So the timeSetEvent's callback function posts a message to the dispatcher which the dispatcher picks up and sends as QEvent::Timer. In Qt, timers fire in the same thread as that of the QObject. timeSetEvent takes a callback function that is called on expiry and will be called from a separate thread. If the interval is less than 20 msecs, Qt tries to use multimedia (aka fast) timers using timeSetEvent.SetTimer sends a WM_TIMER message to the callback function that gets sent as QEvent::Timer to the QObject. If the interval is greater than 20 msecs, it uses SetTimer (with the id that was generated by QAbstractEventDispatcher::registerTimer).
#Qt sync timer to second clock windows
When a timer is registered, QEventDispatcherWin creates a native Windows timer based on the timer interval. The prepare, check and dispatch functions of this custom GSource work on the timer list in the obvious way.Īs noted in QtEventProcessing, Qt creates a hidden window with a callback function to process events. Just like QEventDispatcherUNIX, QEventDispatcherGlib manages timers in a list sorted on expiry time. Glib loop documentation" provides an introduction to how glib loops are written. Qt can be compiled to use the glib event loop (the default) which helps it integrate better with Gtk. Note that Qt does not use the POSIX timer API - timerfd_create (Why? Portability?). After returning from select, the dispatcher "activates" expired timers by sending the associated QObjects a QEvent::Timer. It then provides the timeout to the select() system call as the interval in the first item of the timer list. It first updates the expiry interval for all the registered timers when processEvents() is entered. The dispatcher relies on select() to wait for events on all fds (the X connection, sockets).
The timer is then inserted into a list sorted based on its expiry interval (first item expires first). On systems that do not support monotonic clock, gettimeofday is used. When a timer is registered, it's expiry time is computed by adding the timeout value to monotonic clock (clock_gettime). It maintains a list of timers sorted by their expiry time. On Unix, QEventDispatcherUNIX implements the event dispatcher. If fresh timer ids were to be regenerated on a QObject::moveToThread, then the application needs to be notified about the id changes and this is a hassle for the application programmer. If the timer ids were not unique, the timer ids might clash with existing timers in the new thread.
Moving the timer is simply a matter of unregistering the timer from the old thread's dispatcher and registering in the new thread's dispatcher. This is because when a QObject moves from one thread to another, its timers also move along with it (i.e signals and events are now delivered through the new thread's event loop). The algorithm for timer id allocation is lock-free and can be studied from Brad's blog.Īn important fact is that timer ids are required to be unique even across threads. High level APIs have to emulate a single shot timer by unregistering the timer after the first interval expires. Also, timers keep firing (i.e recurring) until killed and there is no concept of a "single shot" in the event dispatcher interface. A high-level API has to create a new timer to change the interval. QAbstractEventDispatcher::registeredTimers can be used to query the list of timers associated with a QObject.Īt the low level, once you register a timer, its interval cannot be changed and should be considered as immutable. With this approach, the platform event dispatcher maintains the list of timers associated with a QObject. The non-virtual registerTimer (cross-platform) allocates a unique timer id and calls the virtual registerTimer for actually registering/creating the timer in a platform-specific way. Both versions take an argument to QObject that the timer is to be associated with. There are two versions of the registerTimer (one virtual and one non-virtual). It requires registerTimer and unregisterTimer to be implemented for timers. An event dispatcher exists for each thread created using Qt. QAbstractEventDispatcher is the interface for the event/message pump.
QtEventProcessing provides an overview of how platform events are processed by Qt. It simply uses QObject::startTimer() and in the QObject::timerEvent() event handler, it emits the elapsed() signal. QTimer - QTimer is a QObject which emits the elapsed() signal when the timer expires. QObject::killTimer(id) can be used to stop and disarm the timer. The QTimerEvent argument contains the timer id which can be matched against if the QObject uses multiple timers. When the timer expires it receives a QEvent::Timer that can be handled by overriding QObject::timerEvent(QTimerEvent ). QObject::startTimer - Creates a recurring timer for use by any QObject subclass and returns the timer id.
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