Qt高级——QtDBus快速入门

DBus for IPC in Qt

1. Client :: Method Call

1.1. 方法1: 直接使用 Message 传递消息（Low-Level-API）

// 传参数
QDBusMessage msg = QDBusMessage::createMethodCall("com.myabc.service",
                             "/", "com.myabc.interface", "setName");
msg << QString("Bright");
QDBusMessage response = QDBusConnection::sessionBus().call(msg);

// 获取返回值
QDBusMessage msg = QDBusMessage::createMethodCall("com.myabc.service",
                             "/", "com.myabc.interface", "name");
QDBusMessage response = QDBusConnection::sessionBus().call(msg);

// 判断 Method 是否被正确返回
if(response.type() == QDBusMessage::ReplyMessage)
{
    // QDBusMessage的arguments不仅可以用来存储发送的参数，也用来存储返回值
    // 这里取得 checkIn 的返回值
    QString name= response.arguments().takeFirst().toString();
}

1.2. 方法2: 通过 DBusInterface 调用方法（同步+异步）

QDBusInterface interface("com.myabc.service", "/",
                         "com.myabc.interface",
                         QDBusConnection::sessionBus());
if(!interface.isValid())
{
    qDebug() << qPrintable(QDBusConnection::sessionBus().lastError().message());
    exit(1);
}

// 调用远程对象的方法 setName()
interface.call("setName", "Bright");

// 调用 name() 并获取返回值
QDBusReply<QString> reply = interface.call("name");
if(reply.isValid())
{
    QString value = reply.value();
    qDebug() << "value = " << value ;
}

interface::asyncCall( ) 异步调用

QDBusPendingCall async = interface->asyncCall("setName", "Brion");
// or use this: QDBusPendingReply<QString> reply = interface->asyncCall("RemoteMethod");
async.waitForFinished ();
QDBusPendingCallWatcher *watcher = new QDBusPendingCallWatcher(async, this);

QObject::connect(watcher, SIGNAL(finished(QDBusPendingCallWatcher*)),
                 this, SLOT(callFinishedSlot(QDBusPendingCallWatcher*)));

void MyClass::callFinishedSlot(QDBusPendingCallWatcher *call)
{
    QDBusPendingReply<QString> reply = *call;
    if (! reply.isError()) {
        QString name= reply.argumentAt<0>();
        qDebug() << "name = " << name;
    }
    call->deleteLater();
}

1.3. 方法3: 从XML导入代理类

1. 使用工具qdbuscpp2xml从object.h生成XML文件：

   指令: qdbuscpp2xml -M test.h -o com.scorpio.test.xml

   <!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN" "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">;;
   <node>
     <interface name="com.scorpio.test.value">
       <method name="maxValue">
         <arg type="i" direction="out"/>
       </method>
       <method name="minValue">
         <arg type="i" direction="out"/>
       </method>
       <method name="value">
         <arg type="i" direction="out"/>
       </method>
     </interface>
   </node>
   

2. 使用工具qdbusxml2cpp从XML文件生成继承自QDBusInterface的类

   指令: qdbusxml2cpp com.scorpio.test.xml -p valueInterface

   生成两个文件：valueInterface.cpp 和 valueInterface.h & valueInterface.h文件：

   /*
    * This file was generated by qdbusxml2cpp version 0.7
    * Command line was: qdbusxml2cpp com.scorpio.test.xml -p testInterface
    *
    * qdbusxml2cpp is Copyright (C) 2013 Digia Plc and/or its subsidiary(-ies).
    *
    * This is an auto-generated file.
    * Do not edit! All changes made to it will be lost.
    */
   
   #ifndef TESTINTERFACE_H_1526737677
   #define TESTINTERFACE_H_1526737677
   
   #include <QtCore/QObject>
   #include <QtCore/QByteArray>
   #include <QtCore/QList>
   #include <QtCore/QMap>
   #include <QtCore/QString>
   #include <QtCore/QStringList>
   #include <QtCore/QVariant>
   #include <QtDBus/QtDBus>
   
   /*
    * Proxy class for interface com.scorpio.test.value
    */
   class ComScorpioTestValueInterface: public QDBusAbstractInterface
   {
       Q_OBJECT
   public:
       static inline const char *staticInterfaceName()
       { return "com.scorpio.test.value"; }
   
   public:
       ComScorpioTestValueInterface(const QString &service, const QString &path, const QDBusConnection &connection, QObject *parent = 0);
   
