case WM_CLOSE:
      if (client && !client->IsClosing()) {
        CefRefPtr<CefBrowser> browser = client->GetBrowser();
        if (browser.get()) {
          // Notify the browser window that we would like to close it. This
          // will result in a call to ClientHandler::DoClose() if the
          // JavaScript 'onbeforeunload' event handler allows it.
          browser->GetHost()->CloseBrowser(false);

          // Cancel the close.
          return 0;
        }
      }

      // Allow the close.
      break;

browser->GetHost()->CloseBrowser(false);

void ClientHandler::CloseAllBrowsers(bool force_close) {
  if (!CefCurrentlyOn(TID_UI)) {
    // Execute on the UI thread.
    CefPostTask(TID_UI,
        base::Bind(&ClientHandler::CloseAllBrowsers, this, force_close));
    return;
  }

  if (!popup_browsers_.empty()) {
    // Request that any popup browsers close.
    BrowserList::const_iterator it = popup_browsers_.begin();
    for (; it != popup_browsers_.end(); ++it)
      (*it)->GetHost()->CloseBrowser(force_close);
  }

  if (browser_.get()) {
    // Request that the main browser close.
    browser_->GetHost()->CloseBrowser(force_close);
  }
}

void CefBrowserHostImpl::CloseBrowser(bool force_close) {
  if (CEF_CURRENTLY_ON_UIT()) {
    // Exit early if a close attempt is already pending and this method is
    // called again from somewhere other than WindowDestroyed().
    if (destruction_state_ >= DESTRUCTION_STATE_PENDING &&
        (IsWindowless() || !window_destroyed_)) {
      if (force_close && destruction_state_ == DESTRUCTION_STATE_PENDING) {
        // Upgrade the destruction state.
        destruction_state_ = DESTRUCTION_STATE_ACCEPTED;
      }
      return;
    }

    if (destruction_state_ < DESTRUCTION_STATE_ACCEPTED) {
      destruction_state_ = (force_close ? DESTRUCTION_STATE_ACCEPTED :
                                          DESTRUCTION_STATE_PENDING);
    }

    content::WebContents* contents = web_contents();
    if (contents && contents->NeedToFireBeforeUnload()) {
      // Will result in a call to BeforeUnloadFired() and, if the close isn't
      // canceled, CloseContents().
      contents->DispatchBeforeUnload(false);
    } else {
      CloseContents(contents);
    }
  } else {
    CEF_POST_TASK(CEF_UIT,
        base::Bind(&CefBrowserHostImpl::CloseBrowser, this, force_close));
  }
}

void CefBrowserHostImpl::CloseContents(content::WebContents* source) {
  if (destruction_state_ == DESTRUCTION_STATE_COMPLETED)
    return;

  bool close_browser = true;

  // If this method is called in response to something other than
  // WindowDestroyed() ask the user if the browser should close.
  if (client_.get() && (IsWindowless() || !window_destroyed_)) {
    CefRefPtr<CefLifeSpanHandler> handler =
        client_->GetLifeSpanHandler();
    if (handler.get()) {
      close_browser = !handler->DoClose(this);   // CefLifeSpanHandler::DoClose
    }
  }

  if (close_browser) {
    if (destruction_state_ != DESTRUCTION_STATE_ACCEPTED)
      destruction_state_ = DESTRUCTION_STATE_ACCEPTED;

    if (!IsWindowless() && !window_destroyed_) {
      // A window exists so try to close it using the platform method. Will
      // result in a call to WindowDestroyed() if/when the window is destroyed
      // via the platform window destruction mechanism.
      PlatformCloseWindow();
    } else {
      // Keep a reference to the browser while it's in the process of being
      // destroyed.
      CefRefPtr<CefBrowserHostImpl> browser(this);

      // No window exists. Destroy the browser immediately.
      DestroyBrowser();       // CefLifeSpanHandler::OnBeforeClose
      if (!IsWindowless()) {
        // Release the reference added in PlatformCreateWindow().
        Release();
      }
    }
  } else if (destruction_state_ != DESTRUCTION_STATE_NONE) {
    destruction_state_ = DESTRUCTION_STATE_NONE;
  }
}

void CefBrowserHostImpl::DestroyBrowser() {
  CEF_REQUIRE_UIT();

  destruction_state_ = DESTRUCTION_STATE_COMPLETED;

  if (client_.get()) {
    CefRefPtr<CefLifeSpanHandler> handler = client_->GetLifeSpanHandler();
    if (handler.get()) {
      // Notify the handler that the window is about to be closed.
      handler->OnBeforeClose(this);        //`CefLifeSpanHandler::OnBeforeClose
    }
  }
  // 省略
}

=> 表示同步调用 ->异步调用
[os]window close =>
    [个人]browser(s) close =>
        [cef]CefBrowserHostImpl::CloseBrowser =>
            [cef]CefBrowserHostImpl::CloseContents =>
                [实现cef接口]CefLifeSpanHandler::DoClose =>
                [cef]CefLifeSpanHandler::DestroyBrowser =>
                    [实现cef接口]CefLifeSpanHandler::OnBeforeClose =>
[os]阻止window close

void CefBrowserHostImpl::PlatformCloseWindow() {
  if (window_info_.window != NULL) {
    HWND frameWnd = GetAncestor(window_info_.window, GA_ROOT);
    PostMessage(frameWnd, WM_CLOSE, 0, 0);
  }
}

 case WM_CLOSE:
      if (client && !client->IsClosing()) {
// 这次不会进入IF分支

LRESULT CALLBACK CefBrowserHostImpl::WndProc(HWND hwnd, UINT message,
                                             WPARAM wParam, LPARAM lParam) {
// 略
  case WM_DESTROY:
    if (browser) {
      // Clear the user data pointer.
      gfx::SetWindowUserData(hwnd, NULL);

      // Force the browser to be destroyed and release the reference added in
      // PlatformCreateWindow().
      browser->WindowDestroyed();
    }
    return 0;
// 略
  }

void ClientHandler::OnBeforeClose(CefRefPtr<CefBrowser> browser) {
  CEF_REQUIRE_UI_THREAD();

  message_router_->OnBeforeClose(browser);

  if (GetBrowserId() == browser->GetIdentifier()) {
    {
      base::AutoLock lock_scope(lock_);
      // Free the browser pointer so that the browser can be destroyed
      browser_ = NULL;
    }

    if (osr_handler_.get()) {
      osr_handler_->OnBeforeClose(browser);
      osr_handler_ = NULL;
    }
  } else if (browser->IsPopup()) {
    // Remove from the browser popup list.
    BrowserList::iterator bit = popup_browsers_.begin();
    for (; bit != popup_browsers_.end(); ++bit) {
      if ((*bit)->IsSame(browser)) {
        popup_browsers_.erase(bit);
        break;
      }
    }
  }

  if (--browser_count_ == 0) {
    // All browser windows have closed.
    // Remove and delete message router handlers.
    MessageHandlerSet::const_iterator it = message_handler_set_.begin();
    for (; it != message_handler_set_.end(); ++it) {
      message_router_->RemoveHandler(*(it));
      delete *(it);
    }
    message_handler_set_.clear();
    message_router_ = NULL;

    // Quit the application message loop.
    //AppQuitMessageLoop();
  XCefAppManage::Instance()->QuitMessageLoop();
  }
}

void            XCefAppManage::QuitMessageLoop() {
  if (GetCefSettings().multi_threaded_message_loop)
  {
    // Running in multi-threaded message loop mode. Need to execute
    // PostQuitMessage on the main application thread.
    if (NULL == message_wnd__)
    {
      message_wnd__ = ::FindWindow(XWinUtil::GetMessageWindowClassName(XWinUtil::GetParentProcessID()), NULL);
    }
    DCHECK(message_wnd__);
    PostMessage(message_wnd__, WM_COMMAND, ID_QUIT, 0);
  }
  else {
    CefQuitMessageLoop();
  }
}

PostMessage(message_wnd__, WM_COMMAND, ID_QUIT, 0);

    case ID_QUIT:
      PostQuitMessage(0);
      return 0;

=> 表示同步调用 ->异步调用
[os]on window close WM_CLOSE =>
    [个人]browser(s) close =>
        [cef]CefBrowserHostImpl::CloseBrowser =>
            [cef]CefBrowserHostImpl::CloseContents =>
                [实现cef接口]CefLifeSpanHandler::DoClose =>
                [cef]CefBrowserHostImpl::PlatformCloseWindow =>
                    [os] post window close -> WM_CLOSE
[os]阻止window close

post被接收
[os] 第二次on window close WM_CLOSE =>
     [个人] 允许 window close ->WM_DESTROY

[os] 系统群发WM_DESTROY
    [cef] CefBrowserHostImpl::WndProc =>
        [cef & os] 每个browser proc内的 WM_CLOSE 处理
        [cef]CefLifeSpanHandler::DestroyBrowser =>
            [实现cef接口]CefLifeSpanHandler::OnBeforeClose =>
                发起总关闭(个人使用XCefAppManage::QuitMessageLoop)->

退出消息循环
CefShutDown