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

How to Execute a Functor or a Lambda in a Given Thread in Qt, GCD-Style?

Problem:

In Qt, you want to execute a lambda or a functor in any thread that has an event loop, similar to GCD's dispatch queues in Objective-C.

Solution:

The solution centers around delivering an event that wraps the functor to a consumer object residing in the desired thread. This operation is referred to as metacall posting, and it can be implemented in several ways.

Qt 5.10 & Up TL;DR:

You can invoke methods on QObjects or QThreads directly:

// Invoke on the main thread
QMetaObject::invokeMethod(qApp, []{ ... });

// Invoke on an object's thread
QMetaObject::invokeMethod(obj, []{ ... });

// Invoke on a particular thread
QMetaObject::invokeMethod(
    QAbstractEventDispatcher::instance(thread),
    []{ ... });

TL;DR for Functors:

Several template functions are provided to wrap the functor in an appropriate event object, making metacall posting convenient.

// Post to the main thread sync
void execInMainThread_sync(std::function<void(void)> func) {
    if(qApp->thread() == QThread::currentThread())
        func();
    else {
        ((App*) qApp)->invokeGenericExec(func, Qt::BlockingQueuedConnection);
    }
}

// Post to the main thread async
void execInMainThread_async(std::function<void(void)> func) {
    ((App*) qApp)->invokeGenericExec(func, Qt::QueuedConnection);
}

TL;DR for Methods/Slots:

For methods or slots, a macro is defined to handle the metacall posting.

#define POST_METHOD(obj, method) \
    QObject src; \
    QObject::connect(&src, &QObject::destroyed, obj, method, \
                       Qt::QueuedConnection);

Common Code:

The provided code includes common helper functions and macros that abstract the metacall posting implementation for use in different scenarios.

Example:

void test1() {
    QThread t;
    QObject o;
    o.moveToThread(&t);

    // Execute in given object's thread
    postToObject([&]{ o.setObjectName("hello"); }, &o);
    // or
    postToObject(std::bind(&QObject::setObjectName, &o, "hello"), &o);

    // Execute in given thread
    postToThread([]{ qDebug() << "hello from worker thread"; });

    // Execute in the main thread
    postToThread([]{ qDebug() << "hello from main thread"; });
}

Additional Note:

If your target thread does not have an event loop, such as a worker thread, you may need a mechanism to start the event loop within the thread. For example, if using a single-shot timer for metacall posting, you can start the event loop using QThread::exec().

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