more details - when player is paused, stacktrace is:
#0 MediaPlayerTest::playerStateChanged (this=0xbefff584, state=QMediaPlayer::PausedState) at /home/mersdk/share/SailfishOS/projects/osmscout-sailfish/src/MediaPlayer.cpp:33
#1 0xb69cd122 in QtPrivate::QSlotObjectBase::call (a=0xbeffed28, r=<optimized out>, this=0x2a18dfe0) at ../../include/QtCore/../../src/corelib/kernel/qobject_impl.h:124
#2 QMetaObject::activate (sender=sender@entry=0x2a046650, signalOffset=<optimized out>, local_signal_index=local_signal_index@entry=2, argv=argv@entry=0xbeffed28) at kernel/qobject.cpp:3715
#3 0xb69cd43a in QMetaObject::activate (sender=sender@entry=0x2a046650, m=<optimized out>, local_signal_index=local_signal_index@entry=2, argv=argv@entry=0xbeffed28) at kernel/qobject.cpp:3595
#4 0xb6f7cbc8 in QMediaPlayer::stateChanged (this=this@entry=0x2a046650, _t1=<optimized out>, _t1@entry=QMediaPlayer::PausedState) at .moc/moc_qmediaplayer.cpp:707
#5 0xb6f7cfc6 in QMediaPlayerPrivate::_q_stateChanged (this=<optimized out>, ps=QMediaPlayer::PausedState) at playback/qmediaplayer.cpp:211
#6 0xb6f7f5ca in QMediaPlayer::qt_static_metacall (_o=<optimized out>, _c=<optimized out>, _id=<optimized out>, _a=<optimized out>) at playback/qmediaplayer.h:207
#7 0xb69cd258 in QMetaObject::activate (sender=0x2a186ae8, signalOffset=<optimized out>, local_signal_index=local_signal_index@entry=3, argv=argv@entry=0xbeffee30) at kernel/qobject.cpp:3730
#8 0xb69cd43a in QMetaObject::activate (sender=<optimized out>, m=<optimized out>, local_signal_index=local_signal_index@entry=3, argv=argv@entry=0xbeffee30) at kernel/qobject.cpp:3595
#9 0xb6f74c90 in QMediaPlayerControl::stateChanged (this=<optimized out>, _t1=<optimized out>) at .moc/moc_qmediaplayercontrol.cpp:342
#10 0xb3432bc2 in QGstreamerPlayerControl::qt_static_metacall (_o=<optimized out>, _c=<optimized out>, _id=12, _a=<optimized out>) at .moc/moc_qgstreamerplayercontrol.cpp:120
#11 0xb69cd258 in QMetaObject::activate (sender=0x2a187eb0, signalOffset=<optimized out>, local_signal_index=<optimized out>, argv=<optimized out>) at kernel/qobject.cpp:3730
#12 0xb2ef7d02 in ResourcePolicyInt::handleResourcesReleasedByManager (this=0xb2f0911c <(anonymous namespace)::Q_QGS_globalResourcePolicyInt::innerFunction()::holder>) at resourcepolicyint.cpp:430
#13 0xb69cd258 in QMetaObject::activate (sender=0x2a16a800, signalOffset=<optimized out>, local_signal_index=<optimized out>, argv=<optimized out>) at kernel/qobject.cpp:3730
#14 0xb69cd258 in QMetaObject::activate (sender=sender@entry=0x2a186cf8, signalOffset=<optimized out>, local_signal_index=local_signal_index@entry=8, argv=argv@entry=0x0) at kernel/qobject.cpp:3730
#15 0xb69cd43a in QMetaObject::activate (sender=sender@entry=0x2a186cf8, m=<optimized out>, local_signal_index=local_signal_index@entry=8, argv=argv@entry=0x0) at kernel/qobject.cpp:3595
#16 0xb2edfe8e in ResourcePolicy::ResourceEngine::resourcesReleasedByManager (this=this@entry=0x2a186cf8) at moc/moc_resource-engine.cpp:287
#17 0xb2edbf76 in ResourcePolicy::ResourceEngine::receivedRelease (this=this@entry=0x2a186cf8, message=message@entry=0xbefff104) at src/resource-engine.cpp:247
