Sub-Attaq

This example shows Qt's ability to combine the animation framework and the state machine framework to create a game.

The purpose of the game is to destroy all submarines to win the current level. The boat can be controlled using left and right keys. To fire a bomb you can press the up and down keys.

The `main()` Function

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    Q_INIT_RESOURCE(subattaq);

    MainWindow w;
    w.show();

    return app.exec();
}

The MainWindow instance is created and shown.

The `MainWindow` Class

    QMenu *file = menuBar()->addMenu(tr("&File"));

    QAction *newAction = file->addAction(tr("New Game"));
    newAction->setShortcut(QKeySequence(Qt::CTRL | Qt::Key_N));
    QAction *quitAction = file->addAction(tr("Quit"));
    quitAction->setShortcut(QKeySequence(Qt::CTRL | Qt::Key_Q));

    if (QApplication::arguments().contains("-fullscreen")) {
        scene = new GraphicsScene(0, 0, 750, 400, GraphicsScene::Small, this);
        setWindowState(Qt::WindowFullScreen);
    } else {
        scene = new GraphicsScene(0, 0, 880, 630, GraphicsScene::Big, this);
        layout()->setSizeConstraint(QLayout::SetFixedSize);
    }

    view = new QGraphicsView(scene, this);
    view->setAlignment(Qt::AlignLeft | Qt::AlignTop);
    scene->setupScene(newAction, quitAction);
    setCentralWidget(view);

MainWindow extends QMainWindow and contains the GraphicsScene instance. It creates and sets up the menu bar as well.

The `GraphicsScene` Class

There are several state machines in the application. The GraphicsScene state machine handles states related to events outside the actual game scene like the letter animation in the beginning.

The `GraphicsScene` Constructor

    PixmapItem *backgroundItem = new PixmapItem(QStringLiteral("background"), mode);
    backgroundItem->setZValue(1);
    backgroundItem->setPos(0,0);
    addItem(backgroundItem);

    PixmapItem *surfaceItem = new PixmapItem(QStringLiteral("surface"), mode);
    surfaceItem->setZValue(3);
    surfaceItem->setPos(0, sealLevel() - surfaceItem->boundingRect().height() / 2);
    addItem(surfaceItem);

    //The item that displays score and level
    progressItem = new ProgressItem(backgroundItem);

    textInformationItem = new TextInformationItem(backgroundItem);

    textInformationItem->setMessage(QString("Select new game from the menu or press Ctrl+N to start!<br/>Press left or right to move the ship and up to drop bombs."), false);
    textInformationItem->setPos(backgroundItem->boundingRect().center().x() - textInformationItem->boundingRect().size().width() / 2,
                                backgroundItem->boundingRect().height() * 3 / 4);

    //We create the boat
    addItem(boat);
    boat->setPos(this->width()/2, sealLevel() - boat->size().height());
    boat->hide();

The GraphicsScene class contains the background images and the score and level information texts.

`The` setupScene Method

    QStateMachine *machine = new QStateMachine(this);

    //This state is when the player is playing
    PlayState *gameState = new PlayState(this, machine);

    //Final state
    QFinalState *finalState = new QFinalState(machine);

    //Animation when the player enters the game
    QAnimationState *lettersMovingState = new QAnimationState(machine);
    lettersMovingState->setAnimation(lettersGroupMoving);

    //Animation when the welcome screen disappears
    QAnimationState *lettersFadingState = new QAnimationState(machine);
    lettersFadingState->setAnimation(lettersGroupFading);

    //if it is a new game then we fade out the welcome screen and start playing
    lettersMovingState->addTransition(newAction, &QAction::triggered, lettersFadingState);
    lettersFadingState->addTransition(lettersFadingState, &QAnimationState::animationFinished, gameState);

    //New Game is triggered then player starts playing
    gameState->addTransition(newAction, &QAction::triggered, gameState);

    //Wanna quit, then connect to CTRL+Q
    gameState->addTransition(quitAction, &QAction::triggered, finalState);
    lettersMovingState->addTransition(quitAction, &QAction::triggered, finalState);

    //Welcome screen is the initial state
    machine->setInitialState(lettersMovingState);

    machine->start();

    //We reach the final state, then we quit
    connect(machine, &QStateMachine::finished, qApp, &QApplication::quit);

The four state machine states are created with sequential transitions from one to the next. The gameState also has a transition that is triggered by newAction, the new game menu item, or its shortcut key at any point in the application. The gameState state is an instance of the PlayState class.

