Windows: Reliably handle all COM init scenarios on the UI thread

platforms/windows/examples/hello_world.rs

@@ -9,7 +9,6 @@ use windows::{
     Win32::{
         Foundation::*,
         Graphics::Gdi::ValidateRect,
-        System::Com::*,
         System::LibraryLoader::GetModuleHandleW,
         UI::{Input::KeyboardAndMouse::*, WindowsAndMessaging::*},
     },
@@ -229,18 +228,6 @@ fn create_window(title: &str, initial_state: TreeUpdate, initial_focus: NodeId)
 }
 
 fn main() -> Result<()> {
-    // There isn't a single correct answer to the question of how, or whether,
-    // to initialize COM. It's not clear whether this should even matter
-    // for a UI Automation provider that, like AccessKit, doesn't use
-    // COM threading. However, as discussed in #37, it apparently does matter,
-    // at least on some machines. If a program depends on legacy code
-    // that requires a single-threaded apartment (STA), then it should
-    // initialize COM that way. But that's not the case for AccessKit's
-    // examples and tests, and the multi-threaded apartment (MTA)
-    // is apparently more reliable for our use case, so we choose that.
-    unsafe { CoInitializeEx(std::ptr::null_mut(), COINIT_MULTITHREADED) }?;
-    let _com_guard = scopeguard::guard((), |_| unsafe { CoUninitialize() });
-
     let window = create_window(WINDOW_TITLE, get_initial_state(), INITIAL_FOCUS)?;
     unsafe { ShowWindow(window, SW_SHOW) };
 

platforms/windows/src/manager.rs

@@ -18,6 +18,14 @@ pub struct Manager {
 
 impl Manager {
     pub fn new(hwnd: HWND, initial_state: TreeUpdate) -> Self {
+        // It's unfortunate that we have to force UIA to initialize early;
+        // it would be more optimal to let UIA lazily initialize itself
+        // when we receive the first `WM_GETOBJECT`. But if we don't do this,
+        // then on a thread that's using a COM STA, we can get a race condition
+        // that leads to nested WM_GETOBJECT messages and, in some cases,
+        // ATs not realizing that our window natively implements UIA. See #37.
+        force_init_uia();
+
         Self {
             hwnd,
             tree: Tree::new(initial_state),
@@ -58,3 +66,13 @@ impl Manager {
         unsafe { UiaReturnRawElementProvider(self.hwnd, wparam, lparam, el) }
     }
 }
+
+fn force_init_uia() {
+    // `UiaLookupId` is a cheap way of forcing UIA to initialize itself.
+    unsafe {
+        UiaLookupId(
+            AutomationIdentifierType_Property,
+            &ControlType_Property_GUID,
+        )
+    };
+}

platforms/windows/src/tests/mod.rs

@@ -191,19 +191,17 @@ where
 {
     let _lock_guard = MUTEX.lock();
 
-    unsafe { CoInitializeEx(std::ptr::null_mut(), COINIT_MULTITHREADED) }.unwrap();
-    let _com_guard = scopeguard::guard((), |_| unsafe { CoUninitialize() });
-
-    let uia: IUIAutomation =
-        unsafe { CoCreateInstance(&CUIAutomation8, None, CLSCTX_INPROC_SERVER) }?;
-
     let window_mutex: Mutex<Option<HWND>> = Mutex::new(None);
     let window_cv = Condvar::new();
 
     crossbeam_utils::thread::scope(|thread_scope| {
         thread_scope.spawn(|_| {
-            unsafe { CoInitializeEx(std::ptr::null_mut(), COINIT_MULTITHREADED) }.unwrap();
-            let _com_guard = scopeguard::guard((), |_| unsafe { CoUninitialize() });
+            // We explicitly don't want to initialize COM on the provider thread,
+            // because we want to make sure that the provider side of UIA works
+            // even if COM is never initialized on the provider thread
+            // (as is the case if the window is never shown), or if COM is
+            // initialized after the window is shown (as is the case,
+            // at least on some Windows 10 machines, due to IME support).
 
             let window = create_window(window_title, initial_state, initial_focus).unwrap();
 
@@ -232,6 +230,24 @@ where
             unsafe { PostMessageW(window, WM_CLOSE, WPARAM(0), LPARAM(0)) }.unwrap()
         });
 
+        // We must initialize COM before creating the UIA client. The MTA option
+        // is cleaner by far, especially when we want to wait for a UIA event
+        // handler to be called, and there's no reason not to use it here.
+        // Note that we don't initialize COM this way on the provider thread,
+        // as explained above. It's also important that we let the provider
+        // thread do its forced initialization of UIA, in an environment
+        // where COM has not been initialized, before we create the UIA client,
+        // which also triggers UIA initialization, in a thread where COM
+        // _has_ been initialized. This way, we ensure that the provider side
+        // of UIA works even if it is set up in an environment where COM
+        // has not been initialized, and that this sequence of events
+        // doesn't prevent the UIA client from working.
+        unsafe { CoInitializeEx(std::ptr::null_mut(), COINIT_MULTITHREADED) }.unwrap();
+        let _com_guard = scopeguard::guard((), |_| unsafe { CoUninitialize() });
+
+        let uia: IUIAutomation =
+            unsafe { CoCreateInstance(&CUIAutomation8, None, CLSCTX_INPROC_SERVER) }?;
+
         let s = Scope { uia, window };
         f(&s)
     })