Gets the DPI for all awareness mode and older Windows versions

shell/platform/windows/win32_dpi_helper.cc

@@ -4,6 +4,8 @@ namespace flutter {
 
 namespace {
 
+constexpr UINT kDefaultDpi = 96;
+
 template <typename T>
 bool AssignProcAddress(HMODULE comBaseModule, const char* name, T*& outProc) {
   outProc = reinterpret_cast<T*>(GetProcAddress(comBaseModule, name));
@@ -13,60 +15,57 @@ bool AssignProcAddress(HMODULE comBaseModule, const char* name, T*& outProc) {
 }  // namespace
 
 Win32DpiHelper::Win32DpiHelper() {
-  // TODO ensure that this helper works correctly on downlevel builds.
   user32_module_ = LoadLibraryA("User32.dll");
   if (user32_module_ == nullptr) {
     return;
   }
 
-  if (!AssignProcAddress(user32_module_, "EnableNonClientDpiScaling",
-                         enable_non_client_dpi_scaling_)) {
+  if (AssignProcAddress(user32_module_, "GetDpiForWindow",
+                        get_dpi_for_window_)) {
+    dpi_for_window_supported_ = true;
     return;
   }
 
-  if (!AssignProcAddress(user32_module_, "GetDpiForWindow",
-                         get_dpi_for_window_)) {
+  shlib_module_ = LoadLibraryA("Shcore.dll");
+  if (shlib_module_ == nullptr) {
     return;
   }
 
-  if (!AssignProcAddress(user32_module_, "SetProcessDpiAwarenessContext",
-                         set_process_dpi_awareness_context_)) {
-    return;
+  if (AssignProcAddress(shlib_module_, "GetDpiForMonitor",
+                        get_dpi_for_monitor_) &&
+      AssignProcAddress(user32_module_, "MonitorFromWindow",
+                        monitor_from_window_)) {
+    dpi_for_monitor_supported_ = true;
   }
-
-  permonitorv2_supported_ = true;
 }
 
 Win32DpiHelper::~Win32DpiHelper() {
   if (user32_module_ != nullptr) {
     FreeLibrary(user32_module_);
   }
-}
-
-bool Win32DpiHelper::IsPerMonitorV2Available() {
-  return permonitorv2_supported_;
-}
-
-BOOL Win32DpiHelper::EnableNonClientDpiScaling(HWND hwnd) {
-  if (!permonitorv2_supported_) {
-    return false;
+  if (shlib_module_ != nullptr) {
+    FreeLibrary(shlib_module_);
   }
-  return enable_non_client_dpi_scaling_(hwnd);
 }
 
-UINT Win32DpiHelper::GetDpiForWindow(HWND hwnd) {
-  if (!permonitorv2_supported_) {
-    return false;
+UINT Win32DpiHelper::GetDpi(HWND hwnd) {
+  // GetDpiForWindow returns the DPI for any awareness mode. If not available,
+  // fallback to a per monitor, system, or default DPI.
+  if (dpi_for_window_supported_) {
+    return get_dpi_for_window_(hwnd);
   }
-  return get_dpi_for_window_(hwnd);
-}
 
-BOOL Win32DpiHelper::SetProcessDpiAwarenessContext(
-    DPI_AWARENESS_CONTEXT context) {
-  if (!permonitorv2_supported_) {
-    return false;
+  if (dpi_for_monitor_supported_) {
+    HMONITOR monitor = monitor_from_window_(hwnd, MONITOR_DEFAULTTONEAREST);
+    UINT dpi_x = 0, dpi_y = 0;
+    HRESULT result =
+        get_dpi_for_monitor_(monitor, MDT_EFFECTIVE_DPI, &dpi_x, &dpi_y);
+    return SUCCEEDED(result) ? dpi_x : kDefaultDpi;
   }
-  return set_process_dpi_awareness_context_(context);
-}
 
+  HDC hdc = GetDC(hwnd);
+  UINT dpi = GetDeviceCaps(hdc, LOGPIXELSX);
+  ReleaseDC(hwnd, hdc);
+  return dpi;
+}
 }  // namespace flutter

shell/platform/windows/win32_dpi_helper.h

@@ -10,38 +10,34 @@
 
 namespace flutter {
 
-// A helper class for abstracting various Windows DPI related functions across
-// Windows OS versions.
+/// A helper class for abstracting various Windows DPI related functions across
+/// Windows OS versions.
 class Win32DpiHelper {
  public:
   Win32DpiHelper();
 
   ~Win32DpiHelper();
 
