HGE tutorial06

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bool FrameFunc() {     float dt = hge->Timer_GetDelta();     static float t = 0.0f;     float tx, ty;     int id;     static int lastid = 0;       // If ESCAPE was pressed, tell the GUI to finish     if (hge->Input_GetKeyState(HGEK_ESCAPE)) { lastid = 5; gui->Leave(); }       // We update the GUI and take an action if     // one of the menu items was selected     id = gui->Update(dt);     if (id == -1)     {         switch (lastid)         {         case 1:         case 2:         case 3:         case 4:             gui->SetFocus(1);             gui->Enter();             break;           case 5: return true;         }     }     else if (id) { lastid = id; gui->Leave(); }       // Here we update our background animation     t += dt;     tx = 50 * cosf(t / 60);     ty = 50 * sinf(t / 60);       quad.v[0].tx = tx;        quad.v[0].ty = ty;     quad.v[1].tx = tx + 800 / 64; quad.v[1].ty = ty;     quad.v[2].tx = tx + 800 / 64; quad.v[2].ty = ty + 600 / 64;     quad.v[3].tx = tx;        quad.v[3].ty = ty + 600 / 64;       return false; }

在这个代码中，当gui->update(dt)之后，会进行动画的操作。关于gui->update(dt):

If one of the controls has changed it's state, Update returns that control's identificator. If leave method was called and all the controls have finished their animation, Update returns -1. Otherwise it returns 0.

按照这个解释，我们会遇到三种情况：

1）、指向了某个选项，但是没有选择，那么 return  0;

2）、指向并选择了某个选项，那么 return id; 之后会调用leave();然后选项会消失在屏幕上

3）、在2）之后的情况，调用leave()之后会立刻的 return -1。那么在return -1则会默认指向某个选项，并且会刷新使得选项出现。

将原来的if-else if改成了if-else if-else输出了当前指示的选项。