ConnectCPPtoQML.md

Connect CPP to QML

CPP Property setting

1. In car2qml.h, we set variable, status of variable and status of dbus connection.

QDBusReply<qreal> speed;
QDBusReply<qreal> battery;
QDBusReply<qreal> temp;
QDBusReply<bool> brake;

bool speedstatus;
bool batterystatus;
bool tempstatus;
bool brakestatus;

bool dbusstatus;

• variable is declared as QDBusReply, which means that variable will be updated by Dbus Interface.

2. To Expose Attributes of C++ Types to QML, we used Q_PROPERTY macro.

Q_PROPERTY(qreal speed READ getSpeed)
Q_PROPERTY(qreal battery READ getBattery)
Q_PROPERTY(qreal temp READ getTemp)
Q_PROPERTY(bool brake READ getBrake)

Q_PROPERTY(bool speedstatus READ getSpeedStatus CONSTANT)
Q_PROPERTY(bool batterystatus READ getBatteryStatus CONSTANT)
Q_PROPERTY(bool tempstatus READ getTempStatus CONSTANT)
Q_PROPERTY(bool brakestatus READ getBrakeStatus CONSTANT)

Q_PROPERTY(bool dbusstatus READ getDbusStatus CONSTANT)

• It looks like name in Q_PROPERTY and name of variable is same, but they are different.
• Name in Q_PROPERTY is used for exposing to QML and used as property name.
• READ associate function (getVariable) with property (Name).
• (Optional) WRITE can be used for set value of property.
• CONSTANT is for preventing depends on non-NOTIFYable properties error.

Connect to QML

1. Expose C++ types to QML in main.cpp

qmlRegisterType<Car2Qml>("Car2Qml", 1, 0, "Car2Qml");

2. Declare in QML

(1) Import Car2Qml Class as Property of QML in instrumentCluster.qml

 import Car2Qml 1.0

(2) Define Property and Connect Signal of Class in instrumentCluster.qml

Car2Qml{
        id: carinfo

        onSpeedChanged: {instrumentcluster.speed = carinfo.speed}
        onBatteryChanged: {instrumentcluster.battery = carinfo.battery}
        onTempChanged: {instrumentcluster.temp = carinfo.temp}
        onBrakeChanged: {instrumentcluster.brake = carinfo.brake}

        onSpeedstatusChanged: {instrumentcluster.speed_timeout = status}
        onBatterystatusChanged: {instrumentcluster.battery_timeout = status}
        onTempstatusChanged: {instrumentcluster.temp_timeout = status}
        onBrakestatusChanged: {instrumentcluster.brake_timeout = status}

        onDbusstatusChanged: {instrumentcluster.dbus_timeout = status}

    }

• As we set in car2qml.h by using Q_PROPERTY macro, we can access to getVariable and getVaraibleStatus functions by Name in Q_PROPERTY.
• However, we only update variable property by using Q_PROPERTY.
• variableStatus property is updated by signal.

3. About property Initialization in instrumentCluster.qml

property int speed: 0
property int battery: 0
property int temp: 0
property bool brake: false

property bool speed_timeout: carinfo.speedstatus
property bool battery_timeout: carinfo.batterystatus
property bool temp_timeout: carinfo.tempstatus
property bool brake_timeout: carinfo.brakestatus

property bool dbus_timeout: carinfo.dbusstatus

• Here, getVaraibleStatus is used for initialization.
• In this way, we can detect sender status when application starts.

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Reference

1. Exposing Attributes of C++ Types to QML
2. QQmlEngine Class (qmlRegisterType)
3. QDBusReply Class