       ~ComScorpioTestValueInterface();
   
   public Q_SLOTS: // METHODS
       inline QDBusPendingReply<int> maxValue()
       {
           QList<QVariant> argumentList;
           return asyncCallWithArgumentList(QLatin1String("maxValue"), argumentList);
       }
   
       inline QDBusPendingReply<int> minValue()
       {
           QList<QVariant> argumentList;
           return asyncCallWithArgumentList(QLatin1String("minValue"), argumentList);
       }
   
       inline QDBusPendingReply<int> value()
       {
           QList<QVariant> argumentList;
           return asyncCallWithArgumentList(QLatin1String("value"), argumentList);
       }
   
   Q_SIGNALS: // SIGNALS
   };
   
   namespace com {
     namespace scorpio {
       namespace test {
         typedef ::ComScorpioTestValueInterface value;
       }
     }
   }
   #endif
   

   /*
    * This file was generated by qdbusxml2cpp version 0.7
    * Command line was: qdbusxml2cpp com.scorpio.test.xml -p testInterface
    *
    * qdbusxml2cpp is Copyright (C) 2013 Digia Plc and/or its subsidiary(-ies).
    *
    * This is an auto-generated file.
    * This file may have been hand-edited. Look for HAND-EDIT comments
    * before re-generating it.
    */
   
   #include "testInterface.h"
   
   /*
    * Implementation of interface class ComScorpioTestValueInterface
    */
   
   ComScorpioTestValueInterface::ComScorpioTestValueInterface(const QString &service, const QString &path, const QDBusConnection &connection, QObject *parent)
       : QDBusAbstractInterface(service, path, staticInterfaceName(), connection, parent)
   {
   }
   
   ComScorpioTestValueInterface::~ComScorpioTestValueInterface()
   {
   }
   

   调用Proxy类访问Service如下：

   #include <QCoreApplication>
   #include <QDBusMessage>
   #include <QDBusConnection>
   #include <QDBusReply>
   #include <QDBusInterface>
   #include <QDebug>
   #include "testInterface.h"
   
   int main(int argc, char *argv[])
   {
       QCoreApplication a(argc, argv);
       // 初始化自动生成的Proxy类com::scorpio::test::value
       com::scorpio::test::value test("com.scorpio.test",
                                      "/test/objects",
                                      QDBusConnection::sessionBus());
       // 调用value方法
       QDBusPendingReply<int> reply = test.value();
       //qdbusxml2cpp生成的Proxy类是采用异步的方式来传递Message，
       //所以需要调用waitForFinished来等到Message执行完成
       reply.waitForFinished();
       if (reply.isValid())
       {
           int value = reply.value();
           qDebug() << QString("value =  %1").arg(value);
       }
       else
       {
           qDebug() << "value method called failed!";
       }
   
       return a.exec();
   }
   

2. Subscriber :: Signal Catching

2.1. 方法1：BusConnection捕获信号

QDBusConnection::sessionBus().connect("com.brion.service", "/",
                                      "com.brion.interface",
                                      "ageChanged", this,
                                      SLOT(onAgeChanged(int)));

2.2. 方法2：通过Proxy/Interface捕获信号

QDBusInterface *interface = new QDBusInterface("com.brion.service", "/",
                                               "com.brion.interface",
                                               DBusConnection::sessionBus());

QObject::connect(&interface, SIGNAL(ageChanged(int)),
                 object, SLOT(onAgeChanged(int)));

3. Server/Publisher :: Register Service

3.1. 方式1：编写服务类，并注册服务和接口对象

class Person : public QObject
{
    Q_OBJECT

    Q_CLASSINFO("D-Bus Interface", "com.brion.interface")

public:
    explicit Person(QObject *parent = 0);

signals:
    void nameChanged(QString);
    void ageChanged(int);

public slots:
    QString name() const { return m_name; }
    // can‘t be reference
    void setName(QString name) {
        m_name = name;
    }

    int age() const { return m_age; }
    void setAge(int age) {
        m_age = age;
    }

private:
    QString m_name;
    int m_age;
};

// main.cpp
#include <QtDBus/QDBusConnection>
#include <person.h>
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    QDBusConnection sessionBus = QDBusConnection::sessionBus();
    if (sessionBus.registerService("com.brion.service")) {
        sessionBus.registerObject("/", new Person(),
                                  QDBusConnection::ExportAllContents);
    }
    return a.exec();
}

示例2：PingPong Game

Publisher，直接继承自 QDBusAbstractAdaptor：

class Pong: public QDBusAbstractAdaptor
{
    Q_OBJECT
    Q_CLASSINFO("D-Bus Interface", "org.example.QtDBus.ComplexPong.Pong")
    Q_PROPERTY(QString value READ value WRITE setValue)
public:
    QString m_value;
    QString value() const;
    void setValue(const QString &newValue);

