Merge pull request #137 from Unity-Technologies/fix_window_instance_null_crash

Fix window instance null crash

README-ZH.md

 
 如果想在runtime选择是否debug，请修改文件```com.unity.uiwidgets/com.unity.uiwidgets/Runtime/foundation/debug.cs```中的```static bool debugEnableAtRuntimeInternal```
 
+## 使用Window Scope保护外部调用
+如果您在调试时遇到 `AssertionError: Window.instance is null` 或者在调用 `Window.instance` 时得到空指针, 那么您需要
+使用以下方式来保护您的调用，使之可以在正确的Isolate上执行回调逻辑：
+```csharp
+using(Isolate.getScope(the isolate of your App)) {
+    // code dealing with UIWidgets,
+    // e.g. setState(() => {....})
+}
+```
+
+通常，在您使用外部事件，例如来自UIWidgets之外的输入事件、网络传输回调事件时需要做这样的处理。具体的您可以参考我们的 HttpRequestSample 这个样例中的写法。需要注意的是，一般情况下您在UIWidgets框架的内部调用 (如 `Widget.build, State.initState...`)中不需要额外采取上述保护措施。
+
 ## 学习
 
 #### 教程

README.md

 
 If you want to change different mode in runtime, please modify the file “com.unity.uiwidgets/com.unity.uiwidgets/Runtime/foundation/debug.cs” by making “static bool debugEnableAtRuntimeInternal” to true or false. Note that the value is set to false by default.
 
+## Using Window Scope
+If you see the error `AssertionError: Window.instance is null` or null pointer error of `Window.instance`,
+it means the code is not running in the window scope. In this case, you can enclose your code
+with window scope as below:
+```csharp
+using(Isolate.getScope(the isolate of your App)) {
+    // code dealing with UIWidgets,
+    // e.g. setState(() => {....})
+}
+```
+
+This is needed if the code is in methods
+not invoked by UIWidgets. For example, if the code is in `completed` callback of `UnityWebRequest`,
+you need to enclose them with window scope.
+Please see our HttpRequestSample for detail.
+For callback/event handler methods from UIWidgets (e.g `Widget.build, State.initState...`), you don't need do
+it yourself, since the framework ensure it's in window scope.
+
 ## Learn
 
 #### Samples

com.unity.uiwidgets/Runtime/ui/window.cs

 
         public static Window instance {
             get {
-                GCHandle gcHandle = (GCHandle) Window_instance();
+                IntPtr ptr = Window_instance();
+                if (ptr == IntPtr.Zero) {
+                    D.assert(false, () => "AssertionError: Window.instance is null. Please enclose your code with window scope (detailed examples can be found in the README file)");
+                    return null;
+                }
+                
+                GCHandle gcHandle = (GCHandle) ptr;
                 return (Window) gcHandle.Target;
             }
         }

engine/src/lib/ui/ui_mono_state.cc

   return static_cast<UIMonoState*>(MonoState::Current());
 }
 
+bool UIMonoState::EnsureCurrentIsolate() {
+  return MonoState::EnsureCurrentIsolate();
+}
+
 void UIMonoState::SetWindow(std::unique_ptr<Window> window) {
   window_ = std::move(window);
 }

engine/src/lib/ui/ui_mono_state.h

  public:
   static UIMonoState* Current();
 
+  static bool EnsureCurrentIsolate();
+
   Window* window() const { return window_.get(); }
 
   const TaskRunners& GetTaskRunners() const;

engine/src/lib/ui/window/window.cc

 }
 
 UIWIDGETS_API(Mono_Handle) Window_instance() {
+  if (!UIMonoState::EnsureCurrentIsolate()) {
+    return nullptr;
+  }
   return UIMonoState::Current()->window()->mono_window();
 }
 

engine/src/runtime/mono_state.cc

   return isolate_data->get();
 }
 
+bool MonoState::EnsureCurrentIsolate() {
+  return Mono_CurrentIsolate() != nullptr;
+}
+
 MonoState* MonoState::Current() {
   auto isolate_data =
       static_cast<std::shared_ptr<MonoState>*>(Mono_CurrentIsolateData());

engine/src/runtime/mono_state.h

 
   static MonoState* From(Mono_Isolate isolate);
   static MonoState* Current();
+  static bool EnsureCurrentIsolate();
 
   std::weak_ptr<MonoState> GetWeakPtr();