- Qt 5.15
- Qt Remote Objects
- Example 1: Direct Connection using a Static Source
- Qt 5.15.16 Reference Documentation

# Example 1: Direct Connection using a Static Source

1. Create a source object

   To create this Source object, first we create the definition file, `simpleswitch.rep`. This file describes the properties and methods for the object and is input to the Qt Remote Objects Compiler repc. This file only defines interfaces that are necessary to expose to the Replicas.

   `simpleswitch.rep`

        class SimpleSwitch
        {
            PROP(bool currState=false);
            SLOT(server_slot(bool clientState));
        };

   In `simpleswitch.rep`,

   - `currState` holds the current state of the switch.
   - `server_slot()` allows us to interact with the source - it will be connected to the `echoSwitchState(bool newstate)` signal.

   For repc to process this file, add the following line to your `.pro` file:

        REPC_SOURCE = simpleswitch.rep

   The `REPC_SOURCE` variable is only relevant for the Qt Remote Object module, so you need to add it to your project as well:

        QT       += remoteobjects

   repc creates the `rep_SimpleSwitch_source.h` header in the build directory that you specify. For more information, see Source.

   repc creates three helper classes for use with QtRO. For this example, we use the basic: `SimpleSwitchSimpleSource`. It's an abstract class, defined in `rep_SimpleSwitch_source.h`. We derive from it to define our SimpleSwitch implementation class as shown below:

   `simpleswitch.h`

        #ifndef SIMPLESWITCH_H
        #define SIMPLESWITCH_H

        #include "rep_SimpleSwitch_source.h"

        class SimpleSwitch : public SimpleSwitchSimpleSource
        {
            Q_OBJECT
        public:
            SimpleSwitch(QObject *parent = nullptr);
            ~SimpleSwitch();
            virtual void server_slot(bool clientState);
        public Q_SLOTS:
            void timeout_slot();
        private:
            QTimer *stateChangeTimer;
        };

        #endif

   In `simpleswitch.h`,

   - `stateChangeTimer` is a QTimer that is used to toggle the state of our SimpleSwitch.
   - `timeout_slot()` is connected to `stateChangeTimer`'s timeout() signal.
   - `server_slot()` -- which is called automatically on the source whenever any replica calls their version of the slot -- outputs the received value.
   - `currStateChanged(bool)`, defined in the repc-generated `rep_SimpleSwitch_source.h`, is emitted whenever `currState` toggles. In this example, we ignore the signal on the source side, and handle it later on the replica side.

   The definition of our `SwitchState` class is shown below:

   `simpleswitch.cpp`

        #include "simpleswitch.h"

        // constructor
        SimpleSwitch::SimpleSwitch(QObject *parent) : SimpleSwitchSimpleSource(parent)
        {
            stateChangeTimer = new QTimer(this); // Initialize timer
            QObject::connect(stateChangeTimer, &SimpleSwitch::timeout, this, &SimpleSwitch::timeout_slot); // connect timeout() signal from stateChangeTimer to timeout_slot() of simpleSwitch
            stateChangeTimer->start(2000); // Start timer and set timout to 2 seconds
            qDebug() << "Source Node Started";
        }

        //destructor
        SimpleSwitch::~SimpleSwitch()
        {
            stateChangeTimer->stop();
        }

        void SimpleSwitch::server_slot(bool clientState)
        {
            qDebug() << "Replica state is " << clientState; // print switch state echoed back by client
        }

        void SimpleSwitch::timeout_slot()
        {
            // slot called on timer timeout
            if (currState()) // check if current state is true, currState() is defined in repc generated rep_SimpleSwitch_source.h
                setCurrState(false); // set state to false
            else
                setCurrState(true); // set state to true
            qDebug() << "Source State is "<<currState();

        }

2. Create a registry

   Because this example uses a direct connection between nodes, we can omit this step.

3. Create a host node

   The host node is created as shown below:

        QRemoteObjectHost srcNode(QUrl(QStringLiteral("local:switch")));

4. Host source object and remoting

   The following statements instantiate the Source object and pass it on to the host to enable "remoting", which is the process of making an object visible to the QtRO network:

        SimpleSwitch srcSwitch; // create simple switch
        srcNode.enableRemoting(&srcSwitch); // enable remoting

   The contents of `main.cpp` file that implements the steps described above are as follows:

   `main.cpp`

        #include <QCoreApplication>
        #include "simpleswitch.h"

        int main(int argc, char *argv[])
        {
            QCoreApplication a(argc, argv);

            SimpleSwitch srcSwitch; // create simple switch

            QRemoteObjectHost srcNode(QUrl(QStringLiteral("local:switch"))); // create host node without Registry
            srcNode.enableRemoting(&srcSwitch); // enable remoting/sharing

            return a.exec();
        }

   Compile and run this source-side project. The output, without any replicas created, should look as shown below with the switch state toggling between `true` and `false` every two seconds.

