有没有一种方法可以调试针对该对象排队的Qt插槽调用？

Question

An object lives in a thread and have some slots, that are called with

Qt::QueuedConnection 

or

Qt::BlockingQueuedConnection

Is there a way to know how many slot calls are in the queue? Or even know number of each slot calls in the that queue ?

Answer 1

Maybe a bit offtopic, but next pattern may help you (when you need only one slot call per several operations). For example, when you have a lot of unpredictable data changes and you need to update a view.

class SlotProxy : public QObject
{
  Q_OBJECT

signals:
  void triggered();

public:
  explicit SlotProxy( QObject *parent = nullptr );
  void trigger();

private:
  void onTimeout();

private:
  QPointer<QTimer> _timer;
};

SlotProxy::SlotProxy( QObject *parent = nullptr )
  : _timer{ new QTimer{ this } }
{
  _timer->setInterval( 0 );
  _timer->setSingleShot( true );
  connect( _timer, &QTimer::timeout, this, &SlotProxy::onTimeout );
}

void SlotProxy::trigger()
{
  _timer->start( 0 ); // May be not zero;
}

void SlotProxy::onTimeout()
{
  emit triggered();
}

Usage:

class MyForm : public QWidget
{
/*...*/
void updateGui();
};

class Worker : public QObject
{
/*...*/
signals:
  void updateRequest();

public:
  explicit Worker( QObject *parent = nullptr );

private:
  void heavyTask();

private:
  SlotProxy _proxy;
};

Worker::Worker( QObject *parent )
  : QObject{ parent }
{
  connect( _proxy, &SlotProxy::triggered, this, &Worker::updateRequest );
}

void Worker::heavyTask()
{
  for ( auto i = 0; i < 1000; i++ )
  {
    /*...*/
    _proxy.trigger();
  }
}

MyForm form;
Worker worker;

QObject::connect( &worker, &Worker::updateRequest, &form, &MyForm::updateGui );
worker.heavyTask();
// here you will have only 1 slot MyForm::updateGui in queue

Subject to improve. You may call _timer->start( 100 ); with non-zero timeout for better latency, but you need to somehow force emitting a signal periodically. It will do a code a bit more complex, please write a comment if you need help.

Answer 2

See this question for details. You need to monitor (via a filter) the QMetaCallEvent events.