Add a systemPreferences API method to fetch the system ColorizationColor

This is useful for automatical app theming

* Fetches the color from the registry
* Returns an empty string if the fetch fails (a falsey value)

atom/browser/api/atom_api_system_preferences_win.cc

@@ -0,0 +1,93 @@
+// Copyright (c) 2014 GitHub, Inc.
+// Use of this source code is governed by the MIT license that can be
+// found in the LICENSE file.
+
+#include "atom/browser/api/atom_api_system_preferences.h"
+
+#include "base/win/wrapped_window_proc.h"
+#include "ui/base/win/shell.h"
+#include "ui/gfx/win/hwnd_util.h"
+
+namespace atom {
+
+namespace {
+
+const wchar_t kSystemPreferencestWindowClass[] =
+  L"Electron_SystemPreferencesHostWindow";
+
+}  // namespace
+
+namespace api {
+
+bool SystemPreferences::IsAeroGlassEnabled() {
+  return ui::win::IsAeroGlassEnabled();
+}
+
+std::string hexColorDWORDToRGBA(DWORD color) {
+  std::ostringstream stream;
+  stream << std::hex << color;
+  std::string hexColor = stream.str();
+  return hexColor.substr(2) + hexColor.substr(0, 2);
+}
+
+std::string SystemPreferences::GetAccentColor() {
+  DWORD color = 0;
+  BOOL opaque = FALSE;
+
+  if (FAILED(dwmGetColorizationColor(&color, &opaque))) {
+    return "";
+  }
+
+  return hexColorDWORDToRGBA(color);
+}
+
+void SystemPreferences::InitializeWindow() {
+  WNDCLASSEX window_class;
+  base::win::InitializeWindowClass(
+      kSystemPreferencestWindowClass,
+      &base::win::WrappedWindowProc<SystemPreferences::WndProcStatic>,
+      0, 0, 0, NULL, NULL, NULL, NULL, NULL,
+      &window_class);
+  instance_ = window_class.hInstance;
+  atom_ = RegisterClassEx(&window_class);
+
+  // Create an offscreen window for receiving broadcast messages for the system
+  // colorization color.  Create a hidden WS_POPUP window instead of an
+  // HWND_MESSAGE window, because only top-level windows such as popups can
+  // receive broadcast messages like "WM_DWMCOLORIZATIONCOLORCHANGED".
+  window_ = CreateWindow(MAKEINTATOM(atom_),
+                         0, WS_POPUP, 0, 0, 0, 0, 0, 0, instance_, 0);
+  gfx::CheckWindowCreated(window_);
+  gfx::SetWindowUserData(window_, this);
+}
+
+LRESULT CALLBACK SystemPreferences::WndProcStatic(HWND hwnd,
+                                              UINT message,
+                                              WPARAM wparam,
+                                              LPARAM lparam) {
+  SystemPreferences* msg_wnd = reinterpret_cast<SystemPreferences*>(
+      GetWindowLongPtr(hwnd, GWLP_USERDATA));
+  if (msg_wnd)
+    return msg_wnd->WndProc(hwnd, message, wparam, lparam);
+  else
+    return ::DefWindowProc(hwnd, message, wparam, lparam);
+}
+
+LRESULT CALLBACK SystemPreferences::WndProc(HWND hwnd,
+                                        UINT message,
+                                        WPARAM wparam,
+                                        LPARAM lparam) {
+  if (message == WM_DWMCOLORIZATIONCOLORCHANGED) {
+    DWORD new_color = (DWORD) wparam;
+    std::string new_color_string = hexColorDWORDToRGBA(new_color);
+    if (new_color_string != current_color_) {
+      Emit("accent-color-changed", hexColorDWORDToRGBA(new_color));
+      current_color_ = new_color_string;
+    }
+  }
+  return ::DefWindowProc(hwnd, message, wparam, lparam);
+}
+
+}  // namespace api
+
+}  // namespace atom