-  // Check if Windows Per Monitor V2 DPI scaling functionality is available on
-  // current system.
-  bool IsPerMonitorV2Available();
-
-  // Wrapper for OS functionality to turn on automatic window non-client scaling
-  BOOL EnableNonClientDpiScaling(HWND);
-
-  // Wrapper for OS functionality to return the DPI for |HWND|
-  UINT GetDpiForWindow(HWND);
-
-  // Sets the current process to a specified DPI awareness context.
-  BOOL SetProcessDpiAwarenessContext(DPI_AWARENESS_CONTEXT);
+  /// Returns the current DPI. Supports all DPI awareness modes, and is backward
+  /// compatible down to Windows Vista.
+  UINT GetDpi(HWND);
 
  private:
-  using EnableNonClientDpiScaling_ = BOOL __stdcall(HWND);
   using GetDpiForWindow_ = UINT __stdcall(HWND);
-  using SetProcessDpiAwarenessContext_ = BOOL __stdcall(DPI_AWARENESS_CONTEXT);
+  using GetDpiForMonitor_ = HRESULT __stdcall(HMONITOR hmonitor,
+                                              MONITOR_DPI_TYPE dpiType,
+                                              UINT* dpiX,
+                                              UINT* dpiY);
+  using MonitorFromWindow_ = HMONITOR __stdcall(HWND hwnd, DWORD dwFlags);
 
-  EnableNonClientDpiScaling_* enable_non_client_dpi_scaling_ = nullptr;
   GetDpiForWindow_* get_dpi_for_window_ = nullptr;
-  SetProcessDpiAwarenessContext_* set_process_dpi_awareness_context_ = nullptr;
+  GetDpiForMonitor_* get_dpi_for_monitor_ = nullptr;
+  MonitorFromWindow_* monitor_from_window_ = nullptr;
 
   HMODULE user32_module_ = nullptr;
-  bool permonitorv2_supported_ = false;
+  HMODULE shlib_module_ = nullptr;
+  bool dpi_for_window_supported_ = false;
+  bool dpi_for_monitor_supported_ = false;
 };
 
 }  // namespace flutter

shell/platform/windows/win32_window.cc

@@ -6,16 +6,8 @@
 
 namespace flutter {
 
-Win32Window::Win32Window() {
-  // Assume Windows 10 1703 or greater for DPI handling.  When running on a
-  // older release of Windows where this context doesn't exist, DPI calls will
-  // fail and Flutter rendering will be impacted until this is fixed.
-  // To handle downlevel correctly, dpi_helper must use the most recent DPI
-  // context available should be used: Windows 1703: Per-Monitor V2, 8.1:
-  // Per-Monitor V1, Windows 7: System See
-  // https://docs.microsoft.com/en-us/windows/win32/hidpi/high-dpi-desktop-application-development-on-windows
-  // for more information.
-}
+Win32Window::Win32Window() {}
+
 Win32Window::~Win32Window() {
   Destroy();
 }
@@ -26,7 +18,7 @@ void Win32Window::InitializeChild(const char* title,
   Destroy();
   std::wstring converted_title = NarrowToWide(title);
 
-  WNDCLASS window_class = ResgisterWindowClass(converted_title);
+  WNDCLASS window_class = RegisterWindowClass(converted_title);
 
   auto* result = CreateWindowEx(
       0, window_class.lpszClassName, converted_title.c_str(),
@@ -54,7 +46,7 @@ std::wstring Win32Window::NarrowToWide(const char* source) {
   return wideTitle;
 }
 
-WNDCLASS Win32Window::ResgisterWindowClass(std::wstring& title) {
+WNDCLASS Win32Window::RegisterWindowClass(std::wstring& title) {
   window_class_name_ = title;
 
   WNDCLASS window_class{};
@@ -82,14 +74,7 @@ LRESULT CALLBACK Win32Window::WndProc(HWND const window,
                      reinterpret_cast<LONG_PTR>(cs->lpCreateParams));
 
     auto that = static_cast<Win32Window*>(cs->lpCreateParams);
-
-    // Since the application is running in Per-monitor V2 mode, turn on
-    // automatic titlebar scaling
-    BOOL result = that->dpi_helper_->EnableNonClientDpiScaling(window);
-    if (result != TRUE) {
-      OutputDebugString(L"Failed to enable non-client area autoscaling");
-    }
-    that->current_dpi_ = that->dpi_helper_->GetDpiForWindow(window);
+    that->current_dpi_ = that->dpi_helper_->GetDpi(window);
     that->window_handle_ = window;
   } else if (Win32Window* that = GetThisFromHandle(window)) {
     return that->MessageHandler(window, message, wparam, lparam);
@@ -282,7 +267,7 @@ Win32Window::HandleDpiChange(HWND hwnd,
     // The DPI is only passed for DPI change messages on top level windows,
     // hence call function to get DPI if needed.
     if (uDpi == 0) {
-      uDpi = dpi_helper_->GetDpiForWindow(hwnd);
+      uDpi = dpi_helper_->GetDpi(hwnd);
     }
     current_dpi_ = uDpi;
     window->OnDpiScale(uDpi);

shell/platform/windows/win32_window.h

@@ -57,7 +57,7 @@ class Win32Window {
 
   // Registers a window class with default style attributes, cursor and
   // icon.
-  WNDCLASS ResgisterWindowClass(std::wstring& title);
+  WNDCLASS RegisterWindowClass(std::wstring& title);
 
   // OS callback called by message pump.  Handles the WM_NCCREATE message which
   // is passed when the non-client area is being created and enables automatic