How to check if an event loop has pending events outside of a thread?
event-loop, multithreading, qt, qt5, qthread
Solution
What you're showing might be a Qt bug. Alas, you might not need to checking this way if another thread has any pending events.
The only reason I see that you might want to do this is to manage your own thread pool and move objects to threads that are not "busy". You'd keep a list of "busy" and "available" threads. That's what `QAbstractEventDispatcher::aboutToBlock` signal is for. Your thread pool should connect to this signal for every thread it creates, and add the thread to the "available" list upon reception.
If, on the other hand, you're trying to use it to implement some event compression, that's really the most awkward way to go about it. In another answer, I show how to implement custom event compression, and also how to compress signal-slot calls.
Problem
Calling `QCoreApplication::hasPendingEvents()` or `QAbstractEventDispatcher::instance()->hasPendingEvents()` inside of a thread works just fine. However, outside of it, the latter one (with appropriate parameter) always returns `false` (former cannot be used outside, because it refers to the thread from which it is called). Here is a complete code: ``` #include <QCoreApplication> #include <QAbstractEventDispatcher> #include <QThread> #include <QDebug> bool hasPendingEvents(QThread *thread = 0) { return QAbstractEventDispatcher::instance(thread)->hasPendingEvents(); } class MyObject: public QObject { Q_OBJECT public slots: void Run() { qDebug() << __LINE__ << hasPendingEvents() << QCoreApplication::hasPendingEvents(); QThread::sleep(1); } }; int main(int argc, char *argv[]) { QCoreApplication app(argc, argv); QThread thread; MyObject t; t.moveToThread(&thread); thread.start(); for (int i = 0; i<4; ++i) QMetaObject::invokeMethod(&t, "Run", Qt::QueuedConnection); for (int i = 0; i<10; ++i) { QThread::msleep(500); qDebug() << __LINE__ << hasPendingEvents(&thread) << hasPendingEvents(t.thread()); } return 0; } #include "main.moc" ``` Here is the output: ``` 15 true true 31 false false 31 false false 15 true true 31 false false 31 false false 15 true true 31 false false 31 false false 15 false false 31 false false 31 false false 31 false false 31 false false ``` Why doesn't `QAbstractEventDispatcher.hasPendingEvents()` work outside of a thread? Maybe there is an alternative?