View/environment modifier, support for both EnvironmentValues and EnvironmentKey (used for custom storage in the environment)

Examples/Sources/GreetingGeneratorExample/GreetingGeneratorApp.swift

@@ -29,7 +29,8 @@ struct GreetingGeneratorApp: App {
 
                     Toggle("Selectable Greeting", active: $isGreetingSelectable)
                     if let latest = greetings.last {
-                        Text(latest)
+                        EnvironmentDisplay()
+                            .environment(key: TestKey.self, value: latest)
                             .padding(.top, 5)
                             .textSelectionEnabled(isGreetingSelectable)
 
@@ -51,3 +52,15 @@ struct GreetingGeneratorApp: App {
         }
     }
 }
+
+struct EnvironmentDisplay: View {
+    @Environment(TestKey.self) var value: String?
+    var body: some View {
+        Text(value ?? "nil")
+    }
+}
+
+struct TestKey: EnvironmentKey {
+    typealias Value = String?
+    static let defaultValue: Value = nil
+}

Sources/SwiftCrossUI/Environment/Environment.swift

@@ -36,21 +36,26 @@
 /// ```
 @propertyWrapper
 public struct Environment<Value>: DynamicProperty {
-    var keyPath: KeyPath<EnvironmentValues, Value>
+    var keyPath: KeyPath<EnvironmentValues, Value>?
+    var environmentKey: (any EnvironmentKey.Type)?
     var value: Box<Value?>
 
     public func update(
         with environment: EnvironmentValues,
         previousValue: Self?
     ) {
-        value.value = environment[keyPath: keyPath]
+        if let keyPath {
+            value.value = environment[keyPath: keyPath]
+        } else if let environmentKey {
+            value.value = (environment[environmentKey] as! Value)
+        }
     }
 
     public var wrappedValue: Value {
         guard let value = value.value else {
             fatalError(
                 """
-                Environment value \(keyPath) used before initialization. Don't \
+                Environment value \(keyPath.debugDescription) used before initialization. Don't \
                 use @Environment properties before SwiftCrossUI requests the \
                 view's body.
                 """
@@ -63,4 +68,9 @@ public struct Environment<Value>: DynamicProperty {
         self.keyPath = keyPath
         value = Box(value: nil)
     }
+
+    public init<Key: EnvironmentKey>(_ type: Key.Type) where Value == Key.Value {
+        self.environmentKey = type
+        self.value = Box(value: nil)
+    }
 }

Sources/SwiftCrossUI/Environment/EnvironmentValues.swift

@@ -222,6 +222,14 @@ public struct EnvironmentValues {
         environment[keyPath: keyPath] = newValue
         return environment
     }
+
+    /// Returns a copy of the environment with the specified key set to the
+    /// provided new value.
+    public func with<T: EnvironmentKey>(key: T.Type, value: T.Value) -> Self {
+        var environment = self
+        environment[key] = value
+        return environment
+    }
 }
 
 /// A key that can be used to extend the environment with new properties.

Sources/SwiftCrossUI/Views/Modifiers/EnvironmentModifier.swift

@@ -37,3 +37,21 @@ package struct EnvironmentModifier<Child: View>: View {
         )
     }
 }
+
+extension View {
+    /// Modifies the environment of the View its applied to.
+    public func environment<T: EnvironmentKey>(key: T.Type, value: T.Value) -> some View {
+        EnvironmentModifier(self) { environment in
+            environment.with(key: key, value: value)
+        }
+    }
+
+    /// Modifies the environment of the View its applied to
+    public func environment<T>(_ keyPath: WritableKeyPath<EnvironmentValues, T>, _ newValue: T)
+        -> some View
+    {
+        EnvironmentModifier(self) { environment in
+            environment.with(keyPath, newValue)
+        }
+    }
+}