#18 0xb2edc120 in handleReleaseMessage (message=0xbefff104, rs=<optimized out>) at src/resource-engine.cpp:240
#19 0xb2ea60fc in client_method (dcon=<optimized out>, dbusmsg=0x2a084698, user_data=<optimized out>) at dbus-proto.c:732
#20 0xb5719a88 in _dbus_object_tree_dispatch_and_unlock () from /usr/lib/libdbus-1.so.3
#21 0xb570e23e in dbus_connection_dispatch () from /usr/lib/libdbus-1.so.3
#22 0xb2ebb86c in DBUSConnectionEventLoop::dispatch (this=this@entry=0xb2ecd178 <(anonymous namespace)::Q_QGS_classInstance::innerFunction()::holder>) at dbusconnectioneventloop.cpp:111
#23 0xb2ebbbc0 in DBUSConnectionEventLoop::readSocket (this=0xb2ecd178 <(anonymous namespace)::Q_QGS_classInstance::innerFunction()::holder>, fd=<optimized out>) at dbusconnectioneventloop.cpp:84
#24 0xb69cd258 in QMetaObject::activate (sender=sender@entry=0x2a188b40, signalOffset=<optimized out>, local_signal_index=local_signal_index@entry=0, argv=argv@entry=0xbefff300) at kernel/qobject.cpp:3730
#25 0xb69cd43a in QMetaObject::activate (sender=sender@entry=0x2a188b40, m=<optimized out>, local_signal_index=local_signal_index@entry=0, argv=argv@entry=0xbefff300) at kernel/qobject.cpp:3595
#26 0xb6a1e8b8 in QSocketNotifier::activated (this=this@entry=0x2a188b40, _t1=<optimized out>) at .moc/moc_qsocketnotifier.cpp:135
#27 0xb69d4cf8 in QSocketNotifier::event (this=0x2a188b40, e=<optimized out>) at kernel/qsocketnotifier.cpp:260
#28 0xb69b0c44 in doNotify (event=<optimized out>, receiver=<optimized out>) at ../../include/QtCore/../../src/corelib/kernel/qobject.h:137
#29 QCoreApplication::notify (this=<optimized out>, receiver=<optimized out>, event=<optimized out>) at kernel/qcoreapplication.cpp:1076
#30 0xb69b0d52 in QCoreApplication::notifyInternal2 (receiver=0x2a188b40, event=event@entry=0xbefff434) at ../../include/QtCore/../../src/corelib/kernel/qobject.h:137
#31 0xb69e9ca6 in QCoreApplication::sendEvent (event=0xbefff434, receiver=<optimized out>) at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:225
#32 socketNotifierSourceDispatch (source=0x2a0181d8) at kernel/qeventdispatcher_glib.cpp:102
#33 0xb599c38c in g_main_dispatch (context=0x2a041568) at ../glib/gmain.c:3179
#34 g_main_context_dispatch (context=context@entry=0x2a041568) at ../glib/gmain.c:3844
#35 0xb599c524 in g_main_context_iterate (context=context@entry=0x2a041568, block=block@entry=1, dispatch=dispatch@entry=1, self=<optimized out>) at ../glib/gmain.c:3917
#36 0xb599c57c in g_main_context_iteration (context=0x2a041568, may_block=may_block@entry=1) at ../glib/gmain.c:3978
#37 0xb69e99a2 in QEventDispatcherGlib::processEvents (this=0x2a06da88, flags=...) at kernel/qeventdispatcher_glib.cpp:423
#38 0xb69af9ea in QEventLoop::exec (this=this@entry=0xbefff548, flags=..., flags@entry=...) at ../../include/QtCore/../../src/corelib/global/qflags.h:131
#39 0xb69b51e0 in QCoreApplication::exec () at ../../include/QtCore/../../src/corelib/global/qflags.h:111
#40 0x2a0018c4 in main (argc=<optimized out>, argv=<optimized out>) at /home/mersdk/share/SailfishOS/projects/osmscout-sailfish/src/MediaPlayer.cpp:78
…so it is clear that Qt media players are communicating via D-Bus…