modal box for little vGL

static lv_obj_t* mbox, * info;
static const char welcom_info[] = { "Welcome to the modal message box demo!\n"
                                    "Press the button to display a message box." };
static const char in_msg_info[] = {"Notice that you cannot touch "
"the button again while the message box is open."};

static void mbox_evnet_handler(lv_obj_t* obj, lv_event_t event)
{
    if (event == LV_EVENT_DELETE && obj == mbox)
    {
        lv_obj_del_async(lv_obj_get_parent(mbox)); //异步删除message box的父窗口
        mbox = NULL;                                //消息框对象置成空指针
        lv_label_set_text(info, welcom_info);      //设置文本
        printf("LV_EVENT_DELETE\n");
    }
    else if (event == LV_EVENT_VALUE_CHANGED)
    {
        lv_mbox_start_auto_close(mbox, 0);  //销废message box
        printf("LV_EVENT_VALUE_CHANGED\n");
    }
}

static void btn_event_handler(lv_obj_t* obj, lv_event_t event)
{
    if (event == LV_EVENT_CLICKED) //按键已点击
    {
        static lv_style_t modal_style;
        /*Create a full-screen background*/
        lv_style_copy(&modal_style, &lv_style_plain_color);
        /*Set the background's style*/
        modal_style.body.main_color = LV_COLOR_BLACK; 
        modal_style.body.grad_color = LV_COLOR_BLACK;
        modal_style.body.opa = LV_OPA_50;     
        /*Create a base object for the modal background*/
        lv_obj_t* obj = lv_obj_create(lv_scr_act(), NULL);
        lv_obj_set_style(obj, &modal_style);
        lv_obj_set_pos(obj, 0, 0);
        lv_obj_set_size(obj, LV_HOR_RES, LV_VER_RES);
        lv_obj_set_opa_scale_enable(obj, true);

        static const char* btns2[] = {
            "OK","Cancel",""
        };

        mbox = lv_mbox_create(obj, NULL);   //创建消息对话框对象
        lv_mbox_add_btns(mbox, btns2);
        lv_mbox_set_text(mbox, "Hello world!");   //设置消息文本
        lv_obj_align(mbox, NULL, LV_ALIGN_CENTER, 0, 0);
        lv_obj_set_event_cb(mbox, mbox_evnet_handler); //设置消息对话框按键回调

        /*Fade the message box in with an animation*/
        lv_anim_t a;
        lv_anim_init(&a);
        lv_anim_set_time(&a, 500, 0);
        lv_anim_set_values(&a, LV_OPA_TRANSP, LV_OPA_COVER);
        lv_anim_set_exec_cb(&a, obj, (lv_anim_exec_xcb_t)lv_obj_set_opa_scale);
        lv_anim_create(&a);

        lv_label_set_text(info, in_msg_info);  //label文本显示更新
        lv_obj_align(info, NULL, LV_ALIGN_CENTER, 5, -5);

    }
}

void message_box_demo2(void)
{
    /*Create a button,then set its position and event callback*/
    lv_obj_t* btn = lv_btn_create(lv_scr_act(), NULL);
    lv_obj_set_size(btn, 200, 60);
    lv_obj_set_event_cb(btn, btn_event_handler);
    lv_obj_align(btn, NULL, LV_ALIGN_IN_TOP_LEFT, 20, 20);

    /*Create a label on the button*/
    lv_obj_t* label = lv_label_create(btn, NULL);
    lv_label_set_text(label, "Display a message box!");

    /*Create an informative label on the screen*/
    info = lv_label_create(lv_scr_act(), NULL);
    lv_label_set_text(info, welcom_info);   //显示信息
    lv_label_set_long_mode(info, LV_LABEL_LONG_BREAK);
    
    lv_obj_set_width(info, LV_HOR_RES - 10);
    lv_obj_align(info, NULL, LV_ALIGN_IN_BOTTOM_LEFT, 5, -5);
}