Question

我正在编写一个glfw应用程序，我将函数调用函数包装到一个简单的类中。我在设置密钥回调时遇到问题。我的班级被定义为：

class GAME 
{
private:
    bool running;
public:
    GAME();
    int execute();
    void events(int, int);
    int loop();
    int render();
};

执行功能是：

int GAME::execute() 
    {
        glfwOpenWindow(640, 320, 8, 8, 8, 8, 0, 0, GLFW_WINDOW);
        glfwSetWindowTitle("Viraj");
        glfwSetKeyCallback(events);
        running = true;
        while(glfwGetWindowParam(GLFW_OPENED))
        {
            glfwPollEvents();
            loop();
            render();
        }
        return 0;
    }

在Visual Studio 2010上编译以下代码，会出现错误：

error C3867: 'GAME::events': function call missing argument list; use '&GAME::events' to create a pointer to member

使用＆amp; GAME :: events提供：

error C2664: 'glfwSetKeyCallback' : cannot convert parameter 1 from 'void (__thiscall GAME::* )(int,int)' to 'GLFWkeyfun' 1> There is no context in which this conversion is possible

Answer 1

其他答案中提供的代码示例没有描述如何将回调重定向到每个对象的成员函数，可能包含任意数量的对象。让你的班级成为单身人士将限制你的设计，不会扩展到多个glfw窗口。

可扩展的解决方案是将glfw窗口用户指针设置为您的对象，然后在回调中获取它，并调用成员函数：

class MyGlWindow
{
public:
     void mouseButtonPressed();
};

void makeWindow()
{
    GLFWwindow* glfwWindow;
    MyGlWindow* myWindow;

    /* ... Initialize everything here ... */

    glfwSetWindowUserPointer(glfwWindow, myWindow);

    auto func = [](GLFWwindow* w, int, int, int)
    {
        static_cast<MyGlWindow*>(glfwGetWindowUserPointer(w))->mouseButtonPressed( /* ... */ );
    }

    glfwSetMouseButtonCallback(glfwWindow, func);
}

此解决方案更短，适用于任意数量的窗口。

Answer 2

我也遇到了另一个glfw回调函数的问题，但是我并不想将我的类方法声明为static，因为我需要访问其中的成员变量。所以我尝试std::function和std::bind让我能够将实例方法绑定为回调函数，但不幸的是，在使用C回调时它不是一个选项。

GLFW常见问题"How do I use C++ methods as callbacks"中也说明了这个问题的答案：

您不能使用常规方法作为回调，因为GLFW是C库并且不知道对象和这个指针。如果你愿意的话接收回调到C ++对象，使用静态方法或常规函数作为回调，将指针存储到你想要的对象在回调中可以到达的某个位置呼叫并使用它来呼叫对象的方法。

然而，这鼓励我为我的回调类应用Singleton模式并将其集成如下：

我的类的回调方法仍然是静态的，因此可以指定/用作glfw回调
此静态回调方法使用单例并将回调参数传递给实例方法
此实例方法实际上处理回调参数，具有能够访问成员变量的好处

这就是它的样子：

// Input.h (the actual callback class for glfwSetMouseButtonCallback)
class Input 
{
public:
    static Input& getInstance() // Singleton is accessed via getInstance()
    {
        static Input instance; // lazy singleton, instantiated on first use
        return instance;
    }

    static void mouseButtonCallback(int key, int action) // this method is specified as glfw callback
    {
        //here we access the instance via the singleton pattern and forward the callback to the instance method
        getInstance().mouseButtonCallbackImpl(key, action); 
    }

    void mouseButtonCallbackImpl(int key, int action) //this is the actual implementation of the callback method
    {
        //the callback is handled in this instance method           
        //... [CODE here]
    }

private:
    Input(void) // private constructor necessary to allow only 1 instance
    {
    }

    Input(Input const&); // prevent copies
    void operator=(Input const&); // prevent assignments
};

在我的main.cpp中：

Input &hexmap = Input::getInstance(); // initialize the singleton

//The glfw callback is set up as follows:   
glfwSetMouseButtonCallback( &Input::mouseButtonCallback); // specifying the static callback method, which internally forwards it to the instance method

Answer 3

有一种用于指向类成员方法的C ++语法，但是您无法将它们传递给C样式API。 C理解函数调用和每个非静态对象方法，以events为例，用C语言看起来像这样思考：void events(void* this, int, int);意味着除标准参数之外的每个方法都得到this指针默默地传递。

要使events C兼容，请将其设为static void events(int, int);。这样它将遵循C调用语义 - 它不需要传递this指针。您还必须以某种其他方式将对象传递给此回调（如果在回调中需要此对象的数据）。

