Qt Signal Slot Thread Safety
2021年11月13日Register here: http://gg.gg/wv2cj
Signals and slots were one of the distinguishing features that made Qt an exciting and innovative tool back in time. But sometimes you can teach new tricks to an old dog, and QObjects gained a new way to connect between signals and slots in Qt5, plus some extra features to connect to other functions which are not slots. Let’s review how to get the most of that feature. Jul 23, 2013 Introduction. For any C developer who’s used Qt, we’ve grown to love the Signals/Slots idiom it presents for creating clean Observer code. However, it relied on the Qt Moc pre-compiler tool, which meant any project that wanted to use this feature had to use follow along with the Qt idiom, which really made Qt applications look potentially foreign despite being written in C. pyqt update gui from thread
qthread
update ui from thread c++
qt thread example
qt event loop
qt main thread
qt connect
qthread example
i have an multithreaded qt application. when i am doing some processes in mainwindow.cpp, at the same time, i want to update mainwindow.ui from other thread.
i have mythread.h
mythread.cpp
but the problem is that, i cannot reach the ana->ui->horizontalLayout_4->addWidget(label);
how can i do that?
but the problem is that, i cannot reach the ana->ui->horizontalLayout_4->addWidget(label);
Put your UI modifications in a slot in your main window, and connect a thread signal to that slot, chances are it will work. I think only the main thread has access to the UI in Qt. Thus if you want GUI functionality, it must be there, and can be only signaled from other threads.
OK, here is a simple example. BTW, your scenario doesn’t really require to extend QThread - so you are better off not doing it, unless you really have to. That is why in this example I will use a normal QThread with a QObject based worker instead, but the concept is the same if you subclass QThread:
The main UI:
.. and the worker object:
The worker object is created and moved to another thread, then connected to the slot that creates the labels, then its newLabel method is invoked, which is just a wrapper to emit the requestNewLabel signal and pass the path to the image. The signal is then passed from the worker object/thread to the main UI slot along with the image path parameter and a new label is added to the layout.
Since the worker object is created without parent in order to be able to move it to another thread, we also connect the thread destroyed signal to the worker deleteLater() slot.
Is this an acceptable/safe way to update GUI from another thread , I am new to Qt and am writing a program on Windows, using Visual Studio One of the things this program does is create a background thread to I want the main GUI window to update when the external hardware state� Qt - updating main window with second thread. Ask Question Asked 7 years, 1 month ago. Active 7 years, 1 month ago. Viewed 31k times 18. 11. i have an multithreaded
First and foremost, ’you’re doing it wrong’. Normally you want to create a class derived from a QObject and move that class to a new thread object instead of deriving your class from a Qthread
Now to get onto the specifics of your question, you’re not able to directly modify the ui elements of your main GUI thread from a separate thread. You have to connect a signal from your 2nd thread to a slot in your main thread. You can pass any data that you need through this signal/slot connection but you’re unable to directly modify the ui element (which in all honestly you probably do not want to if you intend to keep the frontend of your app separate from the backend). Checkout Qt’s signal and slot documentation for a whole lot more information
Update GUI from another thread, The actual work is done within a separate thread. This way the GUI remains responsive. Everything works great, but I just can’t figure out how to update the GUI from the separate thread. This because I need to develop a application that runs on both Windows and Mac. int main(int argv, char** args). The Qt GUI must run in this thread. All widgets and several related classes, for example QPixmap, don’t work in secondary threads. A secondary thread is commonly referred to as a ’worker thread’ because it is used to offload processing work from the main thread. Simultaneous Access to Data
how can i do that?
You’ve already got the answers to what you should be doing, but not a why, so I’m going to add a why.Qt Signal Slot Thread Safety Tool
The reason you don’t modify GUI elements from another thread is because GUI elements are usually not thread-safe. This means that if both your main GUI thread and your worker thread update the UI, you cannot be certain of the order of what happened when.
For reading data generally this can sometimes be fine (e.g. checking a condition) but generally you do not want this to be case. For writing data, this is almost always the source of very, very stressful bugs which occur ’at random’.
Another answer has remarked on good design principles - not only does constraining your GUI logic to one thread and firing signals to talk to it get rid of your race condition issues, but it also forces you to compartmentalize your code nicely. Presentation logic (the display bit) and data processing logic can then be cleanly separated out, which makes maintaining the two much easier.
