Add URLHashDemo w/ window.onhashchange closure (#288)

* Add `URLHashDemo` w/ `window.onhashchange` closure

Resolves #284

* Assign `.undefined` in HashState.deinit

Sources/TokamakDemo/TokamakDemo.swift

@@ -144,6 +144,7 @@ struct TokamakDemoView: View {
           #if os(WASI)
           Section(header: Text("TokamakDOM")) {
             NavItem("DOM reference", destination: DOMRefDemo())
+            NavItem("URL hash changes", destination: URLHashDemo())
           }
           #endif
         }

Sources/TokamakDemo/URLHashDemo.swift

@@ -0,0 +1,52 @@
+// Copyright 2020 Tokamak contributors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+import JavaScriptKit
+import TokamakDOM
+
+private let location = JSObject.global.location.object!
+private let window = JSObject.global.window.object!
+
+private final class HashState: ObservableObject {
+  var onHashChange: JSClosure!
+
+  @Published var currentHash = location.hash.string!
+
+  init() {
+    let onHashChange = JSClosure { [weak self] _ in
+      self?.currentHash = location.hash.string!
+    }
+
+    window.onhashchange = .function(onHashChange)
+    self.onHashChange = onHashChange
+  }
+
+  deinit {
+    window.onhashchange = .undefined
+    onHashChange.release()
+  }
+}
+
+struct URLHashDemo: View {
+  @StateObject private var hashState = HashState()
+
+  var body: some View {
+    VStack {
+      Button("Assign random location.hash") {
+        location.hash = .string("\(Int.random(in: 0...1000))")
+      }
+      Text("Current location.hash is \(hashState.currentHash)")
+    }
+  }
+}