ObjectEdge

Sources/SwiftUISupport/ObjectEdge.swift

@@ -0,0 +1,117 @@
+import SwiftUI
+
+/**
+ https://forums.developer.apple.com/forums/thread/739163
+ */
+@propertyWrapper
+public struct ObjectEdge<O>: DynamicProperty {
+
+  @State private var box: Box<O> = .init()
+
+  public var wrappedValue: O {
+    if let value = box.value {
+      return value
+    } else {
+      box.value = factory()
+      return box.value!
+    }
+  }
+
+  private let factory: () -> O
+
+  public init(wrappedValue factory: @escaping @autoclosure () -> O) {
+    self.factory = factory
+  }
+
+  private final class Box<Value> {
+    var value: Value?
+  }
+
+}
+
+#if DEBUG
+
+@available(iOS 17, *)
+@Observable
+private final class Model {
+
+  var count: Int = 0
+
+  init(count: Int) {
+    print("Init model")
+    self.count = count    
+  }
+
+  func up() {
+    count += 1
+  }
+}
+
+@available(iOS 17, *)
+private struct Demo: View {
+
+  @ObjectEdge var model: Model = .init(count: 0)
+
+  var body: some View {
+
+    VStack {
+      Text("\(model.count)")
+      Button("Up") {
+        model.up()
+      }
+    }
+  }
+}
+
+@available(iOS 17, *)#Preview{
+  Demo()
+}
+
+struct Box<T> {
+
+  var value: T
+
+  init(_ value: T) {
+    print("Init box")
+    self.value = value
+  }
+}
+
+#Preview("State init value") {
+
+  struct StateInit<Nested: View>: View {
+
+    @State var value: Box<Int> = .init(0)
+
+    private let nested: () -> Nested
+
+    init(@ViewBuilder nested: @escaping () -> Nested) {
+      self.nested = nested
+    }
+
+    var body: some View {
+      VStack {
+        Text(value.value.description)
+        Button("Up") {
+          value.value += 1
+        }
+        nested()
+      }
+      .padding()
+      .background(
+        RoundedRectangle(cornerRadius: 8)
+          .fill(Color.yellow.opacity(0.2))
+      )
+    }
+  }
+
+  return StateInit {
+    StateInit {
+      StateInit {
+        EmptyView()
+      }
+    }
+  }
+}
+
+#endif