How to place lambda function in Qt event loop?

Question

Basically, I need the same thing as in Java:

SwingUtilities.invokeLater(()->{/* function */}); 

Or as in javascript:

 setTimeout(()=>{/* function */}, 0); 

But with Qt and lambda. So, some pseudo code:

 Qt::queuePushMagic([]() { /* function */ }); 

As an additional complication, I need it to work in a multi-threaded context. What I'm actually trying to do is automatically run certain methods in the correct thread. What would the code look like:

 SomeClass::threadSafeAsyncMethod() { if(this->thread() != QThread::currentThread()) { Qt::queuePushMagic([this]()=>{ this->threadSafeAsyncMethod() }); return; } } 

How to do it?

Answer 1

Your problem is How to use Qt to make the QObject method thread safe? Let me tailor the solutions there to your use case. First, let’s give up security checks:

 bool isSafe(QObject * obj) { Q_ASSERT(obj->thread() || qApp && qApp->thread() == QThread::currentThread()); auto thread = obj->thread() ? obj->thread() : qApp->thread(); return thread == QThread::currentThread(); } 

The proposed approach uses a functor and allows the compiler to understand the arguments (if any) in the functor:

 template <typename Fun> void postCall(QObject * obj, Fun && fun) { qDebug() << __FUNCTION__; struct Event : public QEvent { using F = typename std::decay<Fun>::type; F fun; Event(F && fun) : QEvent(QEvent::None), fun(std::move(fun)) {} Event(const F & fun) : QEvent(QEvent::None), fun(fun) {} ~Event() { fun(); } }; QCoreApplication::postEvent( obj->thread() ? obj : qApp, new Event(std::forward<Fun>(fun))); } 

The second approach stores copies of all parameters explicitly inside the event and does not use a functor:

 template <typename Class, typename... Args> struct CallEvent : public QEvent { // See https://stackoverflow.com/a/7858971/1329652 // See also https://stackoverflow.com/a/15338881/1329652 template <int ...> struct seq {}; template <int N, int... S> struct gens { using type = typename gens<N-1, N-1, S...>::type; }; template <int ...S> struct gens<0, S...> { using type = seq<S...>; }; template <int ...S> void callFunc(seq<S...>) { (obj->*method)(std::get<S>(args)...); } Class * obj; void (Class::*method)(Args...); std::tuple<typename std::decay<Args>::type...> args; CallEvent(Class * obj, void (Class::*method)(Args...), Args&&... args) : QEvent(QEvent::None), obj(obj), method(method), args(std::move<Args>(args)...) {} ~CallEvent() { callFunc(typename gens<sizeof...(Args)>::type()); } }; template <typename Class, typename... Args> void postCall(Class * obj, void (Class::*method)(Args...), Args&& ...args) { qDebug() << __FUNCTION__; QCoreApplication::postEvent( obj->thread() ? static_cast<QObject*>(obj) : qApp, new CallEvent<Class, Args...>{obj, method, std::forward<Args>(args)...}); } 

Used as follows:

 struct Class : QObject { int num{}; QString str; void method1(int val) { if (!isSafe(this)) return postCall(this, [=]{ method1(val); }); qDebug() << __FUNCTION__; num = val; } void method2(const QString &val) { if (!isSafe(this)) return postCall(this, &Class::method2, val); qDebug() << __FUNCTION__; str = val; } }; 

Test harness:

 // https://github.com/KubaO/stackoverflown/tree/master/questions/safe-method-40382820 #include <QtCore> // above code class Thread : public QThread { public: Thread(QObject * parent = nullptr) : QThread(parent) {} ~Thread() { quit(); wait(); } }; void moveToOwnThread(QObject * obj) { Q_ASSERT(obj->thread() == QThread::currentThread()); auto thread = new Thread{obj}; thread->start(); obj->moveToThread(thread); } int main(int argc, char ** argv) { QCoreApplication app{argc, argv}; Class c; moveToOwnThread(&c); const auto num = 44; const auto str = QString::fromLatin1("Foo"); c.method1(num); c.method2(str); postCall(&c, [&]{ c.thread()->quit(); }); c.thread()->wait(); Q_ASSERT(c.num == num && c.str == str); } 

Output:

 postCall postCall postCall method1 method2 

The above compiles and works with either Qt 4 or Qt 5.

See also this question , exploring various ways of calling functors in other flow contexts in Qt.