    Pong(QObject *obj) : QDBusAbstractAdaptor(obj)
    { }
signals:
    void aboutToQuit();
public slots:
    QDBusVariant query(const QString &query);
    Q_NOREPLY void quit();
};

//  启动服务
int main(int argc, char **argv)
{
    QCoreApplication app(argc, argv);

    QObject obj;
    Pong *pong = new Pong(&obj);
    QObject::connect(&app, &QCoreApplication::aboutToQuit, pong, &Pong::aboutToQuit);
    pong->setProperty("value", "initial value");
    QDBusConnection::sessionBus().registerObject("/", &obj);

    if (!QDBusConnection::sessionBus().registerService(SERVICE_NAME)) {
        fprintf(stderr, "%s\n",
                qPrintable(QDBusConnection::sessionBus().lastError().message()));
        exit(1);
    }

    app.exec();
    return 0;
}

Subscriber端，监听Publisher的信号：

class Ping: public QObject
{
    Q_OBJECT
public slots:
    void start(const QString &);
public:
    QFile qstdin;
    QDBusInterface *iface;
};

void Ping::start(const QString &name)
{
    if (name != SERVICE_NAME)
        return;

    // open stdin for reading
    qstdin.open(stdin, QIODevice::ReadOnly);

    // find our remote
    iface = new QDBusInterface(SERVICE_NAME, "/", "org.example.QtDBus.ComplexPong.Pong",
                               QDBusConnection::sessionBus(), this);
    if (!iface->isValid()) {
        fprintf(stderr, "%s\n",
                qPrintable(QDBusConnection::sessionBus().lastError().message()));
        QCoreApplication::instance()->quit();
    }

    connect(iface, SIGNAL(aboutToQuit()), QCoreApplication::instance(), SLOT(quit()));

    while (true) {
        printf("Ask your question: ");

        QString line = QString::fromLocal8Bit(qstdin.readLine()).trimmed();
        if (line.isEmpty()) {
            iface->call("quit");
            return;
        } else if (line == "value") {
            QVariant reply = iface->property("value");
            if (!reply.isNull())
                printf("value = %s\n", qPrintable(reply.toString()));
        } else if (line.startsWith("value=")) {
            iface->setProperty("value", line.mid(6));
        } else {
            QDBusReply<QDBusVariant> reply = iface->call("query", line);
            if (reply.isValid())
                printf("Reply was: %s\n", qPrintable(reply.value().variant().toString()));
        }

        if (iface->lastError().isValid())
            fprintf(stderr, "Call failed: %s\n", qPrintable(iface->lastError().message()));
    }
}


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

    if (!QDBusConnection::sessionBus().isConnected()) {
        fprintf(stderr, "Cannot connect to the D-Bus session bus.\n"
                "To start it, run:\n"
                "\teval `dbus-launch --auto-syntax`\n");
        return 1;
    }

    QDBusServiceWatcher serviceWatcher(SERVICE_NAME, QDBusConnection::sessionBus(),
                                       QDBusServiceWatcher::WatchForRegistration);

    Ping ping;
    QObject::connect(&serviceWatcher, &QDBusServiceWatcher::serviceRegistered,
                     &ping, &Ping::start);

    QProcess pong;
    pong.start("./complexpong");

    app.exec();
}

3.2. 方式2：通过 XML 定义并转换为Adapter对象发布服务

方法4: DBusAdapter 生成Adapter类的流程如下：

1. 使用工具 qdbuscpp2xml从test.h生成XML文件

   qdbuscpp2xml -M test.h -o com.scorpio.test.xml

2. 编辑com.scorpio.test.xml，选择需要发布的method，不需要发布的删除。

3. 使用工具qdbusxml2cpp从XML文件生成继承自QDBusInterface的类

   qdbusxml2cpp com.scorpio.test.xml -i test.h -a valueAdaptor

   生成两个文件：valueAdaptor.cpp和valueAdaptor.h & valueAdaptor.h文件：

   /*
    + This file was generated by qdbusxml2cpp version 0.7
    + Command line was: qdbusxml2cpp com.scorpio.test.xml -i test.h -a valueAdaptor
    *
    + qdbusxml2cpp is Copyright (C) 2013 Digia Plc and/or its subsidiary(-ies).
    *
    + This is an auto-generated file.
    + This file may have been hand-edited. Look for HAND-EDIT comments
    + before re-generating it.
    */
   
