How to add a welcome/loading screen with a progress bar?

Question

C++ / C#

Here is the code:

#include "Game.h"
Game::Game()
{
    run=true;
  bonus = false;
  Logic_ = new Logic();
  loading = true;
}
void__ Game::draw_screen()
{
    while(run)
    {
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_COLOR_MATERIAL);
    glEnable(GL_TEXTURE_2D);
    if (Math_->GetWin_() > 0 && !bonus && Math_->keyboard_->getF(3) == true)
    {
      bonus = true;
      Math_->keyboard_->setF(false, 3);
      Math_->keyboard_->setF(false, 0);
      Math_->keyboard_->setF(false, 1);
      Math_->keyboard_->setF(false, 2);
      Math_->keyboard_->setF(false, 4);
      Logic_->SetRandom();
    }
    if ((Math_->GetWin_() > 0 && bonus && Math_->keyboard_->getF(1) == true) || (bonus && Math_->GetWin_() == 0))
    {
      bonus = false;
      Math_->keyboard_->setF(false, 1);
      Math_->keyboard_->setF(false, 0);
      Math_->keyboard_->setF(false, 2);
      Math_->keyboard_->setF(false, 3);
      Math_->keyboard_->setF(false, 4);
    }
    if (!bonus){
      
      Scene1_->ShowDrum();
      //Scene1_->ShowWelcome(loading);
      //Scene_->Show(Math_->GetCountDrums(), Math_->GetRotate(), Math_->GetCountTextureOnDrums(), Math_->GetDrums(),
        //Math_->GetCredits(), Math_->GetWin_(), Math_->GetTotalBet(), Math_->GetLines_(), Math_->GetBet(),
        //Math_->GetLines(), Math_->GetMS(), Math_->keyboard_->GetF());
    }
    else
    {
      Logic_->SetCredits(Math_->GetCredits());
      Logic_->SetWin(Math_->GetWin_());
      Logic_->SetTotalBet(Math_->GetTotalBet());
      Scene2_->ShowBackGround(Math_->keyboard_->GetF(), Logic_->GetRandom(),
        Logic_->GetCredits(), Logic_->GetWin(), Logic_->GetTotalBet());
    }
    glDisable(GL_TEXTURE_2D);
#ifdef _WINDOWS_2
      Math_->keyboard_->keyboard__->Show();
#else
        SDL_GL_SwapWindow(window);
#endif
#ifndef _WINDOWS_2
        Math_->Update(&run,bonus,Logic_,event_);
#else
        Math_->Update(&run,bonus,Logic_);
#endif
    loading = false;
    }
    Exit();
}
void__ Game::setup_opengl(int__ width,int__ height)
{
    glClearColor(0,0,0,0);
    glViewport(0,0,width,height);
    glShadeModel(GL_SMOOTH);
    glCullFace(GL_BACK);
    glFrontFace(GL_CCW);
    glMatrixMode(GL_MODELVIEW);
    gluLookAt(0.0f,0.0f,1.0f,0.0f,0.0f,0.0f,0.0f,1.0f,0.0f);
    gluPerspective(189,0.5,0.6,0.6);
  ilInit(); // Инициализация основной библиотеки
  iluInit(); // Инициализация библиотеки утилит
  ilEnable(IL_CONV_PAL);
    ilEnable(IL_ORIGIN_SET);
    ilSetInteger(IL_ORIGIN_MODE, IL_ORIGIN_UPPER_LEFT);
}
int__ Game::Execute()
{
    Math_=new Math();
    bool fullscreen=Math_->Init();
#ifdef _WINDOWS_2
  if(!Math_->keyboard_->keyboard__->CreateWindow_(L"Tropic Island",Math_->GetResolution().x,Math_->GetResolution().y,32,fullscreen))
    return 0;
#else
    SDL_GLContext context;
    SDL_Init(SDL_INIT_VIDEO);
    window=SDL_CreateWindow("Tropic Island",0,0,Math_->GetResolution().x,Math_->GetResolution().y,SDL_WINDOW_OPENGL | SDL_WINDOW_SHOWN);
    context=SDL_GL_CreateContext(window);
    SDL_GL_SetSwapInterval(1);
#endif
  setup_opengl(Math_->GetResolution().x,Math_->GetResolution().y);

  Scene_ = new Scene();
  Scene2_ = new Scene2();
  Scene2_->SetData();
//	Scene2_->LoadWelcome();
  Scene1_ = new Scene1();
  Scene1_->LoadWelcome();
  Scene1_->LoadDrum();
    draw_screen();
#ifdef _WINDOWS_2
  Math_->keyboard_->keyboard__->KillWindow();
#else
    SDL_GL_DeleteContext(context);
    SDL_DestroyWindow(window);
    SDL_Quit();
#endif
    return 0;
}
void Game::Exit()
{
    run=false;
}
Game::~Game()
{
  //delete run;
    delete Math_;
    delete Scene_;
    delete Scene2_;
  delete Logic_;
}

Due to long loading Scene2_->SetData(); A white screen hangs for about 30 seconds, but you need to show the code from Scene1_-> ShowWelcome ();

Answer 1

The general approach is:
1) Move the loading of data to a separate thread
2) Show a splash screen or whatever
3) In a loop, wait until the loading of the data is completed, remembering to update the loading indicator and "skip" window system events so that the splash screen does not freeze and update . Here you will need some analogue of the event loop.
This is the simplest approach. Various variations are possible here.
By the way, if you use some kind of GUI framework, then an approach similar to that described above is probably implemented there. In SDL, I'm pretty sure there is an event loop in some form. Dig in this direction.