SDL-为什么移动鼠标会更改按钮状态？

debugcn 发表于 Dev

2013年

我在Simple Directmedia层库中遇到问题。下面的代码在按下鼠标按钮时在屏幕上绘制一个块：

SDL_Event event;
while(running){
    while(SDL_PollEvent(&event)){
        while(event.button.state == SDL_PRESSED){

            SDL_PollEvent(&event);

            //where to draw                
            boxRect.x = event.motion.x;
            boxRect.y = event.motion.y;

            //Draw to screen
            SDL_FillRect(display,&boxRect,boxColor);
            SDL_Flip(display);
        }
        // ...
    }
    // ...
}

直到我移动鼠标，它都可以正常工作，为什么移动鼠标会导致event.button.state错误呢？

如何同时使用两者（即在按下按钮的同时保持绘图状态）？

martelel

您的代码存在的问题是您两次调用SDL_PollEvent（在此处记录）。如文档中所述：

如果event不为NULL，则将下一个事件从队列中删除，并存储在event指向的SDL_Event结构中。

重新安排代码，例如摆脱第二个代码SDL_PollEvent，创建适当的流程以单击，移动，释放和从输入泵浦中提取渲染，应该会给您这样的东西：

SDL_Event Event;
while(running)
{
    while(SDL_PollEvent(&Event))
    {
        switch(Event.type)
        {
            // Handle your drawing with a simple state machine:
            case SDL_MOUSEBUTTONDOWN:
            {
                if(Event.button.button == SDL_BUTTON_LEFT)
                {
                    if(stateMachine == Released)
                    {
                        // ... begin drawing
                        stateMachine = Dragging
                    }
                }
                break;
            }
            case SDL_MOUSEMOTION:
            {
                if(stateMachine == Dragging)
                {
                    // ... update the extends of your rect
                }
            }
            case SDL_MOUSEBUTTONUP:
            {
                if(Event.button.button == SDL_BUTTON_LEFT)
                {
                    if(stateMachine != Released)
                    {
                        // ... finalize drawing... add the rect to a list? flush it?
                        stateMachine = Released;
                    }
                }
            }
            case SDL_QUIT:
            {
                running = false;
                break;
            }
        }
    }

    // Outside of your event pumping, update the graphics
    SDL_FillRect(display,&boxRect,boxColor);
    SDL_Flip(display);
}