c++ - StepTimer.GetTotalSeconds() 产生的值并不总是增加

Question

我使用的是 DirectX 11。为简单起见并重现问题，我将问题缩小为以下步骤:

1. 在 Visual Studio 中创建一个新的“DirectX 和 XAML 应用程序 (UWP)”(我使用的是 VS 2017)。

2. 用以下代码替换 Sample3DSceneRenderer::Update 方法:

   void Sample3DSceneRenderer::Update(DX::StepTimer const& timer)
   {
       if (!m_tracking)
       {
           double total = timer.GetTotalSeconds();
   
           // Convert degrees to radians, then convert seconds to rotation angle
           float radiansPerSecond = XMConvertToRadians(m_degreesPerSecond);
           double totalRotation = total * radiansPerSecond;
           float radians = static_cast<float>(fmod(totalRotation, XM_2PI));
   
           DX::DebugTrace(L"num = %4.2f\t%4.2f\n", radians, total);
   
           Rotate(radians);
       }
   }
   

3. 添加以下函数以跟踪“输出”窗口中的值:

   inline void DebugTrace(const wchar_t *format, ...)
   {
       // Generate the message string.
       va_list args;
       va_start(args, format); // initialize the argument list
       wchar_t buffer[1024];
       va_end(args);
   
       OutputDebugStringW(buffer); // this is a Windows function
   }
   

当我运行应用程序时，“输出”窗口显示以下值:

    num = 0.01  0.01
    num = 0.02  0.02
    num = 0.00  0.00 // decreased
    num = 0.00  0.01 // decreased
    num = 0.03  0.04
    num = 0.05  0.06
    num = 0.02  0.02 // decreased
    num = 0.06  0.07
    num = 0.03  0.04 // decreased
    num = 0.07  0.09
    num = 0.04  0.06 // decreased
    num = 0.08  0.11
    num = 0.06  0.07 // decreased
    num = 0.10  0.12 
    num = 0.07  0.09 // decreased
    num = 0.11  0.14
    num = 0.08  0.11 // decreased
    num = 0.12  0.16
    num = 0.10  0.12 // decreased
    num = 0.11  0.14 
    num = 0.14  0.17
    num = 0.12  0.16 // decreased
    num = 0.15  0.19
    num = 0.16  0.21
    num = 0.14  0.17 // decreased
    num = 0.18  0.22
    num = 0.15  0.19 // decreased
    num = 0.16  0.21 
    num = 0.19  0.24
    num = 0.20  0.26
    num = 0.18  0.22 // decreased
    etc.

问题:为什么 TotalSeconds 值会先增加然后减少然后再增加等等？例如:0.01、0.02、0.00、0.01。它们不应该一直增加吗？

Answer 1

错误在 App::OnLaunched 中。模板中的版本正在创建两个 DirectXPage:

• 直接赋值:m_directXPage = ref new DirectXPage();
• 导航:rootFrame->Navigate(TypeName(DirectXPage::typeid), e->Arguments);

我有一个修改后的版本，它仅通过引用导航期间创建的版本来创建一个版本，但我没有花太多时间检查错误情况。

void App::OnLaunched(Windows::ApplicationModel::Activation::LaunchActivatedEventArgs^ e)
{
#if _DEBUG
       if (IsDebuggerPresent())
       {
              DebugSettings->EnableFrameRateCounter = true;
       }
#endif

       auto rootFrame = dynamic_cast<Frame^>(Window::Current->Content);

       // Do not repeat app initialization when the Window already has content,
       // just ensure that the window is active
       if (rootFrame == nullptr)
       {
              // Create a Frame to act as the navigation context and associate it with
              // a SuspensionManager key
              rootFrame = ref new Frame();

              rootFrame->NavigationFailed += ref new Windows::UI::Xaml::Navigation::NavigationFailedEventHandler(this, &App::OnNavigationFailed);

              // Place the frame in the current Window
              Window::Current->Content = rootFrame;
       }

       if (rootFrame->Content == nullptr)
       {
              // When the navigation stack isn't restored navigate to the first page,
              // configuring the new page by passing required information as a navigation
              // parameter
              rootFrame->Navigate(TypeName(DirectXPage::typeid), e->Arguments);
       }

       if (m_directXPage == nullptr)
       {
              m_directXPage = dynamic_cast<DirectXPage^>(rootFrame->Content);
       }

       if (e->PreviousExecutionState == ApplicationExecutionState::Terminated)
       {
              m_directXPage->LoadInternalState(ApplicationData::Current->LocalSettings->Values);
       }

       // Ensure the current window is active
       Window::Current->Activate();
}