How to Execute Functors or Lambdas in a Specific Thread in Qt?

Executing Functors or Lambdas in a Given Thread in Qt, GCD-Style

Problem:

In Objective-C with GCD, one can execute a lambda or function in any thread that spins an event loop. For example, dispatching something to the main thread's queue can be done synchronously or asynchronously:

dispatch_sync(dispatch_get_main_queue(), ^{ /* do sth */ });

dispatch_async(dispatch_get_main_queue(), ^{ /* do sth */ });

How can this be achieved in Qt?

Solution:

Qt 5.10 & Up

For Qt 5.10 and later, the simplest solution is to use QMetaObject::invokeMethod:

QMetaObject::invokeMethod(qApp, []{ ... });

QMetaObject::invokeMethod(obj, []{ ... });

QMetaObject::invokeMethod(QAbstractEventDispatcher::instance(thread),
                         []{ ... });

For functors:

template <typename F>
static void postToObject(F &amp;&amp;fun, QObject *obj = qApp) {
  QMetaObject::invokeMethod(obj, std::forward<F>(fun));
}

template <typename F>
static void postToThread(F &amp;&amp; fun, QThread *thread = qApp->thread()) {
   auto *obj = QAbstractEventDispatcher::instance(thread);
   Q_ASSERT(obj);
   QMetaObject::invokeMethod(obj, std::forward<F>(fun));
}

Qt 5/4

For Qt 5/4, there are several methods:

• Posting via QEvent: Create a custom event FunctorCallEvent and implement a consumer object FunctorCallConsumer.
• Posting via Temporary Object: Use a temporary QObject as a signal source and connect the functor to its destroyed() signal.
• Posting via QMetaCallEvent (Qt 5 Private): Wrap the functor in the private QtPrivate::QFunctorSlotObject and use a QMetaCallEvent.
• Posting via Object Event Consumer: Override the event() method of an object to call the functor.

Common Code:

#ifndef HAS_FUNCTORCALLCONSUMER
namespace FunctorCallConsumer {
   bool needsRunningThread() { return true; }
   QObject * forThread(QThread * thread) {
      Q_ASSERT(thread);
      QObject * target = thread == qApp->thread()
            ? static_cast<QObject*>(qApp) : QAbstractEventDispatcher::instance(thread);
      Q_ASSERT_X(target, "postMetaCall", "the receiver thread must have an event loop");
      return target;
   }
}
#endif

void postMetaCall(QThread * thread, const std::function<void()> &amp; fun) {
   auto receiver = FunctorCallConsumer::forThread(thread);
   QCoreApplication::postEvent(receiver, new FunctorCallEvent(fun, receiver));
}

void postMetaCall(QThread * thread, std::function<void()> &amp;&amp; fun) {
   auto receiver = FunctorCallConsumer::forThread(thread);
   QCoreApplication::postEvent(receiver,
                               new FunctorCallEvent(std::move(fun), receiver));
}

Demonstration:

class Worker : public QThread {
   QMutex m_started;
   void run() {
      m_started.unlock();
      postMetaCall(qApp->thread(), []{
         qDebug() << "worker functor executes in thread" << QThread::currentThread();
      });
      QThread::run();
   }
public:
   Worker(QObject * parent = 0) : QThread(parent) { m_started.lock(); }
   ~Worker() { quit(); wait(); }
   void waitForStart() { m_started.lock(); m_started.unlock(); }
};

int main(int argc, char *argv[])
{
   QCoreApplication a(argc, argv);
   a.thread()->setObjectName("main");
   Worker worker;
   worker.setObjectName("worker");
   qDebug() << "worker thread:" << &amp;worker;
   qDebug() << "main thread:" << QThread::currentThread();
   if (FunctorCallConsumer::needsRunningThread()) {
      worker.start();
      worker.waitForStart();
   }
   postMetaCall(&amp;worker, []{ qDebug() << "main functor executes in thread" << QThread::currentThread(); });
   if (!FunctorCallConsumer::needsRunningThread()) worker.start();
   QMetaObject::invokeMethod(&amp;a, "quit", Qt::QueuedConnection);
   return a.exec();
}

Output:

worker thread: QThread(0x7fff5692fc20, name = "worker")
main thread: QThread(0x7f86abc02f00, name = "main")
main functor executes in thread QThread(0x7fff5692fc20, name = "worker")
worker functor executes in thread QThread(0x7f86abc02f00, name = "main")

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