Answer 4

受到N0vember的回答的启发，我向您展示了更加通用和动态的解决方案：

class MyGlWindow {
public:
    std::function<void(MyGlWindow*)> onClose;
    std::function<void(MyGlWindow*, int, int, int)> onMouseClick = [](auto self, int, int, int) { /*some default behavior*/ };
};

void makeWindow() {
    GLFWwindow* glfwWindow;
    MyGlWindow* myWindow;

    /* ... Initialize everything here ... */

    glfwSetWindowUserPointer(glfwWindow, myWindow);

    #define genericCallback(functionName)\
        [](GLFWwindow* window, auto... args) {\
            auto pointer = static_cast<MyGlWindow*>(glfwGetWindowUserPointer(window));\
            if (pointer->functionName) pointer->functionName(pointer, args...);\
        }

    glfwSetWindowCloseCallback(glfwWindow, genericCallback(onClose));
    glfwSetMouseButtonCallback(glfwWindow, genericCallback(onMouseClick));

    myWindow->onMouseClick = [](auto self, int, int, int) {
        std::cout << "I'm such a rebel" << std::endl;
        self->onClose = [](auto self) {
            std::cout << "I'm such a rebellion" << std::endl;
        };
    };
}

Answer 5

在头文件中，将事件（int，int）设为静态方法。这解决了我的问题。

class GAME 
{
private:
    bool running;
public:
    GAME();
    int execute();
    static void events(int, int); //Changed here to static void 
    int loop();
    int render();
};

Answer 6

我有同样的问题，在阅读完这个帖子后，我想出了一个类似的解决方案。我认为这种方式有点清洁。它基于静态函数，但它嵌套在我们设置所有东西的类中。

标题看起来像这样：

class Application
    {
    public:
        ...
    private:
        ...
        void MousePositionCallback(GLFWwindow* window, double positionX, double positionY);
        void KeyboardCallback(GLFWwindow* window, int key, int scancode, int action, int mods);
        ...
        class GLFWCallbackWrapper
        {
        public:
            GLFWCallbackWrapper() = delete;
            GLFWCallbackWrapper(const GLFWCallbackWrapper&) = delete;
            GLFWCallbackWrapper(GLFWCallbackWrapper&&) = delete;
            ~GLFWCallbackWrapper() = delete;

            static void MousePositionCallback(GLFWwindow* window, double positionX, double positionY);
            static void KeyboardCallback(GLFWwindow* window, int key, int scancode, int action, int mods);
            static void SetApplication(Application *application);
        private:
            static Application* s_application;
        };
    };

源代码：

void Application::GLFWCallbackWrapper::MousePositionCallback(GLFWwindow* window, double positionX, double positionY)
{
    s_application->MousePositionCallback(window, positionX, positionY);
}

void Application::GLFWCallbackWrapper::KeyboardCallback(GLFWwindow* window, int key, int scancode, int action, int mods)
{
    s_application->KeyboardCallback(window, key, scancode, action, mods);
}

void Application::GLFWCallbackWrapper::SetApplication(Application* application)
{
    GLFWCallbackWrapper::s_application = application;
}

Application* Application::GLFWCallbackWrapper::s_application = nullptr;

void Application::MousePositionCallback(GLFWwindow* window, double positionX, double positionY)
{
    ...
}

void Application::KeyboardCallback(GLFWwindow* window, int key, int scancode, int action, int mods)
{
    ...
}

void Application::SetCallbackFunctions()
{
    GLFWCallbackWrapper::SetApplication(this);
    glfwSetCursorPosCallback(m_window, GLFWCallbackWrapper::MousePositionCallback);
    glfwSetKeyCallback(m_window, GLFWCallbackWrapper::KeyboardCallback);
}

Answer 7

对于帮助我解决同样问题的可能解决方案，这是一个有用的讨论，我正在添加我的解决方案以防它有用。

问题陈述

我的情景比BIC，L.Senionis和N0vember所说的更为普遍。特别是，我的用例需要：

通常，回调必须可以访问实例的数据
可以使用一组通用的响应处理程序创建许多应用程序
在应用程序中，可以创建任意数量的窗口
附加到每个窗口的回调集应该与某个可能的响应者库混合并匹配。

建议的解决方案使用

简单的单件设计不再能解决问题。相反，我提供了一个GLFWResponder超类来处理所有的设置复杂性。为了使用类并将响应附加到窗口，这是必需的。

// Implement custom responder
class MyResponder : public GLFWResponder {
  public:
    virtual void cursor_position_callback(GLFWwindow* w, double x, double y) {...}
    ... override relevant callbacks ...
 };

