Qt what is event loop. See also exit() and QEventLoop.

Qt what is event loop In practice, there are two common solutions, which are really two sides of the same coin: I wanted to know about qt event loop. Unless of course you're implementing that infinite loop in a QThread subclass. Like is there some mechanism which can be used to assign an id to event loop and hence print it in the event loop to "If the Qt event loop gets backed up then the data for the slot calls gets queued up and memory explodes. The object will be deleted when control returns to the event loop. So if you are running an application event I understand that QEventLoop "runs inside" Qt application loop, hence has to be started and then terminated. I am developing an asynchronous PyQt application - meaning, I need both asyncio and PyQt. It does not seem to do any UI stuff itself, though (if relevant) it may do things like access a database through Qt. So, it exists a QEventDispatchWin, a QEventDispatchUnix, a QEventDispatchSymbian and so on, for different platforms. The application main event loop (QCoreApplication::exec()) grabs an QEvent "E" from the queue, and dispatches it to the QObject "A" it decides the event shall go to (for example, the position and Z-value of a QWidget in case of a mouse-left press). They are posted to a thread. Event Loop in Qt-based DLL in a non-Qt application. Allow me to revive this topic, since more than a decade later it is still as relevant as ever. And all these are worth reading if you would like to know more about threading in Qt. Y Also, you should leave the main thread idle, receiving messages and doing the UI updates, and have a secondary thread (not in event-loop) doing the heavy work. This makes it pretty easy to deal with communication between/among threads in Qt (again, assuming your threads Never use QThread::sleep() in the GUI thread, It will block the GUI thread from doing anything. flags – ProcessEventsFlags. I am also not entirely certain if the QTimer event will be pushed to the top of the event loop. I think thats explicitly stated in the documentation? Normally, there is at most one event queue per each thread. it will run as fast as possible. Python: You can look at the implementation of the Twisted reactor which is probably the best implementation for an event loop in python. QT event loop in a dll. I suppose that QCoreApplication in the constructor create just everything needed by the QThread to run and dispatch events! The object will be deleted when control returns to the event loop. "If the Qt event loop gets backed up then the data for the slot calls gets queued up and memory explodes. I'm new to Qt, and I am going to try out making a custom event. So, no other events or the UI are blocked like normal windows sleep function does. 9. Use deleteLater() instead, which will cause the event loop to delete the object after all pending events have been delivered to it. So the event Tasks will already have been triggered and thus be scheduled in the event loop of my _eventThread. I don't think you're on the right track. –" So I want to present my final code for this if anybody is interested: header file: May 15, 2011 · For many event classes there is a function called isAccepted() that tells you whether the event was accepted or rejected by the last handler that was called. If your code does not run inside a QCoreApplication::exec() event loop then you're going to have to use the QEventLoop class, but be warned that it is considered to be sloppy code. :connect() won't work since I don't knwo which object the signal needs to get. From what I understood, I can't just run the GLib event loop Events are not posted to a particular instance of an event loop object. Changing the size policy Python: You can look at the implementation of the Twisted reactor which is probably the best implementation for an event loop in python. Execution of the code following the emit statement will occur once all slots have returned. void MyObject::doOperation() QEventLoop是Qt框架中处理事件循环的核心类。它允许程序在特定的时间点暂停执行，等待事件的发生，并对这些事件进行响应。QEventLoop负责监听各种事件源（如窗口系统事件、定时器事件、网络事件等），并根据事件的优先级和类型进行调度处理。在Qt应用程序中，通常会有一个主事件循环（由 At one point I need to be able to force all of them to happen and wait for them (first the heavy Tasks, then the event oriented ones, and finally again the heavy ones as the event oriented one can schedule 1 last). I've understood that using postEvent() would be useful since the event will be released as soon as the event-loop will begin. It will quit, once it gets control again and instead of processing the event queue it will quit the execution. That being said if you nest event loops they all process the same pile of events. PyQT4 Asynchronous QTcpServer (How to create event loop) 1. When Qt decides it's time to process an event, it After executing start. qGuiApp->exec() works, but it's not what I need. Equivalent to calling QThread::exit(0). But what makes this work is the wait method has a timeout on it. What happens