由于Pig采用的是C的代码，我们从main开始分析，从SDL的调用过程基本如下步骤：
1.通过SDL_Init初始化SDL
2.通过SDL_SetVideoMode建立主Surface
3.通过SDL_GetKeyState获取键盘状态数组
4.开始消息循环
5.通过SDL_PollEvent获取消息，并处理直到完成
以下是main函数
/*----------------------------------------------------------
main()
----------------------------------------------------------*/
int main(int argc, char* argv[])
{
SDL_Surface *screen;
GAMESTATE *gs;
int i;
int bpp = 0;
int last_tick, start_time, end_time;
int dashframe;
float logic_fps = 20.0;
int flags = SDL_DOUBLEBUF | SDL_HWSURFACE;
// SDL视频初始化
SDL_Init(SDL_INIT_VIDEO);
// 将函数SDL_Quit压入退出处理程序
atexit(SDL_Quit);
// 设置中断句柄，设置在Ctrl+Break和程序中止的时候，调用breakhandler
// 在breakhandler中重新设置了中断处理句柄，并将中断标志设为1
signal(SIGTERM, breakhandler);
signal(SIGINT, breakhandler);
// 处理参数
for(i = 1; i < argc; ++i)
{
if(strncmp(argv[i], "-s", 2) == 0)
flags &= ~SDL_DOUBLEBUF;
else if(strncmp(argv[i], "-f", 2) == 0)
flags |= SDL_FULLSCREEN;
else
bpp = atoi(&argv[i][1]);
}
// 设置Video Mode的宽度和高度，由于bpp=0，所以使用当前的颜色数
screen = SDL_SetVideoMode(SCREEN_W, SCREEN_H, bpp, flags);
if(!screen)
{
fprintf(stderr, "Failed to open screen!\n");
return 1;
}
// 设置窗口的标题和ICON的名称
SDL_WM_SetCaption("Fixed Rate Pig", "Pig");
// 隐藏鼠标
SDL_ShowCursor(0);
// 初始化游戏状态结构
gs = init_all(screen);
if(!gs)
return 1;
// 取得键盘状态，键盘状态是一个Uint8的数组，包含所有按键的当前状态
// 使用例子if ( gs->keys[SDLK_RETURN] ) ... <RETURN> is pressed.
gs->keys = SDL_GetKeyState(&i);
// 初始化逻辑桢的相关信息
gs->logic_frames = 0;
gs->rendered_frames = 0;
gs->pe->before_objects = before_objects;
// 初始化Pig engine中的精灵和原图片
pig_start(gs->pe, 0);
gs->refresh_screen = gs->pe->pages;
// 记录当前的时间
start_time = last_tick = SDL_GetTicks();
// 开始消息处理循环
while(gs->running)
{
int tick;
float frames, dt;
SDL_Event ev;
/* Handle input */
// SDL_PollEvent有点类似PeekMessage
while(SDL_PollEvent(&ev) > 0)
// 处理键盘和鼠标事件
handle_input(gs, &ev);
// 空函数
handle_keys(gs);
// 判断程序是否被中断
// 前面通过signal设置当Ctrl+Break或者程序结束的中断发生时，break_received要设置为1
// 注意这里并没有直接中断消息循环，所以在处理完本次循环的操作以后才能真正退出
if(break_received)
gs->running = 0;
/* Calculate time since last update */
// 这里用来计算和上次消息循环间隔的时间，并通过显示桢数参数来确定需要显示的逻辑桢的ID
tick = SDL_GetTicks();
dt = (tick - last_tick) * 0.001;
frames = dt * logic_fps;
/* Run the game logic */
// 显示逻辑桢
pig_animate(gs->pe, frames);
/*
* Limit the dashboard frame rate to 15 fps
* when there's no wobbling going on.
*
* The 'dashframe' deal is about keeping the
* pages in sync on a double buffered display.
*/
if(gs->lives_wobble || gs->score_wobble ||
(gs->dashboard_time > 1.0/15.0))
{
dashframe = gs->pe->pages;
gs->dashboard_time = 0;
}
if(dashframe)
{
--dashframe;
dashboard(gs);
}
/* Update sprites */
if(gs->refresh_screen)
{
--gs->refresh_screen;
pig_refresh_all(gs->pe);
}
else
pig_refresh(gs->pe);
/* Make the new frame visible */
pig_flip(gs->pe);
/* Update statistics, timers and stuff */
++gs->rendered_frames;
gs->lives_wobble_time += dt;
gs->score_wobble_time += dt;
gs->dashboard_time += dt;
// 记录时间方便最后的计算
last_tick = tick;
// nice似乎是某种效果的标志，目前还不确定
if(gs->nice)
SDL_Delay(10);
}
/* Print some statistics */
end_time = SDL_GetTicks();
i = end_time - start_time;
printf(" Total time running: %d ms\n", i);
if(!i)
i = 1;
printf("Average rendering frame rate: %.2f fps\n",
gs->rendered_frames * 1000.0 / i);
printf(" Average logic frame rate: %.2f fps\n",
gs->logic_frames * 1000.0 / i);
// 释放Pig engine
pig_close(gs->pe);
return 0;
}
注意：虽然代码中设置了窗体的标题，却没有窗体生成的部分，难道SDL自动创建了窗体？还是隐藏在未展开的代码中？相信随着代码的不断展开应该会更清楚的。 
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