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)
+                        LatestGreetingDisplay()
+                            .environment(\.latestGreeting, latest)
                             .padding(.top, 5)
                             .textSelectionEnabled(isGreetingSelectable)
 
@@ -51,3 +52,24 @@ struct GreetingGeneratorApp: App {
         }
     }
 }
+
+/// This intermediate view exists to show the usage of custom environment keys. In reality it is not necessary.
+struct LatestGreetingDisplay: View {
+    @Environment(\.latestGreeting) var value: String?
+
+    var body: some View {
+        Text(value ?? "nil")
+    }
+}
+
+struct LatestGreetingKey: EnvironmentKey {
+    typealias Value = String?
+    static let defaultValue: Value = nil
+}
+
+extension EnvironmentValues {
+    var latestGreeting: String? {
+        get { self[LatestGreetingKey.self] }
+        set { self[LatestGreetingKey.self] = newValue }
+    }
+}

Sources/SwiftCrossUI/Environment/Environment.swift

@@ -50,7 +50,7 @@ public struct Environment<Value>: DynamicProperty {
         guard let value = value.value else {
             fatalError(
                 """
-                Environment value \(keyPath) used before initialization. Don't \
+                Environment value at \(keyPath) used before initialization. Don't \
                 use @Environment properties before SwiftCrossUI requests the \
                 view's body.
                 """

Sources/SwiftCrossUI/Environment/EnvironmentValues.swift

@@ -239,7 +239,7 @@ public struct EnvironmentValues {
 }
 
 /// A key that can be used to extend the environment with new properties.
-public protocol EnvironmentKey {
+public protocol EnvironmentKey<Value> {
     /// The type of value the key can hold.
     associatedtype Value
     /// The default value for the key.

Sources/SwiftCrossUI/Views/Modifiers/EnvironmentModifier.swift

@@ -51,3 +51,14 @@ package struct EnvironmentModifier<Child: View>: View {
         )
     }
 }
+
+extension View {
+    /// 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)
+        }
+    }
+}