The `PlayState` Class

The PlayState class is a QState derived class that handles the state when the game is in progress.

    machine = new QStateMachine;

    //This state is active when the player is playing
    LevelState *levelState = new LevelState(scene, this, machine);

    //This state is active when the player is actually playing but the game is not paused
    QState *playingState = new QState(levelState);
    levelState->setInitialState(playingState);

    //This state is active when the game is paused
    PauseState *pauseState = new PauseState(scene, levelState);

    //We have one view, it receives the key press events
    QKeyEventTransition *pressPplay = new QKeyEventTransition(scene->views().at(0), QEvent::KeyPress, Qt::Key_P);
    pressPplay->setTargetState(pauseState);
    QKeyEventTransition *pressPpause = new QKeyEventTransition(scene->views().at(0), QEvent::KeyPress, Qt::Key_P);
    pressPpause->setTargetState(playingState);

    //Pause "P" is triggered, when the player pauses the game
    playingState->addTransition(pressPplay);

    //To get back to playing when the game has been paused
    pauseState->addTransition(pressPpause);

    //This state is active when player has lost
    LostState *lostState = new LostState(scene, this, machine);

    //This state is active when player has won
    WinState *winState = new WinState(scene, this, machine);

    //If boat has been destroyed then the game is finished
    levelState->addTransition(scene->boat, &Boat::boatExecutionFinished,lostState);

    //This transition checks if we have won or not
    WinTransition *winTransition = new WinTransition(scene, this, winState);

    //If boat has been destroyed then the game is finished
    levelState->addTransition(winTransition);

    //This state is for an animation when the score changes
    UpdateScoreState *scoreState = new UpdateScoreState(levelState);

    //This transition updates the score when a submarine is destroyed
    UpdateScoreTransition *scoreTransition = new UpdateScoreTransition(scene, this, levelState);
    scoreTransition->setTargetState(scoreState);

    //If the boat has been destroyed then the game is finished
    playingState->addTransition(scoreTransition);

    //We go back to play state
    scoreState->addTransition(playingState);

    //We start playing!!!
    machine->setInitialState(levelState);

    //Final state
    QFinalState *finalState = new QFinalState(machine);

    //This transition is triggered when the player presses space after completing a level
    CustomSpaceTransition *spaceTransition = new CustomSpaceTransition(scene->views().at(0), this, QEvent::KeyPress, Qt::Key_Space);
    spaceTransition->setTargetState(levelState);
    winState->addTransition(spaceTransition);

    //We lost so we should reach the final state
    lostState->addTransition(lostState, &QState::finished, finalState);

    scene->textInformationItem->hide();
    machine->start();

The PlayState state machine handles higher level game logic like pausing the game and updating the score.

The playingState state is a QState instance that is active while the user is actively playing the game. The pauseState is set up with transitions to and from playingState, which are triggered by pressing the p key. The lostState is created with a transition to it, which is triggered when the boat is destroyed. The winState is also created here with a transition to and from the levelState.

The `LevelState` Class

void LevelState::initializeLevel()
{
    //we re-init the boat
    scene->boat->setPos(scene->width()/2, scene->sealLevel() - scene->boat->size().height());
    scene->boat->setCurrentSpeed(0);
    scene->boat->setCurrentDirection(Boat::None);
    scene->boat->setBombsLaunched(0);
    scene->boat->show();
    scene->setFocusItem(scene->boat, Qt::OtherFocusReason);
    scene->boat->run();

    scene->progressItem->setScore(game->score);
    scene->progressItem->setLevel(game->currentLevel + 1);

    const GraphicsScene::LevelDescription currentLevelDescription = scene->levelsData.value(game->currentLevel);
    for (const QPair<int,int> &subContent : currentLevelDescription.submarines) {

        GraphicsScene::SubmarineDescription submarineDesc = scene->submarinesData.at(subContent.first);

        for (int j = 0; j < subContent.second; ++j ) {
            SubMarine *sub = new SubMarine(submarineDesc.type, submarineDesc.name, submarineDesc.points);
            scene->addItem(sub);
            int random = QRandomGenerator::global()->bounded(15) + 1;
            qreal x = random == 13 || random == 5 ? 0 : scene->width() - sub->size().width();
            qreal y = scene->height() -(QRandomGenerator::global()->bounded(150) + 1) - sub->size().height();
            sub->setPos(x,y);
            sub->setCurrentDirection(x == 0 ? SubMarine::Right : SubMarine::Left);
            sub->setCurrentSpeed(QRandomGenerator::global()->bounded(3) + 1);
        }
    }
}

The components of the scene are initialized based on what level the player has reached.

Example project @ code.qt.io

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Sub-Attaq

The main() Function

The MainWindow Class

The GraphicsScene Class

The GraphicsScene Constructor

The setupScene Method

The PlayState Class