我该如何使用SDL2 iOS应用程序事件？

我目前正在使用SDL2 Library和C编写一个iPhone应用程序,而且大部分内容都很顺利.不幸的是,文档似乎在某些方面相当薄,特别是iOS特定的功能.我是使用SDL2的新手,这让事情变得非常困难.到目前为止,一切都有效,但我对一个问题感到难过. SDL2定义了六种专门用于移动应用程序的事件类型. README-ios.txt文件描述了它们并使用它们：

int HandleAppEvents(void *userdata,SDL_Event *event)
{
    switch (event->type)
    {
    case SDL_APP_TERMINATING:
        /* Terminate the app.
           Shut everything down before returning from this function.
        */
        return 0;
    case SDL_APP_LOWMEMORY:
        /* You will get this when your app is paused and iOS wants more memory.
           Release as much memory as possible.
        */
        return 0;
    case SDL_APP_WILLENTERBACKGROUND:
        /* Prepare your app to go into the background.  Stop loops,etc.
           This gets called when the user hits the home button,or gets a call.
        */
        return 0;
    case SDL_APP_DIDENTERBACKGROUND:
        /* This will get called if the user accepted whatever sent your app to the background.
           If the user got a phone call and canceled it,you'll instead get an    SDL_APP_DIDENTERFOREGROUND event and restart your loops.
           When you get this,you have 5 seconds to save all your state or the app will be terminated.
           Your app is NOT active at this point.
        */
        return 0;
    case SDL_APP_WILLENTERFOREGROUND:
       /* This call happens when your app is coming back to the foreground.
           Restore all your state here.
       */
        return 0;
    case SDL_APP_DIDENTERFOREGROUND:
        /* Restart your loops here.
           Your app is interactive and getting cpu again.
        */
        return 0;
    default:
        /* No special processing,add it to the event queue */
        return 1;
    }
}

int main(int argc,char *argv[])
{
    SDL_SetEventFilter(HandleAppEvents,NULL);

    //... run your main loop

    return 0;
}

我对这段代码有几个问题.

SDL_SetEventFilter()有什么作用？我阅读了SDL Wiki页面,看起来特别含糊.

实际上,HandleAppEvents()函数如何工作？例如,如果我有这样的代码：

int main(int argc,char* argv[])
{
    //Initialize SDL,etc...
    SDL_SetEventFilter(HandleAppEvents,NULL);


    //I've got some SDL_Textures and windows and things...
    SDL_Window* my_window;
    SDL_Renderer* windoWrend;
    SDL_Texture* tex1,tex2,tex3;

    //Primitive game loop
    while(game_is_running){
        handle_input();
        do_logic();
        update_screen();
    }

    destroy_all_my_data();
    SDL_Quit();
    return 0;
}

例如,当我收到SDL_APP_WILLENTERBACKGROUND时,应该在HandleAppEvents()或main()中放置什么样的代码来销毁内存或停止我的游戏循环？

假设tex2是可消耗的,如果应用程序收到SDL_APP_LOWMEMORY,则可以将其删除.如何在不弄乱其他数据的情况下从HandleAppEvents()中删除tex2？

userdata指针中有什么？

当我的应用程序进入后台时,我应该将纹理转换为曲面,并将它们作为bmps保存在../tmp/目录中,还是当应用程序回到前台时它们仍然在内存中？

我希望我的混乱问题有某种意义.如果有一个地方我可以找到SDL2的全面文档,那就太棒了.

谢谢参观！