c - SDL - 为什么移动鼠标会改变按钮状态？

Question

我在使用 Simple Directmedia Layer 库时遇到问题。按下鼠标按钮时，以下代码在屏幕上绘制一个 block :

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);
        }
        // ...
    }
    // ...
}

在我移动鼠标之前一切正常，为什么移动鼠标会使 event.button.state 不正确？

如何同时使用两者(即在按下按钮时继续绘图)？

Answer 1

您的代码存在的问题是您调用了两次 SDL_PollEvent(已记录 here)。如文档中所述:

If event is not NULL, the next event is removed from the queue and stored in the SDL_Event structure pointed to by event.

稍微重新安排一下您的代码，例如去掉第二个 SDL_PollEvent，为点击、移动、释放和从输入抽取中提取渲染创建适当的流程，您应该会得到如下结果:

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);
}