Expose DPI helper functions for Runner apps to use

ci/licenses_golden/licenses_flutter

@@ -1108,6 +1108,8 @@ FILE: ../../../flutter/shell/platform/windows/client_wrapper/include/flutter/flu
 FILE: ../../../flutter/shell/platform/windows/client_wrapper/include/flutter/flutter_view_controller.h
 FILE: ../../../flutter/shell/platform/windows/client_wrapper/include/flutter/plugin_registrar_windows.h
 FILE: ../../../flutter/shell/platform/windows/client_wrapper/plugin_registrar_windows_unittests.cc
+FILE: ../../../flutter/shell/platform/windows/dpi_utils.h
+FILE: ../../../flutter/shell/platform/windows/dpi_utils.cc
 FILE: ../../../flutter/shell/platform/windows/flutter_windows.cc
 FILE: ../../../flutter/shell/platform/windows/key_event_handler.cc
 FILE: ../../../flutter/shell/platform/windows/key_event_handler.h
@@ -1117,8 +1119,6 @@ FILE: ../../../flutter/shell/platform/windows/platform_handler.h
 FILE: ../../../flutter/shell/platform/windows/public/flutter_windows.h
 FILE: ../../../flutter/shell/platform/windows/text_input_plugin.cc
 FILE: ../../../flutter/shell/platform/windows/text_input_plugin.h
-FILE: ../../../flutter/shell/platform/windows/win32_dpi_helper.cc
-FILE: ../../../flutter/shell/platform/windows/win32_dpi_helper.h
 FILE: ../../../flutter/shell/platform/windows/win32_flutter_window.cc
 FILE: ../../../flutter/shell/platform/windows/win32_flutter_window.h
 FILE: ../../../flutter/shell/platform/windows/win32_task_runner.cc

shell/platform/windows/BUILD.gn

@@ -43,6 +43,8 @@ source_set("flutter_windows_source") {
   sources = [
     "angle_surface_manager.cc",
     "angle_surface_manager.h",
+    "dpi_utils.h",
+    "dpi_utils.cc",
     "flutter_windows.cc",
     "key_event_handler.cc",
     "key_event_handler.h",
@@ -51,8 +53,6 @@ source_set("flutter_windows_source") {
     "platform_handler.h",
     "text_input_plugin.cc",
     "text_input_plugin.h",
-    "win32_dpi_helper.cc",
-    "win32_dpi_helper.h",
     "win32_flutter_window.cc",
     "win32_flutter_window.h",
     "win32_task_runner.cc",

shell/platform/windows/dpi_utils.cc

@@ -0,0 +1,124 @@
+#include "dpi_utils.h"
+
+#include <ShellScalingApi.h>
+
+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));
+  return *outProc != nullptr;
+}
+
+}  // namespace
+
+/// A helper class for abstracting various Windows DPI related functions across
+/// Windows OS versions.
+class Win32DpiHelper {
+ public:
+  Win32DpiHelper();
+
+  ~Win32DpiHelper();
+
+  /// Returns the current DPI. Supports all DPI awareness modes, and is backward
+  /// compatible down to Windows Vista. If |hwnd| is nullptr, returns the DPI
+  /// for the nearest monitor is available. Otherwise, returns the system's DPI.
+  UINT GetDpi(HWND);
+
+  /// Enables scaling of non-client UI (scrolling bars, title bars, etc). Only
+  /// supported on Per-Monitor V1 DPI awareness mode.
+  BOOL EnableNonClientDpiScaling(HWND hwnd);
+
+ private:
+  using GetDpiForWindow_ = UINT __stdcall(HWND);
+  using GetDpiForMonitor_ = HRESULT __stdcall(HMONITOR hmonitor,
+                                              MONITOR_DPI_TYPE dpiType,
+                                              UINT* dpiX,
+                                              UINT* dpiY);
+  using MonitorFromWindow_ = HMONITOR __stdcall(HWND hwnd, DWORD dwFlags);
+  using EnableNonClientDpiScaling_ = BOOL __stdcall(HWND hwnd);
+
+  GetDpiForWindow_* get_dpi_for_window_ = nullptr;
+  GetDpiForMonitor_* get_dpi_for_monitor_ = nullptr;
+  MonitorFromWindow_* monitor_from_window_ = nullptr;
+  EnableNonClientDpiScaling_* enable_non_client_dpi_scaling_ = nullptr;
+
+  HMODULE user32_module_ = nullptr;
+  HMODULE shlib_module_ = nullptr;
+  bool dpi_for_window_supported_ = false;
+  bool dpi_for_monitor_supported_ = false;
+};
+
+Win32DpiHelper::Win32DpiHelper() {
+  user32_module_ = LoadLibraryA("User32.dll");
+  shlib_module_ = LoadLibraryA("Shcore.dll");
+  if (shlib_module_ == nullptr && user32_module_ == nullptr) {
+    return;
+  }
+
+  dpi_for_window_supported_ =
+      (AssignProcAddress(user32_module_, "GetDpiForWindow",
+                         get_dpi_for_window_) &&
+       AssignProcAddress(user32_module_, "EnableNonClientDpiScaling",
+                         enable_non_client_dpi_scaling_));
+  dpi_for_monitor_supported_ =
+      (AssignProcAddress(shlib_module_, "GetDpiForMonitor",
+                         get_dpi_for_monitor_) &&
+       AssignProcAddress(user32_module_, "MonitorFromWindow",
+                         monitor_from_window_));
+}
+
+Win32DpiHelper::~Win32DpiHelper() {
+  if (user32_module_ != nullptr) {
+    FreeLibrary(user32_module_);
+  }
+  if (shlib_module_ != nullptr) {
+    FreeLibrary(shlib_module_);
+  }
+}
+
+BOOL Win32DpiHelper::EnableNonClientDpiScaling(HWND hwnd) {
+  if (enable_non_client_dpi_scaling_ == nullptr) {
+    return false;
+  }
+  return enable_non_client_dpi_scaling_(hwnd);
+}
+
+UINT Win32DpiHelper::GetDpi(HWND hwnd) {
+  // GetDpiForWindow returns the DPI for any awareness mode. If not available,
+  // or no |hwnd| is provided. fallback to a per monitor, system, or default
+  // DPI.
+  if (dpi_for_window_supported_ && hwnd != nullptr) {
+    return get_dpi_for_window_(hwnd);
+  }
+
+  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;
+  }
+  HDC hdc = GetDC(hwnd);
+  UINT dpi = GetDeviceCaps(hdc, LOGPIXELSX);
+  ReleaseDC(hwnd, hdc);
+  return dpi;
+}
+
+Win32DpiHelper* GetHelper() {
+  static Win32DpiHelper* dpi_helper = new Win32DpiHelper();
+  return dpi_helper;
+}
+
+UINT GetDpiForHWND(HWND hwnd) {
+  return GetHelper()->GetDpi(hwnd);
+}
+
+BOOL EnableNonClientDpiScaling(HWND hwnd) {
+  return GetHelper()->EnableNonClientDpiScaling(hwnd);
+}
+}  // namespace flutter

shell/platform/windows/dpi_utils.h

@@ -0,0 +1,23 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "Windows.h"
+
+#ifndef FLUTTER_SHELL_PLATFORM_WINDOWS_DPI_UTILS_H_
+#define FLUTTER_SHELL_PLATFORM_WINDOWS_DPI_UTILS_H_
+
+namespace flutter {
+
+/// Returns the current DPI. Supports all DPI awareness modes, and is backward
+/// compatible down to Windows Vista. If |hwnd| is nullptr, returns the DPI for
+/// the nearest monitor is available. Otherwise, returns the system's DPI.
+UINT GetDpiForHWND(HWND hwnd);
+
+/// Enables scaling of non-client UI (scrolling bars, title bars, etc). Only
+/// supported on Per-Monitor V1 DPI awareness mode.
+BOOL EnableNonClientDpiScaling(HWND hwnd);
+
+}  // namespace flutter
+
+#endif  // FLUTTER_SHELL_PLATFORM_WINDOWS_DPI_UTILS_H_

