Add support for @Dependency(MyDependency.self)

Sources/Dependencies/Dependency.swift

@@ -97,6 +97,42 @@ 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.[\Key.self], file: file, fileID: fileID, line: line)
+  }
+
   /// The current value of the dependency property.
   public var wrappedValue: Value {
     #if DEBUG
@@ -119,6 +155,13 @@ public struct Dependency<Value>: @unchecked Sendable, _HasInitialValues {
   }
 }
 
+fileprivate extension DependencyValues {
+  subscript<Key: TestDependencyKey>(_: KeyPath<Key, Key>) -> Key.Value {
+    get { self[Key.self] }
+    set { self[Key.self] = 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

Sources/Dependencies/DependencyValues.swift

@@ -100,6 +100,12 @@ public struct DependencyValues: Sendable {
     #endif
   }
 
+  @_disfavoredOverload
+  public subscript<Key: TestDependencyKey>(type: Key.Type) -> Key.Value {
+    get { self[type] }
+    set { self[type] = newValue }
+  }
+
   /// Accesses the dependency value associated with a custom key.
   ///
   /// This subscript is typically only used when adding a computed property to ``DependencyValues``
@@ -319,17 +325,21 @@ private final class CachedValues: @unchecked Sendable {
                 """
             )
 
+            var argument: String {
+              "\(function)" == "subscript(_:)" ? "\(typeName(Key.self)).self" : "\\.\(function)"
+            }
+
             runtimeWarn(
               """
-              "@Dependency(\\.\(function))" has no live implementation, but was accessed from a \
-              live context.
+              @Dependency(\(argument)) has no live implementation, but was accessed from a live \
+              context.
 
               \(dependencyDescription)
 
-              Every dependency registered with the library must conform to "DependencyKey", and \
+              Every dependency registered with the library must conform to 'DependencyKey', and \
               that conformance must be visible to the running application.
 
-              To fix, make sure that "\(typeName(Key.self))" conforms to "DependencyKey" by \
+              To fix, make sure that '\(typeName(Key.self))' conforms to 'DependencyKey' by \
               providing a live implementation of your dependency, and make sure that the \
               conformance is linked with this current application.
               """,
@@ -338,7 +348,11 @@ private final class CachedValues: @unchecked Sendable {
             )
           }
         #endif
-        return Key.testValue
+        let value = Key.testValue
+        if !DependencyValues.isSetting {
+          self.cached[cacheKey] = AnySendable(value)
+        }
+        return value
       }
 
       self.cached[cacheKey] = AnySendable(value)

Tests/DependenciesTests/DependencyKeyTests.swift

@@ -154,6 +154,41 @@ final class DependencyKeyTests: XCTestCase {
       }
     #endif
   }
+
+  func testDependencyKeyCascading_ImplementOnlyLive_NamedType() {
+    #if DEBUG && !os(Linux) && !os(WASI) && !os(Windows)
+      withDependencies {
+        $0.context = .test
+      } operation: {
+        @Dependency(LiveKey.self) var missingTestDependency: Int
+        let line = #line - 1
+        XCTExpectFailure {
+          XCTAssertEqual(42, missingTestDependency)
+        } issueMatcher: { issue in
+          issue.compactDescription == """
+            @Dependency(LiveKey.self) has no test implementation, but was accessed from a test \
+            context:
+
+              Location:
+                DependenciesTests/DependencyKeyTests.swift:\(line)
+              Key:
+                LiveKey
+              Value:
+                Int
+
+            Dependencies registered with the library are not allowed to use their default, live \
+            implementations when run from tests.
+
+            To fix, override 'LiveKey.self' 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
+  }
 }
 
 private enum LiveKey: DependencyKey {

Tests/DependenciesTests/DependencyTests.swift

@@ -206,6 +206,21 @@ final class DependencyTests: XCTestCase {
     XCTAssertEqual(user1.id, UUID(0))
     XCTAssertEqual(user2.id, UUID(1))
   }
+
+  func testDependencyType() {
+    struct MyDependency: TestDependencyKey {
+      static var testValue: Int { 0 }
+    }
+    @Dependency(MyDependency.self) var int
+
+    XCTAssertEqual(int, 0)
+
+    withDependencies {
+      $0[MyDependency.self] = 42
+    } operation: {
+      XCTAssertEqual(int, 42)
+    }
+  }
 }
 
 private class Model {

Tests/DependenciesTests/DependencyValuesTests.swift

@@ -27,7 +27,7 @@ final class DependencyValuesTests: XCTestCase {
         }
       } issueMatcher: {
         $0.compactDescription == """
-          "@Dependency(\\.missingLiveDependency)" has no live implementation, but was accessed \
+          @Dependency(\\.missingLiveDependency) has no live implementation, but was accessed \
           from a live context.
 
             Location:
@@ -37,10 +37,44 @@ final class DependencyValuesTests: XCTestCase {
             Value:
               Int
 
-          Every dependency registered with the library must conform to "DependencyKey", and that \
+          Every dependency registered with the library must conform to 'DependencyKey', and that \
           conformance must be visible to the running application.
 
-          To fix, make sure that "TestKey" conforms to "DependencyKey" by providing a live \
+          To fix, make sure that 'TestKey' conforms to 'DependencyKey' by providing a live \
+          implementation of your dependency, and make sure that the conformance is linked with \
+          this current application.
+          """
+      }
+    #endif
+  }
+
+  func testMissingLiveValue_Type() {
+    #if DEBUG && !os(Linux) && !os(WASI) && !os(Windows)
+      var line = 0
+      XCTExpectFailure {
+        withDependencies {
+          $0.context = .live
+        } operation: {
+          line = #line + 1
+          @Dependency(TestKey.self) var missingLiveDependency: Int
+          _ = missingLiveDependency
+        }
+      } issueMatcher: {
+        $0.compactDescription == """
+          @Dependency(TestKey.self) has no live implementation, but was accessed from a live \
+          context.
+
+            Location:
+              DependenciesTests/DependencyValuesTests.swift:\(line)
+            Key:
+              TestKey
+            Value:
+              Int
+
+          Every dependency registered with the library must conform to 'DependencyKey', and that \
+          conformance must be visible to the running application.
+
+          To fix, make sure that 'TestKey' conforms to 'DependencyKey' by providing a live \
           implementation of your dependency, and make sure that the conformance is linked with \
           this current application.
           """
@@ -169,19 +203,15 @@ final class DependencyValuesTests: XCTestCase {
               XCTExpectFailure {
                 $0.compactDescription.contains(
                   """
-                  @Dependency(\\.reuseClient)" has no live implementation, but was accessed from a \
+                  @Dependency(\\.reuseClient) has no live implementation, but was accessed from a \
                   live context.
                   """
                 )
               }
             #endif
             XCTAssertEqual(reuseClient.count(), 0)
             reuseClient.setCount(-42)
-            XCTAssertEqual(
-              reuseClient.count(),
-              0,
-              "Don't cache dependency when using a test value in a live context"
-            )
+            XCTAssertEqual(reuseClient.count(), -42)
           }
 
           XCTAssertEqual(reuseClient.count(), 42)
@@ -205,8 +235,8 @@ final class DependencyValuesTests: XCTestCase {
             XCTExpectFailure {
               $0.compactDescription.contains(
                 """
-                @Dependency(\\.reuseClient)" has no live implementation, but was accessed from a live \
-                context.
+                @Dependency(\\.reuseClient) has no live implementation, but was accessed from a \
+                live context.
                 """
               )
             }