交通灯

交通灯示例展示了如何使用Qt State Machine Overview实现交通灯的控制流。

在这个示例中，我们编写了一个 TrafficLightWidget 类。交通灯有三个灯：红灯、黄灯和绿灯。交通灯以一定的间隔从一个灯转换到另一个灯（红灯到黄灯到绿灯到黄灯再到红灯）。

class LightWidget : public QWidget
{
    Q_OBJECT
    Q_PROPERTY(bool on READ isOn WRITE setOn)
public:
    explicit LightWidget(const QColor &color, QWidget *parent = nullptr)
        : QWidget(parent), m_color(color)
    {
    }

    bool isOn() const { return m_on; }

    void setOn(bool on)
    {
        if (on == m_on)
            return;
        m_on = on;
        update();
    }

public slots:
    void turnOff() { setOn(false); }
    void turnOn() { setOn(true); }

protected:
    void paintEvent(QPaintEvent *) override
    {
        if (!m_on)
            return;
        QPainter painter(this);
        painter.setRenderHint(QPainter::Antialiasing);
        painter.setBrush(m_color);
        painter.drawEllipse(rect());
    }

private:
    QColor m_color;
    bool m_on = false;
};

LightWidget 类表示交通灯中的一个灯。它提供了一个on 属性和两个槽：turnOn() 和 turnOff()，分别用于打开和关闭交通灯。该窗口小部件将自己涂上传给构造函数的颜色。

class TrafficLightWidget : public QWidget
{
    Q_OBJECT
public:
    explicit TrafficLightWidget(QWidget *parent = nullptr) : QWidget(parent)
    {
        auto vbox = new QVBoxLayout(this);
        m_red = new LightWidget(Qt::red);
        vbox->addWidget(m_red);
        m_yellow = new LightWidget(Qt::yellow);
        vbox->addWidget(m_yellow);
        m_green = new LightWidget(Qt::green);
        vbox->addWidget(m_green);
        auto pal = palette();
        pal.setColor(QPalette::Window, Qt::black);
        setPalette(pal);
        setAutoFillBackground(true);
    }

    LightWidget *redLight() const { return m_red; }
    LightWidget *yellowLight() const { return m_yellow; }
    LightWidget *greenLight() const { return m_green; }

private:
    LightWidget *m_red;
    LightWidget *m_yellow;
    LightWidget *m_green;
};

TrafficLightWidget 类表示交通灯的视觉部分；它是一个包含三个垂直排列的灯的 widget，并为这些灯提供了访问函数。

QState *createLightState(LightWidget *light, int duration, QState *parent = nullptr)
{
    auto lightState = new QState(parent);
    auto timer = new QTimer(lightState);
    timer->setInterval(duration);
    timer->setSingleShot(true);
    auto timing = new QState(lightState);
    QObject::connect(timing, &QAbstractState::entered, light, &LightWidget::turnOn);
    QObject::connect(timing, &QAbstractState::entered, timer, QOverload<>::of(&QTimer::start));
    QObject::connect(timing, &QAbstractState::exited, light, &LightWidget::turnOff);
    auto done = new QFinalState(lightState);
    timing->addTransition(timer, &QTimer::timeout, done);
    lightState->setInitialState(timing);
    return lightState;
}

createLightState() 函数创建了一个状态，当进入该状态时，灯会亮起，当退出该状态时，灯会熄灭。该状态使用一个计时器，正如我们将要看到的，超时用于从一个 LightState 过渡到另一个 LightState。下面是灯光状态的状态图：

class TrafficLight : public QWidget
{
    Q_OBJECT
public:
    explicit TrafficLight(QWidget *parent = nullptr) : QWidget(parent)
    {
        auto vbox = new QVBoxLayout(this);
        auto widget = new TrafficLightWidget;
        vbox->addWidget(widget);
        vbox->setContentsMargins(QMargins());

        auto machine = new QStateMachine(this);
        auto redGoingYellow = createLightState(widget->redLight(), 3000);
        redGoingYellow->setObjectName("redGoingYellow");
        auto yellowGoingGreen = createLightState(widget->yellowLight(), 1000);
        yellowGoingGreen->setObjectName("yellowGoingGreen");
        redGoingYellow->addTransition(redGoingYellow, &QState::finished, yellowGoingGreen);
        auto greenGoingYellow = createLightState(widget->greenLight(), 3000);
        greenGoingYellow->setObjectName("greenGoingYellow");
        yellowGoingGreen->addTransition(yellowGoingGreen, &QState::finished, greenGoingYellow);
        auto yellowGoingRed = createLightState(widget->yellowLight(), 1000);
        yellowGoingRed->setObjectName("yellowGoingRed");
        greenGoingYellow->addTransition(greenGoingYellow, &QState::finished, yellowGoingRed);
        yellowGoingRed->addTransition(yellowGoingRed, &QState::finished, redGoingYellow);

        machine->addState(redGoingYellow);
        machine->addState(yellowGoingGreen);
        machine->addState(greenGoingYellow);
        machine->addState(yellowGoingRed);
        machine->setInitialState(redGoingYellow);
        machine->start();
    }
};

TrafficLight 类将 TrafficLightWidget 与状态机相结合。状态图有四种状态：红到黄、黄到绿、绿到黄和黄到红。初始状态是红到黄；当该状态的定时器超时时，状态机就会转换为黄到绿。同样的过程会在其他状态中重复。这就是状态图的样子：

int main(int argc, char **argv)
{
    QApplication app(argc, argv);

    TrafficLight widget;
    widget.resize(110, 300);
    widget.show();

    return app.exec();
}

main()函数构造了一个交通灯并显示出来。

示例项目 @ code.qt.io

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