Is it possible to get a return value from a QRemoteObject Dynamic Replica slot?

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06-10-2024

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Can You Get a Return Value From a QRemoteObject Dynamic Replica Slot?

Understanding the Problem:

Imagine you're building a complex application with Qt, using QRemoteObject to communicate between different parts of your system. You want to invoke a method on a remote object and get a result back, but the documentation seems unclear. The question arises: can you actually receive a return value from a dynamically-created replica slot?

The Scenario:

Let's say you have a simple remote object, MyRemoteObject, with a method calculateSum:

class MyRemoteObject : public QObject, public QRemoteObject
{
    Q_OBJECT

public:
    MyRemoteObject(QObject *parent = nullptr) : QObject(parent), QRemoteObject(parent) {}

    int calculateSum(int a, int b)
    {
        return a + b;
    }

signals:
    void sumReady(int sum);

public slots:
    void onSumReady(int sum)
    {
        qDebug() << "Sum received: " << sum;
    }
};

Now, on your client side, you want to use the calculateSum method and get the result:

// Client side code
MyRemoteObject *remoteObject = new MyRemoteObject;
QObject *replica = QRemoteObject::dynamicCall("MyRemoteObject", "calculateSum", Q_ARG(int, 10), Q_ARG(int, 20));

// How to get the calculated sum here?

The Answer: Not Directly!

The short answer is no, you cannot directly obtain a return value from a dynamic replica slot created using QRemoteObject::dynamicCall.

The Explanation:

The way QRemoteObject::dynamicCall works is by:

1. Creating a temporary replica object of the specified remote object type.
2. Invoking the method on the replica with the provided arguments.
3. Disposing of the temporary replica once the invocation is complete.

This means the replica object exists only for the duration of the call and is not persistent. It lacks the ability to store or return a value back to the caller.

The Solution: Use Signals:

Instead of trying to get a return value, you need to rely on signals. Here's how:

1. Modify the MyRemoteObject class: Add a signal to emit the calculated sum:

   signals:
       void sumReady(int sum);
   

2. Emit the signal in the calculateSum method:

   int calculateSum(int a, int b)
   {
       int sum = a + b;
       emit sumReady(sum);
       return sum;
   }
   

3. Connect to the signal on the client side:

   // Client side code
   QObject::connect(replica, SIGNAL(sumReady(int)), this, SLOT(onSumReady(int)));
   

Now, whenever the calculateSum method is invoked on the replica, the sumReady signal will be emitted and caught by your onSumReady slot. You can then access the calculated sum within this slot.

Key Takeaways:

• You cannot directly receive return values from dynamically created replicas.
• Use signals to communicate results from remote objects back to the client.
• This ensures a consistent and robust communication pattern between your client and remote objects.

Additional Resources:

• Qt Documentation on QRemoteObject: https://doc.qt.io/qt-6/qremoteobject.html
• Qt Documentation on Signals and Slots: https://doc.qt.io/qt-6/signalsandslots.html

By understanding how QRemoteObject works and using signals effectively, you can build robust and scalable distributed applications with Qt.