At this stage you might think: heck, this threads business is farrrrrr too much work! I’ll just avoid that. To see why this is a bad idea, implement a file copy program in a single thread with a simple progress bar telling you how far along the copy is. Run it on a large file. On Windows, after a while, the application will ’go white’ (or on XP I think it goes gray) and will be ’not responding’. This is very literally what is happening.
GUI applications internally mostly work on the variation of ’one big loop’ processing and dispatching messages. Windows, for example, measures response time to those messages. If a message takes too long to get a response, Windows then decides it is dead, and takes over. This is documented in GetMessage().
So whilst it may seem like quite a bit of work, Signals/Slots (an event-driven model) is basically the way to go - another way to think of this is that it is totally acceptable for your threads to generate ’events’ for the UI too - such as progress updates and the like.
Qt5 Tutorial QThreads - Gui Thread - 2020, Qt5 Tutorial: Gui Thread. to the onValueChanged() which will update the count label on the dialog box. We’ll use Qt Gui Application with QDialog. int main( int argc, char *argv[]) { QApplication a(argc, argv); Dialog w; w.show(); return� Starting with Qt 4.4, there are extensions available to let us do parallel programming: these are provided by QThreadPool and Qt Concurrent. The first possible course of action is to use so called runnables—simple classes whose instances can be executed by a thread. Qt implements runnables through its QRunnable class.
Multithreading with Qt, The QThread is the central class for of the Qt threading system. is executed in a separate thread that has to notify the UI to update some state: a result available in MainWindow::handleResult), whereas the second connect� Cross-thread operation not valid: Control ’Main’ accessed from a thread other than the thread it was created on. I use .net 4 with Winform C#. Thanks a lot for answers. News: The problem is this line: trd.Join(); This line block my GUI and the lable was not update. There are methods to control the finish of thread and updating the label until
Effective Threading Using Qt, There are three main ways I’ve seen people handle basic threading in their Qt applications. The first is using system threads, either pthread or Windows threads . I don’t The second worker could take arguments if you need it to. The MainWindow connects and uses this to, well, update the count. finished� Qt will only let you create widgets from the main GUI thread, this is explicitly mentioned in the docs (emphasis mine): As mentioned, each program has one thread when it is started. This thread is called the ’main thread’ (also known as the ’GUI thread’ in Qt applications). The Qt GUI must run in this thread.
Qt: Access GUI From Another Thread?, Qt does provide an Event system that’s thread-safe, but writing all those Well, for example, one of my worker threads needs to update the status to write another class with signals that match all the slots of the MainWindow,� With the main window form selected, change the title of the window to Time. This is done using the right bottom pane in Qt Creator next to windowTitle. 3. Add a Timer to a Qt Creator Project. A QTimer is used in this project to create a 1s timer tick that calls a function every 1s time period. 3.1 Add a QTimer to the Main Window Class Qt Signal Slot ExampleComments
*If you are like me and you have like 10 minutes till the deadline, here is a more hackish solution: add a dummy button in the main window (width and height 0) , whenever you need to update the ui from the worker emit a click () event in the worker and overwrite the click handler for that button to do the updates.
*can you give an example for connect? i can not decide elements of connect.
*@abby - I added a quick example how to achieve what you need.
*I tried this but instead of loading an image I just put some computations in newLabelText() (had to modify newLabel(),requestNewLabel()) to see if this actually resolves a common problem of freezing the UI when using threads when doing extensive background work (as the author suggests he/she wants to do). And indeed it does freeze the whole thing for a bit (I put an LCD Number control and a button that I use for counting up and displaying the result on the LCD just to check if it does indeed freeze). Which also means that if we load a very large image, this might lead to blocking the UI.
*I guess your GUIUpdater is leaked unless, you add this as parent.
*are there any way to update mainwindow.ui from worker thread? Because i really need it.
