Add state playgrounds (State, Environment, Observable)

Author: colemancda

Demo/App.swiftpm/Sources/StatePlaygrounds.swift

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+#if canImport(AndroidSwiftUI)
+import AndroidSwiftUI
+#else
+import SwiftUI
+#endif
+
+import Observation
+
+struct StatePlayground: View {
+    @State private var parentRenders = 0
+    var body: some View {
+        ScrollView {
+            VStack(alignment: .leading, spacing: 0) {
+                Example("Parent re-renders: \(parentRenders)") {
+                    Button("Re-render parent") { parentRenders += 1 }
+                }
+                Example("Child keeps its own state") {
+                    StateChild()
+                }
+            }
+        }
+    }
+}
+
+struct StateChild: View {
+    @State private var localCount = 0
+    var body: some View {
+        VStack(alignment: .leading, spacing: 8) {
+            Text("Local count: \(localCount)")
+            Button("Bump child") { localCount += 1 }
+            Text("Survives parent re-renders")
+        }
+    }
+}
+
+@Observable
+final class CounterModel {
+    var value = 0
+}
+
+struct EnvironmentPlayground: View {
+    @State private var model = CounterModel()
+    var body: some View {
+        ScrollView {
+            VStack(alignment: .leading, spacing: 0) {
+                Example("Injected object") {
+                    EnvReader().environment(model)
+                }
+                Example("Read here too") {
+                    EnvReader().environment(model)
+                }
+            }
+        }
+    }
+}
+
+struct EnvReader: View {
+    @Environment(CounterModel.self) private var model
+    var body: some View {
+        VStack(alignment: .leading, spacing: 8) {
+            Text("Shared value: \(model.value)")
+            Button("Increment shared") { model.value += 1 }
+        }
+    }
+}
+
+struct ObservablePlayground: View {
+    @State private var model = CounterModel()
+    var body: some View {
+        ScrollView {
+            VStack(alignment: .leading, spacing: 0) {
+                Example("Observation drives updates") {
+                    VStack(alignment: .leading, spacing: 8) {
+                        Text("Observable value: \(model.value)")
+                        Button("Increment") { model.value += 1 }
+                        Text("Mutating the model re-evaluates automatically")
+                    }
+                }
+            }
+        }
+    }
+}