Add support for SwiftUI environment (pointfreeco#13)

* Add support for SwiftUI environment

* Update ExampleApp.swift

* Update ExampleApp.swift

* Update Environment.swift

* Update Environment.swift

* Update Environment.swift

* wip

* README

Author: stephencelis

README.md

@@ -69,6 +69,35 @@ set up correctly:
 > 🟣 Runtime Warning: Perceptible state was accessed but is not being tracked. Track changes to
 > state by wrapping your view in a 'WithPerceptionTracking' view.
 
+### Bindable
+
+SwiftUI's `@Bindable` property wrapper has also been backported to support perceptible objects. You
+can simply qualify the property wrapper with the `Perception` module:
+
+```swift
+struct FeatureView: View {
+  @Perception.Bindable var model: FeatureModel
+
+  // ...
+}
+```
+
+### Environment
+
+SwiftUI's `@Environment` property wrapper and `environment` view modifier's support for observation
+has also been backported to support perceptible objects using the exact same APIs:
+
+```swift
+struct FeatureView: View {
+  @Environment(Settings.self) var settings
+
+  // ...
+}
+
+// In some parent view:
+.environment(settings)
+```
+
 ## Community
 
 If you want to discuss this library or have a question about how to use it to solve

Sources/Perception/Environment.swift

@@ -0,0 +1,74 @@
+import SwiftUI
+
+@available(iOS, introduced: 13, obsoleted: 17)
+@available(macOS, introduced: 10.15, obsoleted: 14)
+@available(tvOS, introduced: 13, obsoleted: 17)
+@available(watchOS, introduced: 6, obsoleted: 10)
+@available(visionOS, unavailable)
+extension Environment {
+  /// Creates an environment property to read a perceptible object from the environment.
+  ///
+  /// A backport of SwiftUI's `Environment.init` that takes an observable object.
+  ///
+  /// - Parameter objectType: The type of the `Perceptible` object to read from the environment.
+  @_disfavoredOverload
+  public init(_ objectType: Value.Type) where Value: AnyObject & Perceptible {
+    self.init(\.[unwrap: \Value.self])
+  }
+
+  /// Creates an environment property to read a perceptible object from the environment, returning
+  /// `nil` if no corresponding object has been set in the current view's environment.
+  ///
+  /// A backport of SwiftUI's `Environment.init` that takes an observable object.
+  ///
+  /// - Parameter objectType: The type of the `Perceptible` object to read from the environment.
+  @_disfavoredOverload
+  public init<T: AnyObject & Perceptible>(_ objectType: T.Type) where Value == T? {
+    self.init(\.[\T.self])
+  }
+}
+
+@available(iOS, introduced: 13, obsoleted: 17)
+@available(macOS, introduced: 10.15, obsoleted: 14)
+@available(tvOS, introduced: 13, obsoleted: 17)
+@available(watchOS, introduced: 6, obsoleted: 10)
+@available(visionOS, unavailable)
+extension View {
+  /// Places a perceptible object in the view’s environment.
+  ///
+  /// A backport of SwiftUI's `View.environment` that takes an observable object.
+  ///
+  /// - Parameter object: The object to set for this object's type in the environment, or `nil` to
+  ///   clear an object of this type from the environment.
+  /// - Returns: A view that has the specified object in its environment.
+  @_disfavoredOverload
+  public func environment<T: AnyObject & Perceptible>(_ object: T?) -> some View {
+    self.environment(\.[\T.self], object)
+  }
+}
+
+private struct PerceptibleKey<T: Perceptible>: EnvironmentKey {
+  static var defaultValue: T? { nil }
+}
+
+extension EnvironmentValues {
+  fileprivate subscript<T: Perceptible>(_: KeyPath<T, T>) -> T? {
+    get { self[PerceptibleKey<T>.self] }
+    set { self[PerceptibleKey<T>.self] = newValue }
+  }
+
+  fileprivate subscript<T: Perceptible>(unwrap _: KeyPath<T, T>) -> T {
+    get {
+      guard let object = self[\T.self] else {
+        fatalError(
+          """
+          No perceptible object of type \(T.self) found. A View.environment(_:) for \(T.self) may \
+          be missing as an ancestor of this view.
+          """
+        )
+      }
+      return object
+    }
+    set { self[\T.self] = newValue }
+  }
+}