   #ifndef VALUEADAPTOR_H_1526742670
   #define VALUEADAPTOR_H_1526742670
   
   #include <QtCore/QObject>
   #include <QtDBus/QtDBus>
   #include "test.h"
   class QByteArray;
   template<class T> class QList;
   template<class Key, class Value> class QMap;
   class QString;
   class QStringList;
   class QVariant;
   
   /*
    + Adaptor class for interface com.scorpio.test.value
    */
   class ValueAdaptor: public QDBusAbstractAdaptor
   {
       Q_OBJECT
       Q_CLASSINFO("D-Bus Interface", "com.scorpio.test.value")
       Q_CLASSINFO("D-Bus Introspection", ""
   "  <interface name=\"com.scorpio.test.value\">\n"
   "    <method name=\"maxValue\">\n"
   "      <arg direction=\"out\" type=\"i\"/>\n"
   "    </method>\n"
   "    <method name=\"minValue\">\n"
   "      <arg direction=\"out\" type=\"i\"/>\n"
   "    </method>\n"
   "  </interface>\n"
           "")
   public:
       ValueAdaptor(QObject *parent);
       virtual ~ValueAdaptor();
   
   public: // PROPERTIES
   public Q_SLOTS: // METHODS
       int maxValue();
       int minValue();
   Q_SIGNALS: // SIGNALS
   };
   
   #endif
   

   /*
    + This file was generated by qdbusxml2cpp version 0.7
    + Command line was: qdbusxml2cpp com.scorpio.test.xml -i test.h -a valueAdaptor
    *
    + qdbusxml2cpp is Copyright (C) 2013 Digia Plc and/or its subsidiary(-ies).
    *
    + This is an auto-generated file.
    + Do not edit! All changes made to it will be lost.
    */
   
   #include "valueAdaptor.h"
   #include <QtCore/QMetaObject>
   #include <QtCore/QByteArray>
   #include <QtCore/QList>
   #include <QtCore/QMap>
   #include <QtCore/QString>
   #include <QtCore/QStringList>
   #include <QtCore/QVariant>
   
   /*
    + Implementation of adaptor class ValueAdaptor
    */
   
   ValueAdaptor::ValueAdaptor(QObject *parent)
       : QDBusAbstractAdaptor(parent)
   {
       // constructor
       setAutoRelaySignals(true);
   }
   
   ValueAdaptor::~ValueAdaptor()
   {
       // destructor
   }
   
   int ValueAdaptor::maxValue()
   {
       // handle method call com.scorpio.test.value.maxValue
       int out0;
       QMetaObject::invokeMethod(parent(), "maxValue", Q_RETURN_ARG(int, out0));
       return out0;
   }
   
   int ValueAdaptor::minValue()
   {
       // handle method call com.scorpio.test.value.minValue
       int out0;
       QMetaObject::invokeMethod(parent(), "minValue", Q_RETURN_ARG(int, out0));
       return out0;
   }
   

   调用Adaptor类注册Object对象如下：

   #include <QCoreApplication>
   #include <QDBusConnection>
   #include <QDebug>
   #include <QDBusError>
   #include "test.h"
   #include "valueAdaptor.h"
   
   int main(int argc, char *argv[]){
       QCoreApplication a(argc, argv);
       QDBusConnection connection = QDBusConnection::sessionBus();
   
       test object(60);
       //ValueAdaptor是qdbusxml2cpp生成的Adaptor类
       ValueAdaptor valueAdaptor(&object);
       if (!connection.registerService("com.scorpio.test"))
       {
           qDebug() << connection.lastError().message();
           exit(1);
       }
       connection.registerObject("/test/objects", &object);
       return a.exec();
   }
   

3.3. 其他：自启动DBus服务项

D-Bus系统提供了一种机制可以在访问某个service时，自动把应用程序运行起来。

需要在/usr/share/dbus-1/services下面建立com.scorpio.test.service文件，文件的内容如下：

**[D-****BUS Service]**
Name=com.scorpio.test
Exec=/path/to/scorpio/test

在访问test的method前，不必手动运行应用程序。

4. QtDBus 在 PyQt5 中的应用

常用类即其方法，请参考脑图。

4.1. 定义服务类

这里提供一个实例代码（PingPong在PyQt5中的实现）：

import sys

from PyQt5.QtCore import QCoreApplication, pyqtSlot, QObject
from PyQt5.QtDBus import QDBusConnection, QDBusInterface, QDBusReply, QDBusAbstractAdaptor

class Pong(QObject):
    @pyqtSlot(str, result=str)
    def pong_method(self, args):
        print("Get a proxy-method call... the args is: ", args)
        return ‘ping("%s") got called.‘ % args

