How to Execute a Functor or Lambda in a Specific Qt Thread?

How to execute a functor or a lambda in a given thread in Qt

In Qt, it is possible to execute a lambda or a functor in a specific thread using metacall posting. Here's how it's done:

1. Create a functoid to wrap the function to be executed:
   Pass the function to be executed as an argument to the std::function class as shown:

    std::function<void()> myFunction = []() {
    // Code to be executed in the specified thread
   };

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2. Post the metacall to the specified thread:
   Use the postMetaCall function from the FunctorCallConsumer namespace to post the metacall to the desired thread. The function takes a thread pointer and a function pointer as arguments.

   FunctorCallConsumer::postMetaCall(&myThread, myFunction);

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   • Note:
     In Qt 5.10 and later, you can directly use QMetaObject::invokeMethod to execute a function in a specified thread:

   QMetaObject::invokeMethod(&myThread, myFunction);

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3. Implement a metacall event consumer:
   The final step is to implement a QEvent subclass that will receive and execute the metacall. The FunctorCallEvent class can be defined as follows:

   class FunctorCallEvent : public QEvent {
   public:
    FunctorCallEvent(std::function<void()> fun) : QEvent(QEvent::None), m_fun(fun) {}
    void execute() { m_fun(); }
   
   private:
    std::function<void()> m_fun;
   };
   

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4. Connect the event consumer to the thread:
   Once the FunctorCallEvent class has been defined, connect it to the event loop of the specified thread using QCoreApplication::installEventFilter.

   QCoreApplication::instance()->installEventFilter(new FunctorCallEventConsumer(&myThread));

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Example:
Here's a complete example that demonstrates how to execute a lambda in a specified thread:

#include <QtCore>

class MyThread : public QThread {
public:
    void run() override {
        // Execute the lambda in this thread
        m_lambda();
    }

    void setLambda(std::function<void()> lambda) {
        m_lambda = lambda;
    }

private:
    std::function<void()> m_lambda;
};

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

    // Create a new thread
    MyThread thread;

    // Create a lambda to execute in the thread
    std::function<void()> lambda = []() {
        qDebug() << "Lambda executed in thread" << QThread::currentThread();
    };

    // Set the lambda on the thread
    thread.setLambda(lambda);

    // Start the thread
    thread.start();

    return app.exec();
}

Output:

Lambda executed in thread QThread(0x7f8632802640, name = "MyThread")

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