*you would update the ui element in the slot that was connected to the signal from the different thread
*Ah that infamous ’You’re doing it wrong’ blog post. I strongly disagree with him. See woboq.com/blog/qthread-you-were-not-doing-so-wrong.html
*There is nothign wrong in extending a QThread. It was even used in examples in Qt.Qt Connect Signal SlotHot Questions
Register here: http://gg.gg/wv2cj
https://diarynote.indered.space
Signals and slots were one of the distinguishing features that made Qt an exciting and innovative tool back in time. But sometimes you can teach new tricks to an old dog, and QObjects gained a new way to connect between signals and slots in Qt5, plus some extra features to connect to other functions which are not slots. Let’s review how to get the most of that feature. Jul 23, 2013 Introduction. For any C developer who’s used Qt, we’ve grown to love the Signals/Slots idiom it presents for creating clean Observer code. However, it relied on the Qt Moc pre-compiler tool, which meant any project that wanted to use this feature had to use follow along with the Qt idiom, which really made Qt applications look potentially foreign despite being written in C. pyqt update gui from thread
qthread
update ui from thread c++
qt thread example
qt event loop
qt main thread
qt connect
qthread example
i have an multithreaded qt application. when i am doing some processes in mainwindow.cpp, at the same time, i want to update mainwindow.ui from other thread.
i have mythread.h
mythread.cpp
but the problem is that, i cannot reach the ana->ui->horizontalLayout_4->addWidget(label);
how can i do that?
but the problem is that, i cannot reach the ana->ui->horizontalLayout_4->addWidget(label);
Put your UI modifications in a slot in your main window, and connect a thread signal to that slot, chances are it will work. I think only the main thread has access to the UI in Qt. Thus if you want GUI functionality, it must be there, and can be only signaled from other threads.
OK, here is a simple example. BTW, your scenario doesn’t really require to extend QThread - so you are better off not doing it, unless you really have to. That is why in this example I will use a normal QThread with a QObject based worker instead, but the concept is the same if you subclass QThread:
The main UI:
.. and the worker object:
The worker object is created and moved to another thread, then connected to the slot that creates the labels, then its newLabel method is invoked, which is just a wrapper to emit the requestNewLabel signal and pass the path to the image. The signal is then passed from the worker object/thread to the main UI slot along with the image path parameter and a new label is added to the layout.
Since the worker object is created without parent in order to be able to move it to another thread, we also connect the thread destroyed signal to the worker deleteLater() slot.
Is this an acceptable/safe way to update GUI from another thread , I am new to Qt and am writing a program on Windows, using Visual Studio One of the things this program does is create a background thread to I want the main GUI window to update when the external hardware state� Qt - updating main window with second thread. Ask Question Asked 7 years, 1 month ago. Active 7 years, 1 month ago. Viewed 31k times 18. 11. i have an multithreaded
First and foremost, ’you’re doing it wrong’. Normally you want to create a class derived from a QObject and move that class to a new thread object instead of deriving your class from a Qthread
Now to get onto the specifics of your question, you’re not able to directly modify the ui elements of your main GUI thread from a separate thread. You have to connect a signal from your 2nd thread to a slot in your main thread. You can pass any data that you need through this signal/slot connection but you’re unable to directly modify the ui element (which in all honestly you probably do not want to if you intend to keep the frontend of your app separate from the backend). Checkout Qt’s signal and slot documentation for a whole lot more information
Update GUI from another thread, The actual work is done within a separate thread. This way the GUI remains responsive. Everything works great, but I just can’t figure out how to update the GUI from the separate thread. This because I need to develop a application that runs on both Windows and Mac. int main(int argv, char** args). The Qt GUI must run in this thread. All widgets and several related classes, for example QPixmap, don’t work in secondary threads. A secondary thread is commonly referred to as a ’worker thread’ because it is used to offload processing work from the main thread. Simultaneous Access to Data
how can i do that?
You’ve already got the answers to what you should be doing, but not a why, so I’m going to add a why.Qt Signal Slot Thread Safety Tool
The reason you don’t modify GUI elements from another thread is because GUI elements are usually not thread-safe. This means that if both your main GUI thread and your worker thread update the UI, you cannot be certain of the order of what happened when.
For reading data generally this can sometimes be fine (e.g. checking a condition) but generally you do not want this to be case. For writing data, this is almost always the source of very, very stressful bugs which occur ’at random’.
Another answer has remarked on good design principles - not only does constraining your GUI logic to one thread and firing signals to talk to it get rid of your race condition issues, but it also forces you to compartmentalize your code nicely. Presentation logic (the display bit) and data processing logic can then be cleanly separated out, which makes maintaining the two much easier.