The subsequent steps are for creating the replica side of the network, which in this example gets the state of switch from the Source and echoes it back.

### Replica Code

1. Use repc to add a replica to your project

   We use the same API definition file as we did on the source side, `SimpleSwitch.rep`, to create a Replica header file using the repc. Include the following line in your client side `.pro` file, specifying a `.rep` file input:

        REPC_REPLICA = simpleswitch.rep

   The repc tool generates a `rep_SimpleSwitch_replica.h` file in the build directory. For more information, see Replica.

2. Create a node to connect with the source's host node

   The following code instantiates the second node on the network and connects it with the source host node:

        QRemoteObjectNode repNode; // create remote object node
        repNode.connectToNode(QUrl(QStringLiteral("local:switch"))); // connect with remote host node

3. Call the node's acquire() to create a pointer to a replica

   First, we instantiate a replica:

        QSharedPointer<SimpleSwitchReplica> ptr;
        ptr.reset(repNode.acquire<SimpleSwitchReplica>()); // acquire replica of source from host node

   Note: acquire() returns a pointer to the replica, but doesn't manage its lifetime. This example shows the recommended process of wrapping the returned pointer in a QSharedPointer or QScopedPointer to ensure that the pointer is always deleted properly.

   `main.cpp` implements the steps described above and instantiates our object:

   `main.cpp`

        #include <QCoreApplication>
        #include "client.h"

        int main(int argc, char *argv[])
        {
            QCoreApplication a(argc, argv);

            QSharedPointer<SimpleSwitchReplica> ptr; // shared pointer to hold source replica

            QRemoteObjectNode repNode; // create remote object node
            repNode.connectToNode(QUrl(QStringLiteral("local:switch"))); // connect with remote host node

            ptr.reset(repNode.acquire<SimpleSwitchReplica>()); // acquire replica of source from host node

            Client rswitch(ptr); // create client switch object and pass reference of replica to it

            return a.exec();
        }

   The complete declaration and definition for the `Client` class is as follows:

   `client.h`

        #ifndef _CLIENT_H
        #define _CLIENT_H

        #include <QObject>
        #include <QSharedPointer>

        #include "rep_SimpleSwitch_replica.h"

        class Client : public QObject
        {
            Q_OBJECT
        public:
            Client(QSharedPointer<SimpleSwitchReplica> ptr);
            ~Client();
            void initConnections();// Function to connect signals and slots of source and client

        Q_SIGNALS:
            void echoSwitchState(bool switchState);// this signal is connected with server_slot(..) on the source object and echoes back switch state received from source

        public Q_SLOTS:
            void recSwitchState_slot(); // slot to receive source state
        private:
            bool clientSwitchState; // holds received server switch state
            QSharedPointer<SimpleSwitchReplica> reptr;// holds reference to replica

         };

        #endif

   `client.cpp`

        #include "client.h"

        // constructor
        Client::Client(QSharedPointer<SimpleSwitchReplica> ptr) :
            QObject(nullptr),reptr(ptr)
        {
            initConnections();
            //We can connect to SimpleSwitchReplica Signals/Slots
            //directly because our Replica was generated by repc.
        }

        //destructor
        Client::~Client()
        {
        }

        void Client::initConnections()
        {
            // initialize connections between signals and slots

            // connect source replica signal currStateChanged() with client's recSwitchState() slot to receive source's current state
            QObject::connect(reptr.data(), &SimpleSwitchReplica::currStateChanged, this, &Client::recSwitchState_slot);
            // connect client's echoSwitchState(..) signal with replica's server_slot(..) to echo back received state
            QObject::connect(this, &Client::echoSwitchState, reptr.data(), &SimpleSwitchReplica::server_slot);
        }

        void Client::recSwitchState_slot()
        {
            qDebug() << "Received source state "<<reptr.data()->currState();
            clientSwitchState = reptr.data()->currState();
            Q_EMIT echoSwitchState(clientSwitchState); // Emit signal to echo received state back to server
        }

   Compiling and running this example together with the source-side example generates the following output: