How to implement a game-loop with C++ and QtQuick

Question

I'm developing a game with QtQuick 2 (Qt5.2) QML and C++. I want most of the game-logic in C++ (I don't want to do it with JS), and I'm trying to use QStateMachines for a lot of the logic.

Now my question is, how do I implement a proper game-loop in that scenario?

The game has for example objects that are moving from and to certain waypoints on the screen, so I think I can't do everything state/event-based. At some point I need to say "my object should move 10 pixels in direction x every second". So for example when my object is in its state "moving", it should move by a certain amount every second and then of course check for some conditions if it has to switch the state (e.g. to "attacking").

Now all the demos in the Qt-examples and on the web seem to be either fully event-based (e.g. four-in-a-row-wins-like) or written in JavaScript. So I am a bit lost here.

One idea I could find was to create a QTimer with a timer of e.g. 30ms and connect that QTimer's timeout() signal to an advance() slot of every moving object, and start that timer before 'return app.exec();'. Like this:

QTimer timer;
QObject::connect(&timer, SIGNAL(timeout()), &scene, SLOT(advance()));
timer.start(1000 / 33);
return app.exec();

and then each object (e.g. the Mouse) has a

void Mouse::advance(int step)

However, this requires a QGraphicsScene and I'm not sure how well that goes with a QtQuick/QML project on Android/iOS.

Is that a good solution? Or is my view of the problem somehow wrong and I don't need a game loop to accomplish my goal?

The solution shouldn't use any desktop-only stuff from Qt, i.e. it should work on Android, iOS and desktops.

Answer 1

That's the way to go: QTimer. Here you find some detailed example on it:

• A typical loop for game in Qt:

  int main(int argc, char* argv[]) { 
  //    init stuff
          while(game.isRunning()) {
                  a.processEvents(); //(a is a QApplication created during the init, should use a better name i guess)
                  QTime currentTime= QTime::currentTime();
                  int timeSinceLastUpdate = lastUpdate.msecsTo(currentTime);
                  while(timeSinceLastUpdate>updateTimeStep){
                          game.update();
                          timeSinceLastUpdate-=updateTimeStep;    
                          lastUpdate=lastUpdateaddMSecs(updateTimeStep);
                  }
                  renderer.setInterpolateFraction(static_cast<float>(timeSinceLastUpdate)/static_cast<float>updateTimeStep);
                  renderer.renderGameObjects();
                  renderer.renderGUI();
                  renderer.swap();    
          }
          a.exit();
          return 0; 
  }
  

Source: Game loop in Qt

• Making a Simple Game Loop with QTimer
• Qt as a game engine

That's should be enough info for you to get started.

Answer 2

Usual game loop of simple game can look like this (not sure if I understand you correctly though).

Each class that represents game object have 2 public methods: update(); and render(); On each call of QTimer object you iterate over all game objects and call their update method. After it you repeat the same for render method();

In update methods each object decides what to do on game map (move/shot/stand/...) and changes its coordinates/properties. In render methods each object just draws itself on display.