDependencyKey>(key: HashableType<Key>) -> Key.Value {
+    get { self[Key.self, file: key.file, line: key.line] }
+    set { self[Key.self, file: key.file, line: key.line] = newValue }
+  }
+}
+
 protocol _HasInitialValues {
   var initialValues: DependencyValues { get }
 }

Sources/Dependencies/DependencyKey.swift

@@ -58,7 +58,7 @@
 /// implementation module.
 ///
 /// See the <doc:LivePreviewTest> article for more information.
-public protocol DependencyKey: TestDependencyKey {
+public protocol DependencyKey<Value>: TestDependencyKey {
   /// The live value for the dependency key.
   ///
   /// This is the value used by default when running the application in a simulator or on a device.
@@ -118,7 +118,7 @@ public protocol DependencyKey: TestDependencyKey {
 /// return a default value suitable for Xcode previews, or the ``testValue``, if left unimplemented.
 ///
 /// See ``DependencyKey`` to define a static, default value for the live application.
-public protocol TestDependencyKey {
+public protocol TestDependencyKey<Value> {
   /// The associated type representing the type of the dependency key's value.
   associatedtype Value: Sendable = Self
 
@@ -210,25 +210,30 @@ extension DependencyKey {
               \(typeName(Value.self))
           """
       )
-      let dependencyName =
+
+      let (argument, override) =
         DependencyValues.currentDependency.name
-        .map { "@Dependency(\\.\($0))" }
-        ?? "A dependency"
+        .map {
+          "\($0)" == "subscript(_:)"
+            ? ("@Dependency(\(typeName(Self.self)).self)", "'\(typeName(Self.self)).self'")
+            : ("@Dependency(\\.\($0))", "'\($0)'")
+        }
+        ?? ("A dependency", "the dependency")
+
       XCTFail(
         """
-        \(dependencyName) has no test implementation, but was accessed from a test context:
+        \(argument) has no test implementation, but was accessed from a test context:
 
         \(dependencyDescription)
 
         Dependencies registered with the library are not allowed to use their default, live \
         implementations when run from tests.
 
-        To fix, override \
-        \(DependencyValues.currentDependency.name.map { "'\($0)'" } ?? "the dependency") with a \
-        test value. If you are using the Composable Architecture, mutate the 'dependencies' \
-        property on your 'TestStore'. Otherwise, use 'withDependencies' to define a scope for the \
-        override. If you'd like to provide a default value for all tests, implement the \
-        'testValue' requirement of the 'DependencyKey' protocol.
+        To fix, override \(override) with a test value. If you are using the \
+        Composable Architecture, mutate the 'dependencies' property on your 'TestStore'. \
+        Otherwise, use 'withDependencies' to define a scope for the override. If you'd like to \
+        provide a default value for all tests, implement the 'testValue' requirement of the \
+        'DependencyKey' protocol.
         """
       )
     #endif