Enable UIA in Window

shell/platform/windows/BUILD.gn

@@ -122,7 +122,10 @@ source_set("flutter_windows_source") {
 
   public_configs = [ ":relative_angle_headers" ]
 
-  defines = [ "FLUTTER_ENGINE_NO_PROTOTYPES" ]
+  defines = [
+    "FLUTTER_ENGINE_NO_PROTOTYPES",
+    "FLUTTER_ENGINE_USE_UIA",
+  ]
 
   public_deps = [
     "//flutter/fml:string_conversion",
@@ -227,6 +230,8 @@ executable("flutter_windows_unittests") {
 
   public_configs = [ "//flutter:config" ]
 
+  defines = [ "FLUTTER_ENGINE_USE_UIA" ]
+
   deps = [
     ":flutter_windows_fixtures",
     ":flutter_windows_headers",

shell/platform/windows/testing/mock_window.h

@@ -67,6 +67,8 @@ class MockWindow : public Window {
 
   MOCK_METHOD0(GetAxFragmentRootDelegate, ui::AXFragmentRootDelegateWin*());
 
+  MOCK_METHOD3(OnGetObject, LRESULT(UINT, WPARAM, LPARAM));
+
   void CallOnImeComposition(UINT const message,
                             WPARAM const wparam,
                             LPARAM const lparam);

shell/platform/windows/window.h

@@ -141,9 +141,9 @@ class Window : public KeyboardManager::WindowDelegate {
   //
   // The primary use of this function is to supply Windows with wrapped
   // semantics objects for use by Windows accessibility.
-  LRESULT OnGetObject(UINT const message,
-                      WPARAM const wparam,
-                      LPARAM const lparam);
+  virtual LRESULT OnGetObject(UINT const message,
+                              WPARAM const wparam,
+                              LPARAM const lparam);
 
   // Called when IME composing begins.
   virtual void OnComposeBegin() = 0;

shell/platform/windows/window_unittests.cc

@@ -360,5 +360,23 @@ TEST(MockWindow, UnknownPointerTypeSkipsDirectManipulation) {
   window.InjectWindowMessage(DM_POINTERHITTEST, MAKEWPARAM(pointer_id, 0), 0);
 }
 
+// Test that the root UIA object is queried by WM_GETOBJECT.
+TEST(MockWindow, GetObjectUia) {
+  MockWindow window;
+  bool uia_called = false;
+  ON_CALL(window, OnGetObject)
+      .WillByDefault(Invoke([&uia_called](UINT msg, WPARAM wpar, LPARAM lpar) {
+#ifdef FLUTTER_ENGINE_USE_UIA
+        uia_called = true;
+#endif  // FLUTTER_ENGINE_USE_UIA
+        return static_cast<LRESULT>(0);
+      }));
+  EXPECT_CALL(window, OnGetObject).Times(1);
+
+  window.InjectWindowMessage(WM_GETOBJECT, 0, UiaRootObjectId);
+
+  EXPECT_TRUE(uia_called);
+}
+
 }  // namespace testing
 }  // namespace flutter