shell/platform/windows/flutter_windows.cc

@@ -15,6 +15,7 @@
 #include "flutter/shell/platform/common/cpp/client_wrapper/include/flutter/plugin_registrar.h"
 #include "flutter/shell/platform/common/cpp/incoming_message_dispatcher.h"
 #include "flutter/shell/platform/embedder/embedder.h"
+#include "flutter/shell/platform/windows/dpi_utils.h"
 #include "flutter/shell/platform/windows/key_event_handler.h"
 #include "flutter/shell/platform/windows/keyboard_hook_handler.h"
 #include "flutter/shell/platform/windows/platform_handler.h"
@@ -182,6 +183,14 @@ HWND FlutterDesktopViewGetHWND(FlutterDesktopViewRef view) {
   return view->window->GetWindowHandle();
 }
 
+UINT FlutterDesktopViewGetDpiForHWND(HWND hwnd) {
+  return flutter::GetDpiForHWND(hwnd);
+}
+
+BOOL FlutterDesktopEnableNonClientDpiScaling(HWND hwnd) {
+  return flutter::EnableNonClientDpiScaling(hwnd);
+}
+
 FlutterDesktopEngineRef FlutterDesktopRunEngine(const char* assets_path,
                                                 const char* icu_data_path,
                                                 const char** arguments,

shell/platform/windows/public/flutter_windows.h

@@ -77,6 +77,15 @@ FlutterDesktopProcessMessages(FlutterDesktopViewControllerRef controller);
 // Return backing HWND for manipulation in host application.
 FLUTTER_EXPORT HWND FlutterDesktopViewGetHWND(FlutterDesktopViewRef view);
 
+// Gets the DPI for a given |hwnd|, depending on the supported APIs per
+// windows version and DPI awareness mode. If nullptr is passed, returns the DPI
+// of the nearest monitor.
+FLUTTER_EXPORT UINT FlutterDesktopViewGetDpiForHWND(HWND hwnd);
+
+// Use only for apps that support Per-Monitor V1 awereness mode. Not necessary
+// for Per-Monitor V2.
+FLUTTER_EXPORT BOOL FlutterDesktopEnableNonClientDpiScaling(HWND hwnd);
+
 // Runs an instance of a headless Flutter engine.
 //
 // The |assets_path| is the path to the flutter_assets folder for the Flutter

shell/platform/windows/win32_window.cc

@@ -4,9 +4,16 @@
 
 #include "flutter/shell/platform/windows/win32_window.h"
 
+#include "dpi_utils.h"
+
 namespace flutter {
 
-Win32Window::Win32Window() {}
+Win32Window::Win32Window() {
+  // Calling GetDpiForHWND with no HNWD returns the DPI from the nearest
+  // monitor, which is the best option as an initial DPI. If Per-Monitor V2 is
+  // supported, |current_dpi_| should be updated in the WM_DPICHANGED message.
+  current_dpi_ = GetDpiForHWND(nullptr);
+}
 
 Win32Window::~Win32Window() {
   Destroy();
@@ -74,7 +81,7 @@ LRESULT CALLBACK Win32Window::WndProc(HWND const window,
                      reinterpret_cast<LONG_PTR>(cs->lpCreateParams));
 
     auto that = static_cast<Win32Window*>(cs->lpCreateParams);
-    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);
@@ -267,7 +274,7 @@ 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_->GetDpi(hwnd);
+      uDpi = GetDpiForHWND(hwnd);
     }
     current_dpi_ = uDpi;
     window->OnDpiScale(uDpi);

shell/platform/windows/win32_window.h

@@ -11,7 +11,6 @@
 #include <memory>
 #include <string>
 
-#include "flutter/shell/platform/windows/win32_dpi_helper.h"
 
 namespace flutter {
 
@@ -172,11 +171,6 @@ class Win32Window {
   // Member variable to hold the window title.
   std::wstring window_class_name_;
 
-  // Member variable referencing an instance of dpi_helper used to abstract some
-  // aspects of win32 High DPI handling across different OS versions.
-  std::unique_ptr<Win32DpiHelper> dpi_helper_ =
-      std::make_unique<Win32DpiHelper>();
-
   // Set to true to be notified when the mouse leaves the window.
   bool tracking_mouse_leave_ = false;