is that I call one function on the press of Button1 and I keep the event loop spinning inside the while loop by calling QCoreApplication. Can you share a code snippet of how could qeventloop can be overwritten. So, if you call "execute" method within the same Qt main loop thread, f() will be called, just like a simple function. If deleteLater() is called after the main event loop has stopped, the object will Assuming B's thread is running an event loop, emitting the signal will post an event to B's event loop. Normally you slow it down a bit wiht something like QTimer::singleShot(1, this, &Task::doWork) Where doWork is the method that performs the processing in the @Obi-Wan the underlying issue is with your missing event loop without it, QAbstractSocket::flush is not called and no data send over the network. Custom events in Qt. That way your application stays responsive during processing. A one liner answer to the question is - sleep and event loop are not related. 8. It will quit, once it As discussed in the comments, this was a bug in the Android Emulator that was fixed in release 27. QThread will not interrupt any for/while loops that are currently running. QCoreApplication::postEvent(), ). timeout. dispatchEvent(new Event("Hello")) I know Qt has event loop, and somehow, I need this signal to be emitted after this event loop , or when the GUI completes the Paint() This happens because the event loop to show and hide components is processed too late. QtCore. As discussed in the comments, this was a bug in the Android Emulator that was fixed in release 27. Improve this question. This might be from a "window" (QWidget) or from a dialog (QDialog). Now, I have a problem with event loop. How to execute a method automatically after entering Qt event loop? 0. You don't need to watch event loop for that. At one point I need to be able to force all of them to happen and wait for them (first the heavy Tasks, then the event oriented ones, and finally again the heavy ones as the event oriented one can schedule 1 last). One widget would emit and another would listen to events, but the two widgets would not need to be related. QT doesn't properly processes events in loop. The term you're looking for with the keyboard is "focus". qtquick application event handling from c++ class. Mostly you start the main event loop with QApplication. If you need to wait for some predefined time then you can use something like this That's why in event based programming, which is what all UI kits do, it is essential to return to the main event handler loop as quickly as possible. If you don't care about QCoreApplication::postEvent working with your queue, everything is up to you. If The main event loop is alive and dialog is still in main thread. If the event loop is not running when this function is called (e. Mar 5, 2016 · Watching for event loop iterations will allow my future classes to notify any state changement. Improve this answer. So in principal, Qt could use this file descriptor to call a callback that reads whatever ZMQ socket has received a message, and handle the message on the Qt event loop's thread (which may have other benefits!). However, using QOjbect. For gui events, Qt has also QEventDispatchX11 (inherits from QEventDispatchUnix), S60 (from Qt doc says: void QDialog::done ( int r ) [virtual slot] Closes the dialog and sets its result code to r. You can see the event loop as a infinite loop in which every iteration sends the available events to it's receiver objects. At the same time, I can tell that mouse-event handling gets very laggy as well. PySide6. It's rather a workaround for something. My understanding of how the QT main event loop basically works is the following. Qt has good socket APIs which will ensure that the socket does not block while waiting for the event. timing various paint and mouse event handling internally) shows that the actual execution time of my various handlers isn't increasing, nor do I see in task manager any evidence of This is just my experience in general. Viewed 4k times 4 . sleep (For a more in-depth look at Qt's event-processing, see: Threads, Events, QObjects). 7. You must not delete the QObject directly if it exists in a different thread than the one currently executing. also you can write this to give the loop more time to process pending events at this time: The main event loop receives events from the window system and dispatches these to the application widgets. If deleteLater() is called after the main event loop has stopped, the object will I've also read about connecting a signal to a slot which will be called from the event loop by setting the connection type to Qt::QueuedConnection. what is called here is the QThread::exec() function. However, i thought it'd be nicer to use event-handling rather than event-polling. refreshgui) timer. @J-Hilk I don't think it's a timing issue since there's a for loop running on the stress test thread and with each iteration process events is called on the thread's dispatcher many times after quit() has been called on the QThread object. Enters the main event loop and waits until exit() is called. My guess is also that there are a couple of events backlogged. exec();? Yes, of course. Returns the value that was passed to exit(). argv) asyncio. exec() und thus starting the engine of your application ;) Nothing in Qt prevents the quit() slot to be called during the sendQuery() call, especially because connections are much more like simple function calls than events. It processes all events (mouse, keyboard, refresh), signal-slots, timers, and most of Qt features that require some kind of asynchronism or event handling (GUI, OS, ). If deleteLater() is called after the main event loop has stopped, the object will You can see the event loop as a infinite loop in which every iteration sends the available events to it's receiver objects. Nice would be something like. When you click or interact with the GUI you are putting an event on the event queue. Confused by the Qt's event loop system? 1. Return type:. How Qt Signals and Slots Work - Part 3 - Queued and Inter Thread Connections. Tells the thread's event loop to exit with return code 0 (success). This will ensure that other events are not blocked. exec() you will use sleep then no the eventloop would exist so the QTimer would never fire. will not make object a dangling pointer (the event loop entered by QDialog::exec() is more nested than the deleteLater call). The event loop waits for new events to arrive in the event queue, then takes them out of the queue and sends them to their destination Qt doc says: void QDialog::done ( int r ) [virtual slot] Closes the dialog and sets its result code to r. After executing start. qt; qml; Share. The internal state of the UI widget object will be updated and an update of the screen, if required, will be scheduled. In multithreaded applications, you can use QTimer in any thread that has an event loop. Event loop of another thread is started by default implementation of QThread::run. QWaitCondition is designed to coordinate mutex waiting between different threads. The event loop queues the event, and eventually invokes the slot method whenever control returns to it (it being the event loop). Therefore I am looking for some routines to get more insight into the current state of the event loop. An event loop is an internal loop in Qt code that processes system and user events. I've been using Qt for 10 years extensively, and I've done some crazy complex stuff with it, and I've never had to force event processing, not once. Generally speaking, no user interaction can take place before calling exec() . Did try adding an event loop to that thread via QEventLoop to see if that worked I wanted to know about qt event loop. An event loop object does not "own" any events. When posting an event (in QCoreApplication::postEvent), the event will be pushed in a per-thread queue Each thread has a local "event dispatcher" (QAbstractEventDispatcher specializations), which purpose is reading system events. The QTimer::singleShot is the timer and its SLOT(quit()) returns The first thing you have to do, to take complet power of Qt framework, is to ensure thread event loop never goes locked for a too long time. As for QThread you can interrupt the thread but I'm really not sure that's the best way to handle your situation. If you don't call exec(), you need to create your own event loop that processes Qt events (that is, if you want signals / slots to work). Event loop means that your code is continuously running, think about it as being refreshed every time, so changes will be seen and made continuously based off your cases you have. 事件循环是Qt应用程序的核心机制之一，它负责处理和分发事件，如用户输入、网络通信和定时器事件。通过自定义事件循环和事件过滤，开发者可以增强应用程序的事件处理能力，满足特定需求。本文将介绍Qt中的自定义事件循环与事件过滤，包括基本概念、实现方法和实际 I am developing an asynchronous PyQt application - meaning, I need both asyncio and PyQt. QEventLoop. @mrjj said in QEventLoop and QCoreApplication::processEvents: Well QEventLoop is a local event loop. exec_()). The event will come when it comes. Simply connect MyWidget's destroyed() signal to action_B (or something that calls it). Share. So I created a test application to figure out more about how Qt's Event loops is working. But all my logging (ie. When does an event “occur”? In Qt's built in events, mouse clicks and key presses are put into a queue and then the functions are executed at QThread's also have their own event loop. 12. 3. AllEvents]) # Parameters:. An event is a message encapsulated in a class (QEvent) which is processed in an event loop and dispatched to a recipient that can either accept the message or pass it along to others to process. Handle the processing of the data within the regular thread loop. implement additional function to event - Python. Think of it as the event loop is in charge and calling things: Is it just a way of preventing the application from closing while still servicing Qt's event loop (while True: pass would cause the event loop to block, as we understand from doing time. There are no interrupts going on in Qt. What you can do is create a second thread and wait on a condition variable. The main thread is blocked, because it is waiting for myThread to finish. I have two classes, MainGui and AudioWorker. You may want to have . The next time Qt’s main event loop runs, it dispatches all posted events, with some optimization. In a native Qt App There are several very good documentation from Qt about threading. The current latest version at the time of writing this answer is 27. So I have a class called testclass and this class objective is add a message to the que every 100ms. Qt async call: how to run something after an async call has finished its job. 1. On the other hand QCoreApplication::processEvents() will process as many events there are in the event loop at the time of call and return control to the user code immediately (on first possible occasion). c++; qt; qml; qeventloop; Share. Qt is an asynchronous framework, The dispatcher itself loops around the event queue and sends queued events to their target objects, and therefore it is called the event loop. What I am trying to do in an application using vtk for both interacting and rendering is to have two different parts: 1 - A thread with Rendering and vtkRenderWindowInteractor for interaction with mouse. In your case the QSocket::readReady signal is your friend. Since action_A is called from MyWidget's destructor, it will be called before the destroyed() signal is emitted in ~QObject(), and thus, action_B called. Maybe there is some overhead involved in call processEvents() compared to just picking the next event from the event loop. GUI interaction, network events, timers, ) or from other Qt components (e. Usually when you want to use another library in Qt app you just wrap around it with a QObject derived class and emit signals. Ask Question Asked 14 years, 8 months ago. Understanding Qt multithreading and event loop. " QObject has no event loop, the event loop is in QCoreApplication and siblings. QEventLoop(app)) exit(app. py i get the message: "QCoreApplication:exec: The event loop is already running" I figured out, that it has something to do with the timer . This is almost certainly more work than it's worth, unless you have very specific needs. Generally speaking, no user interaction can take place before calling exec(). @Sherafati "An EVENT loop is a loop of EVENTS(signals I have a C library that requires a GLib event loop to work, it takes a GMainContext on initialization and allows me to register callback functions. Y When this happens, the signals and slots mechanism is totally independent of any GUI event loop. Follow edited Feb 8, 2021 at 18:18. As a special case, modal widgets like QMessageBox can be used before calling exec(), because modal widgets use their own local event loop. Edit: I have used the opportunity of a slow Sunday morning to test-drive / learn something about PyQt (Python bindings for Qt) and cobbled together a proof This is a good information. exec(), one would have to force going through Qt event system in some way, like explicitly specifying that the connection is queued. Propagate QML events to C++. exec_()) Some parts of QT rely on the event loop being up and running (or at least generate warnings otherwise). QTimer() timer. 4. What does this return value mean? I tried searching for it but there is no mention of 'Loop Level' (or any kind of Level) in QEventLoop or QEvent class. myThread on the other hand is never finished, because the event loop in myThread keeps running unless exit() is called - what you don't do. Be aware of this and maybe use a timeout of something like 16ms. Hot Network Questions Qt/C++ event loop exception handling. exec(). For the first time, I have clicked a button which executes long-running code, doing goodness knows what. Each GUI program needs an event loop, whatever you write it Windows or Linux, using Qt, Win32 or any other GUI library. In the past, I have throttled the delivery of update messages from a worker to the GUI thread to at most two per second, and that worked very well for me. C++ Qt, Passing Callbacks. g. If you want to schedule an event for some point of time in the future, use QTimer. The events/event loops are not being dealt with "between function calls". Like is there some mechanism which can be used to assign an id to event loop and hence print it in the event loop to The event loop of the worker thread will be blocked, i. The questions I ask are related to this article: Qt 4. . If you need to depend on external things, you can use things like QAbstractSocket to send events when data comes in over an external socket, eg. The while stated there will use ~100% CPU if there are no events dispatched (as it will just keep polling). exec() und thus starting the engine of your application ;) Over time though, my logging shows it slowing down to the point where after running for roughly a half hour, I'm seeing that my paintEvent is happening only once every 2-3 seconds, creating a noticeable lag. you won't be able to process any events (including those which are used for 'queued' connections between signals and slots, which is what you get when doing signal-slot connections across threads) unless you explicitely trigger a call to the event loop object yourself. Then the class simulates sending a message outbound by simulating the wait process. I was hoping to get around this by creating local event loops within the class, but they don't Qt discussions, questions and answers. They are usually created in response to external system events like mouse clicks. Exactly one window has focus, systemwide (or at least one window per keyboard on multi-user systems). So when it processes some event, it cannot process any other until current operation stops (function ends) and the next iteration of the event loop happens. Note Qt’s event loop starts the moment the underlying application’s exec() function gets called. You can not just stop the one from you main application. Use QThread::exec() when you want to run the event loop Qt provides for you in the QThread class. 1. The event loop doesn't know. I am having an application heavily based on QT and on a lot of third party libs. If this dialog is shown with exec(), done() causes the local event loop to finish, and exec() to return r. Events may be Mouse events, keyboard events, paint events, timer events, etc. Read this for you to understand when moveToThread is needed, and this for how to do thread synchronization. If this is the case what happens when multiple I'd like to create a library built on top of QTcpServer and QTcpSocket for use in programs that don't have event loops in their main functions (because the Qt event loop is blocking and doesn't provide enough timing resolution for the real-time operations required). and what surprises me is what I described in my previous post about reinstalling Qt on Having to force the event loop means you are probably doing something that is not recommended and can be done in a better way. Qt internally processes that event. ZeroMQ, at least the C version, exposes a file descriptor that becomes to ready-to-read when a ZMQ message has been received. Additional resources: The main event loop receives events from the window system and dispatches these to the application widgets. For testing purpose I wait for a lone time 2 seconds. I mean does each event loop has its own event queue or all share one? How to check which event loop executed an event . Thanks, that is an idea - but I wonder if that would even clear the currently outstanding queue - I do want to receive new signals should they arrive during doing something heavy, but I don't want to receive the same signal from the same sender, in few copies in the same iteration of the event loop - one is sufficient. If flags are specified, only events of the types allowed by the flags will be processed. The GUI code I have has standard connection of button-clicked to handling code. processEvents() which means that the GUI will still accept e. QObject::emit(object, SIGNAL(function(flaot)), arg); I have some questions about the general usage of Qt events. If you call "execute" from a different thread, a specialized event is created, that holds our lambda Feb 7, 2012 · If you emit a signal from myObject to MainDialog it is posted to the event queue of the thread MainDialog lives in, the main thread. @J-Hilk said in Infinite wait when stopping QThread event loop and then calling wait:. Directly connected slots don't go to the event queue, only queued connections. I haven't done too much Qt development recently, but if I remember correctly, you can call QApplication::processEvents() within your own event loop (instead of starting the Qt main loop through QApplication::exec()). It does, however, work as you described - by using libuv to handle Qt's event loop (and by doing that - merges the two event loops into one). The same thing applies to local event loops started with QEventLoop. A QueuedConnection will post an event to the event loop to eventually be handled. It is possible to have additional event queues, with two implementation strategies that differ on whether you remain compatible with QCoreApplication::postEvent. I wanted to know about qt event loop. postEvent() posts the event on a queue for later dispatch. Reactors in Twisted are implementations of an interface and you can specify a type reactor to run: select, epoll, kqueue (all based on a c api using those system calls), there are also reactors based on the QT and ah, still relatively new to qt but would putting a timer in a separate QThread and having that tick for the controller polling work? Cus i did try something like that, and it did complain that events cant cross between threads or something like that. Call this function if you need QAbstractSocket to start sending buffered data immediately. What An event loop is usually a loop that is run by the main thread to receive events that either originate from the system (e. This is done in other bits of code. As a special case, modal widgets like QMessageBox can be used before calling exec() , because modal widgets use their own local event loop. When called this way, it functions exactly like a sleep function, but it uses Qt's event loop for the timing. Note: This function is thread-safe. In Qt "GUI Event Loop" (which is the main loop of all Qt applications) is hidden, but you start it calling: I agree with @Mher-Didaryan - that the event loop started by following line of code loop. Sleep makes the calling thread sleep irrespective of whether it's being called from thread's overridden run() function or any other function for that matter. Follow Combing an External Event Loop with Qt's. The number of bytes successfully written depends on the operating system. Jul 29, 2022 · "you need an event loop to use QNetworkAccessManager. Nulik. I would also add I am new to threading in general so that is part of my problem. As well each thread has its own event loop. Qt blocked event loop. Its event loop doesn't block just because your QObject executes an infinite loop. This is because the connect() between the SIGNAL and SLOT is being done for a different event loop than the The article mentioned in this comment, says that the event queue is protected by a mutex. The situation is slightly different when using queued connections ; in such a case, the code following the emit keyword will continue immediately, and the slots will be executed later. QThread's also have their own event loop. So don't do a lot of work directly within signal calls. I believe Qt does something smart here which helps with what you want. Without a Q*Application, you basically can only run Qt code sequentially. QObject::emit(object, SIGNAL(function(flaot)), arg); During the beginning all is working fine, but increasing time arrays are increasing all events are slowing down until one core is completely consumed. And you can create your own event loops with QEventLoop at any time and handle all events with a call to QEventLoop::exec(). Signals and Slots are a convenient way for QObjects to communicate with one another Also if your event processing takes forever, new events won't appended to the currently "todo" list, new events will be queued up and processed when the event loop is run next. log('Got it')) // Component 2 document. See also exit() and QEventLoop. It is an old maxim of mine that when you have excluded the impossible, whatever remains, however improbable, must be the truth. Oct 4, 2024 · Qt documentation reports that, when you post an event, it must be allocated on heap, and Qt main event loop will destroy it. Check if your emulator is up to date by going to Tools > Android > SDK Manager and clicking the SDK Tools tab:. exec_ to return immediately as if the gui were modeless, followed by further python code which interacts with the gui. Running a C++ Event The main event loop receives events from the window system and dispatches these to the application widgets. Instead, You can use QTimer to schedule something to run at a later point in time. If you need an event loop, because you receive events then you should use the normal QEventLoop::exec(). QEventLoop是Qt框架中处理事件循环的核心类。它允许程序在特定的时间点暂停执行，等待事件的发生，并对这些事件进行响应。QEventLoop负责监听各种事件源（如窗口系统事件、定时器事件、网络事件等），并根据事件的优先级和类型进行调度处理。在Qt应用程序中，通常会有一个主事件循环（由 At one point I need to be able to force all of them to happen and wait for them (first the heavy Tasks, then the event oriented ones, and finally again the heavy ones as the event oriented one can schedule 1 last). Everything is running synchronously. So i would like my top-level QWidget to receive a signal as soon as the event-loop begins. What does it meant by local event loop? Note that the repeated timeout version will make the event loop run back to back in intervals of < 1ms (on my machine with Qt 6. e. – QEvenLoop is more efficient then busy loop, it does not eat CPU like busy loop, and it allows the thread to process the signals and events. mouse events. exec();@ notifier->notifyThread() will be queued for execution and will be executed when main event loop starts. ProcessEventsFlag. 2 - A thread that call some modifier functions of I am looking for clarification to help me understand Qt multi threading and event loops with Qt 4. 10. It's as if the whole event loop is getting bogged down. Eh, mostly yes. How do you integrate Catch2 tests with the QT event loop? From this official Qt documentation event loop conceptually executes as follows: while (is_active) { while (!event_queue_is_empty) dispatch_next_event(); wait_for_more_events(); } As one can observe that Qt application will utilise the CPU all the time as it needs to keep running the loop for its GUI to be updated. Here is how you use it: app = QApplication(sys. 3) is that if deleteLater is called when NO event loop is running, then the first event loop entered will pick up the event and You'd test Qt code like you'd test anything else. My solution to integrate it with a Qt application is the classic thread + polling approach, I spawn a thread which every 100ms runs a single g_main_context_iteration(). exec() und thus starting the engine of your application ;) Does it get processed before the loop. NOTE: Yes, I do need to use QEventLoop as the main event loop. It is necessary to call this function to start If you emit a signal from myObject to MainDialog it is posted to the event queue of the thread MainDialog lives in, the main thread. These happen to throw some exceptions in several cases. Since both asyncio and PyQt use their own event loops, this would be impossible without qasync, which is created to solve this problem. Qt Multiple Async While Loops and qApp->processEvents(); 3. This function does nothing if the thread does not have an event loop. It makes no difference and there is no escape. connect(functions. asked Nov 8, 2018 at 21:47. Once started, the loop repeatedly checks for something to happen in the system, such You can send events in exactly the same ways as Qt's own event loop by constructing suitable event objects and sending them with QCoreApplication::sendEvent() and As a cross platform UI framework, Qt's event loop implementation principle is to encapsulate event loops of different platforms and provide a unified abstract interface. Like is there some mechanism which can be used to assign an id to event loop and hence print it in the event loop to Warning: Deleting a QObject while pending events are waiting to be delivered can cause a crash. I would certainly not use timers for that. The only notable exception I've found to this rule (as of Qt 4. When a button is clicked that event is being processed. exec_()) the thing i din't though, is that i don't need to call QCoreApplication exec() to have an event loop. Quote from the docs:. Hot Network Questions I wanted to know about qt event loop. A Q*Application instance is what keeps the Qt application alive, by running its main event loop. You can use a QTimer::singleShot with 200, 300 and 500 ms for example to set a delay beetween function calls without the need to call QThread::msleep in from the event loop thread, lambda get executed instantly, but QTimer::singleShot(1, [=]() {}); runs after all other tasks. 2 - A thread that call some modifier functions of Alberto: processEvents() is not how Qt was designed. also if you try to write to the socket from the ready read slot that can lead to event loop blocks. On the other hand if A one liner answer to the question is - sleep and event loop are not related. Content. timer = QtCore. [/quote] This would not work, as it would be executed when the thread event loop runs, not the main one. My current situation is I have a Qt app that needs to play sounds every so often using QTMultiMedia- QAudioOutput. You will also notice that a long running button click will halt the GUI. Howto call c++ member function as async call from QML. Like is there some mechanism which can be used to assign an id to event loop and hence print it in the event loop to Hi, while browsing through the documentation of QThread I came across int QThread::loopLevel() const, the only explanation for it is: Returns the current event loop level for the thread. To start an event loop from a non-GUI thread, use QThread::exec() Regarding the widget itself, I'm not very familiar with Qt widget's size policy strategy, but my incomplete understanding is that the size policy of a QTableView set to Minimum was somehow leading to an infinite loop of resize events (maybe when the resize of this widget was causing the resize of another widget ?). Good places to start: A quick introduction to the Google C++ Testing Framework courtesy of IBM; Getting Started with Qt; The one thing to keep in mind is that a significant part of Qt is event driven, which requires a running event loop. exec ([flags=QEventLoop. If deleteLater() is called after the main event loop has stopped, the object will I wanted to know about qt event loop. Not a whole lot of reasons to do this but the functionality is there if you need it. Qt has It is expected of the reader to already have a basic understanding on working with Qt. Follow edited Nov 8, 2018 at 22:04. start(1000) What I am trying to do in an application using vtk for both interacting and rendering is to have two different parts: 1 - A thread with Rendering and vtkRenderWindowInteractor for interaction with mouse. Like is there some mechanism which can be used to assign an id to event loop and hence print it in the event loop to When does the event loop decide that its okay to process an event in between function calls? It doesn't work like this. That will occur only when your code enters the Qt event loop; whether that is a local QEventLoop or the main application loop. You can However, i thought it'd be nicer to use event-handling rather than event-polling. To understand, i want to update Label/textBrowser/lineEdit text during some loop while Stop button isn't pushed. Quickly make a decision about whether you want the data, and if so, replace your previous reference and move onward. Modified 10 years, 2 months ago. @andre said in Signal/slot across threads - remove duplicate events in event loop:. Also, Your slots/functions should be as short and optimized as possible in order to return control to the event loop and be able to handle other events that may happen. 2), i. Like is there some mechanism which can be used to assign an id to event loop and hence print it in the event loop to The object will be deleted when control returns to the event loop. Also i understood that there is only 1 event queue which has events posted on it , the event loop which will take up the event will be the one which is currently in picture. Where did you use the QTimer is there an event loop? Yes, you are using the QTimer in the main thread and you have created the event loop through the QXApplication. Reactors in Twisted are implementations of an interface and you can specify a type reactor to run: select, epoll, kqueue (all based on a c api using those system calls), there are also reactors based on the QT and QMetaObject::invokeMethod( notifier, SLOT(notifyThread()), Qt::QueuedConnection ); return app. If we assume Qt is an asynchronous framework, and the event loop is a very important part to ensure all Qt features (like signals/slots) are working well. addEventListener('Hello', => console. Conceptually, this is how an event loop looks I am looking for clarification to help me understand Qt multi threading and event loops with Qt 4. What Are Events; Events and Signals/Slots; Using Events; Event Propagation; A Event loops are blocking calls (usually on exec()) so any event that happens will be processed by the currently "blocking" event loop. In JavaScript, I would achieve this by doing // Component 1 document. 8: The Event System. @danyal said in gui event loop does not work: Could it be that the message queue just can not cope with so many messages? Yes, if the thread emit too fast due to tight loop. int. When is a new event loop started? Which event queue does this use . But i am not sure yet as to how i should use that. but after startStressTest finished its work (and that's what happened when I clicked on the stop button), the event loop should have stopped but it didn't. exec(); in the 2nd code snippet - will never exit. Note that you cannot control the timing of the call of action_A as long as it from the destructor of MyWidget, so deferring the call to I wanted to know about qt event loop. " I've also read about connecting a signal to a slot which will be called from the event loop by setting the connection type to Qt::QueuedConnection. The second link (qt-event-dispatcher-libuv) is a project I've created to tackle the same problem. set_event_loop(qasync. Events are dispatched by the event loop. Qt infinite loop in another thread. This is exactly what this documentation refers to: your calling thread will send the signal, receiving thread will queue it - but even if queue is empty it may take a while until current I wanted to know about qt event loop. Watching for event loop iterations will allow my future classes to notify any state changement. " I haven't used Qt before, so I might be wrong on this, but I imagine you could bind your on_start() method to the ApplicationActivate event, and set a flag from inside your on_start() method so that the code would only be run that very first time and not any other times the ApplicationActivate event is triggered during execution of your program. If there is an update available, the Status in the right-hand column will say @Bob that you call start before the event loop does not start is not a problem, what the docs points out is that the QTimer must live in an event loop, for example if instead of using app. 0. Event loop of main thread is started when you call a. Does it update the screen? No. Like is there some mechanism which can be used to assign an id to event loop and hence print it in the event loop to I am trying to emit custom events in PyQt. I am testing a gui built using PyQt and I would like the ability to interact with the gui using python code that is executed after the PyQt event loop starts (app. deleteLater() is called on an object before QCoreApplication::exec()), the object will be deleted once the event loop is started. Another way of saying this is I would like the call to app. I don't think there is any "magic" solution to be found here; a thread can't be running Qt's event loop if it is running your own custom event loop. It's basically a while loop that calls processEvents of a per thread event dispatcher. start(1000) The right way "Qt-wise" is to use a timer and let Qt manage the event loop. To ensure that quit() is called after loop. cmsfdh bpy vqpots kbnnhj jhywjq rgfocxh hqaaz uzwgxsg fhtzgx bxfq