    @pyqtSlot()
    def pong_method_without_params(self):
        print("Get a proxy-method call...")


if __name__ == "__main__":
    app = QCoreApplication(sys.argv)
    bus = QDBusConnection.sessionBus()  # sessionBus()
    if not bus.isConnected():
        raise Exception("Fail to connect to Session-Bus.")
    # register the service
    if not bus.registerService(‘org.example.QtDBus.PingPong‘):  # 注册 BusName
        raise Exception("Fail to register the service.")

    pong = Pong()  # 创建对象 -> dbus server object
    print("-->> ", isinstance(pong, QDBusAbstractAdaptor))
    bus.registerObject("/",  # this is the object-path, must be start from ‘/‘
                        "pong.anything.namespace",  # this is the interface [choose to name it]
                        pong,  # this is the server obj, but you must prepare it before
                        # QDBusConnection.ExportAllSlots)  # 注册所有slot作为proxy-method
                        QDBusConnection.ExportAllContents)
    sys.exit(app.exec_())

4.2. 结合XML定义Adapter类

class Car(QObject):
    def turn_left(self, degree: int):
        print("The car turn left [{}] degree.".format(degree))

    def turn_right(self, degree: int):
        print("The car turn right [{}] degree.".format(degree))

    def turn_back(self):
        print("The car is turnning back.")


class CarInterface(QDBusAbstractAdaptor):
    Q_CLASSINFO("D-Bus Interface", ‘org.HallGarden.Examples.Interface‘)
    Q_CLASSINFO("D-Bus Introspection", ‘‘
            ‘  <interface name="org.HallGarden.Examples.Interface">\n‘
            ‘    <method name="turn_left">\n‘
            ‘      <arg direction="in" type="i"/>\n‘
            ‘    </method>\n‘
            ‘    <method name="turn_right">\n‘
            ‘      <arg name="degree" direction="in" type="i"/>\n‘
            ‘      <arg name="any" direction="out" type="i"/>\n‘
            ‘    </method>\n‘
            ‘    <method name="turn_back"/>\n‘
            ‘  </interface>\n‘
            ‘‘)

    def __init__(self, parent):
        super().__init__(parent)
        self.setAutoRelaySignals(True)

    @pyqtSlot(int)
    def turn_left(self, degree):
        self.parent().turn_left(degree)

    @pyqtSlot(int)
    def turn_right(self, degree):
        self.parent().turn_right(degree)
        return 30

    @pyqtSlot()
    def turn_back(self):
        self.parent().turn_back()


if __name__ == "__main__":
    app = QCoreApplication(sys.argv)

    car = Car()
    CarInterface(car)  # 装饰car对象，而新生成的对象并没实际应用

    connection = QDBusConnection.sessionBus()
    connection.registerService(‘org.example.CarExample‘)
    connection.registerObject(‘/Car‘, car)

    sys.exit(app.exec_())

5. 思考

1. 在 freedesktop 体系中，将interface至于Object下层。于是对于binding的设计，一般的（比如dbus-python）proxy可以获得object的代理，而interface依旧是对象的namespace，故而在proxy下层。但Qt的命名似乎不太一样——它所谓的 Interface 对象与代理很相似，而Qt概念中的代理，一般是通过 XML 转换生成的。

2. 一般的binding做到Proxy就可以了，而且一般Proxy确实够用了；而Qt又设计了Adapter类，用于将DBus信号绑定QObject，其意义何在？

3. 关于dbus消息传递时的载体是二进制流——那么它解码后的内容是中间格式的XML吗？亦或是给底层 lib 库的C语言数据结构？

4. 关于XML是为了实现跨平台、跨语言。但问题是，流程不应该是将代码的结构，例如根据 QDBusConnection.ExportAllContents 的性质，将 @pyqtSlot 槽函数抽象为中间代码，也就是XML，再将XML发送给daemon读取形成总线服务或信号。但实际上这个过程却颠倒过来了——如果你使用QDBusAdapter，你需要将xml文本写入，或者通过 qdbusxml2cpp 再生成C++代码。除了逻辑上的颠倒，更麻烦的是，没有任何工具提供了静态编译验证XML语法或语义的 debug 调试工具。一旦你不小心在中少写了一个“/”结束符，程序并不报错，只是服务项一直不正常~ 呃！

QtDbus的API及示例