At this stage you might think: heck, this threads business is farrrrrr too much work! I’ll just avoid that. To see why this is a bad idea, implement a file copy program in a single thread with a simple progress bar telling you how far along the copy is. Run it on a large file. On Windows, after a while, the application will ’go white’ (or on XP I think it goes gray) and will be ’not responding’. This is very literally what is happening.
GUI applications internally mostly work on the variation of ’one big loop’ processing and dispatching messages. Windows, for example, measures response time to those messages. If a message takes too long to get a response, Windows then decides it is dead, and takes over. This is documented in GetMessage().
So whilst it may seem like quite a bit of work, Signals/Slots (an event-driven model) is basically the way to go - another way to think of this is that it is totally acceptable for your threads to generate ’events’ for the UI too - such as progress updates and the like.
Qt5 Tutorial QThreads - Gui Thread - 2020, Qt5 Tutorial: Gui Thread. to the onValueChanged() which will update the count label on the dialog box. We’ll use Qt Gui Application with QDialog. int main( int argc, char *argv[]) { QApplication a(argc, argv); Dialog w; w.show(); return� Starting with Qt 4.4, there are extensions available to let us do parallel programming: these are provided by QThreadPool and Qt Concurrent. The first possible course of action is to use so called runnables—simple classes whose instances can be executed by a thread. Qt implements runnables through its QRunnable class.
Multithreading with Qt, The QThread is the central class for of the Qt threading system. is executed in a separate thread that has to notify the UI to update some state: a result available in MainWindow::handleResult), whereas the second connect� Cross-thread operation not valid: Control ’Main’ accessed from a thread other than the thread it was created on. I use .net 4 with Winform C#. Thanks a lot for answers. News: The problem is this line: trd.Join(); This line block my GUI and the lable was not update. There are methods to control the finish of thread and updating the label until
Effective Threading Using Qt, There are three main ways I’ve seen people handle basic threading in their Qt applications. The first is using system threads, either pthread or Windows threads . I don’t The second worker could take arguments if you need it to. The MainWindow connects and uses this to, well, update the count. finished� Qt will only let you create widgets from the main GUI thread, this is explicitly mentioned in the docs (emphasis mine): As mentioned, each program has one thread when it is started. This thread is called the ’main thread’ (also known as the ’GUI thread’ in Qt applications). The Qt GUI must run in this thread.
Qt: Access GUI From Another Thread?, Qt does provide an Event system that’s thread-safe, but writing all those Well, for example, one of my worker threads needs to update the status to write another class with signals that match all the slots of the MainWindow,� With the main window form selected, change the title of the window to Time. This is done using the right bottom pane in Qt Creator next to windowTitle. 3. Add a Timer to a Qt Creator Project. A QTimer is used in this project to create a 1s timer tick that calls a function every 1s time period. 3.1 Add a QTimer to the Main Window Class Qt Signal Slot ExampleComments
*If you are like me and you have like 10 minutes till the deadline, here is a more hackish solution: add a dummy button in the main window (width and height 0) , whenever you need to update the ui from the worker emit a click () event in the worker and overwrite the click handler for that button to do the updates.
*can you give an example for connect? i can not decide elements of connect.
*@abby - I added a quick example how to achieve what you need.
*I tried this but instead of loading an image I just put some computations in newLabelText() (had to modify newLabel(),requestNewLabel()) to see if this actually resolves a common problem of freezing the UI when using threads when doing extensive background work (as the author suggests he/she wants to do). And indeed it does freeze the whole thing for a bit (I put an LCD Number control and a button that I use for counting up and displaying the result on the LCD just to check if it does indeed freeze). Which also means that if we load a very large image, this might lead to blocking the UI.
*I guess your GUIUpdater is leaked unless, you add this as parent.
*are there any way to update mainwindow.ui from worker thread? Because i really need it.
*you would update the ui element in the slot that was connected to the signal from the different thread
*Ah that infamous ’You’re doing it wrong’ blog post. I strongly disagree with him. See woboq.com/blog/qthread-you-were-not-doing-so-wrong.html
*There is nothign wrong in extending a QThread. It was even used in examples in Qt.Qt Connect Signal SlotHot Questions
Register here: http://gg.gg/wv2cj
https://diarynote.indered.space
コメント