// in main ************************************************

// Assuming initialized GLFWwindow* my_window and my_other_window

MyResponder resp;
MyResponder resp2;  // Can be another subclass of GLFWResponder

// Two responders can respond to same window
resp.respond_to(my_window, GLFWResponder::CURSOR_POSITION);
resp2.respond_to(my_window, GLFWResponder::CURSOR_POSITION);

// One responder can respond to multiple windows
resp2.respond_to(my_other_window, GLFWResponder::CURSOR_POSITION);

// One window can have different handlers for different events
resp.respond_to(my_other_window, GLFWResponder::CURSOR_ENTER);

建议的解决方案实施

以下是GLFWResponder实现的草图，功能齐全，但有一些TODO。可能会对性能产生一些影响，我还没有调查过。

// GLFWResponder.h ************************************************
/**
 * Responder superclass that allows subclasses to handle events from multiple
 * GLFW windows (which have only C API for callbacks).
 * Callbacks are automatically cleaned up when responder goes out of scope.
 */
class GLFWResponder {
 public:
  virtual ~GLFWResponder();

  // Interface -----------------------------------
  enum GLFWEventType {
    CURSOR_POSITION = 0,
    CURSOR_ENTER = 1
    // TODO: add support for other callbacks
  };

  void respond_to(GLFWwindow* window, GLFWEventType event);

  bool does_respond_to(GLFWwindow* window, GLFWEventType event) const;

  // Subclasses implement ------------------------
  virtual void cursor_position_callback(GLFWwindow* window, double xpos, double ypos);

  virtual void cursor_enter_callback(GLFWwindow* window, int entered);

  // TODO: add support for other callbacks


  // Under the hood ------------------------------
  static std::set<GLFWResponder*> getResponders(GLFWwindow* windo, GLFWEventType event);

 private:
  // Windows and events that this instance responds to
  std::set<std::pair<GLFWwindow*, GLFWEventType> > enabled_events_;

  // Global responders keyed by events they respond to
  // (each responder knows which windows it responds to)
  static std::map<GLFWEventType, std::set<GLFWResponder*> > responders_;
}; 

// GLFWResponder.cpp **************************************************
namespace {

void cursor_position_callback_private(GLFWwindow* window, double xpos, double ypos) {
  for (GLFWResponder* r : GLFWResponder::getResponders(window, GLFWResponder::CURSOR_POSITION)) {
    r->cursor_position_callback(window, xpos, ypos);
  }
}

void cursor_enter_callback_private(GLFWwindow* window, int entered) {
  for (GLFWResponder* r : GLFWResponder::getResponders(window, GLFWResponder::CURSOR_ENTER)) {
    r->cursor_enter_callback(window, entered);
  }
}

} // namespace

std::map<GLFWResponder::GLFWEventType, std::set<GLFWResponder*> > GLFWResponder::responders_;

GLFWResponder::~GLFWResponder() {
  for (auto& pr : responders_) {
    pr.second.erase(this);
  }
  // TODO: also clean up window's callbacks
}

void GLFWResponder::respond_to(GLFWwindow* window, GLFWResponder::GLFWEventType event) {
  enabled_events_.insert(std::make_pair(window, event));
  responders_[event].insert(this);
  if (event == CURSOR_POSITION) {
    glfwSetCursorPosCallback(window, cursor_position_callback_private);
  } else if (event == CURSOR_ENTER) {
    glfwSetCursorEnterCallback(window, cursor_enter_callback_private);
  } else {
    // TODO: add support for other callbacks
    LOG(FATAL) << "Unknown GLFWResponder event: " << event;
  }
}

bool GLFWResponder::does_respond_to(GLFWwindow* window, GLFWEventType event) const {
  return enabled_events_.find(std::make_pair(window, event)) != enabled_events_.end();
}

std::set<GLFWResponder*> GLFWResponder::getResponders(
    GLFWwindow* window, GLFWEventType event) {
  std::set<GLFWResponder*> result;
  auto it = responders_.find(event);
  if (it != responders_.end()) {
    for (GLFWResponder* resp : it->second) {
      if (resp->does_respond_to(window, event)) {
        result.insert(resp);
      }
    }
  }
  return result;
}

void GLFWResponder::cursor_position_callback(
    GLFWwindow* window, double xpos, double ypos) {
  // TODO: fail with message "GLFWResponder::do_respond called on a subclass that does not implement a handler for that event"
}

void GLFWResponder::cursor_enter_callback(GLFWwindow* window, int entered) {
  // TODO: fail with message "GLFWResponder::do_respond called on a subclass that does not implement a handler for that event"
}

指向一个类成员的函数--glfw setKeycallback

7 个答案: