I am making something in a Qt Widget Project, coded in C++. Because of what I need to do, I need infinite looping, and after doing my research, I realized that infinite looping in an object's event doesn't work, and instead, I need to use a some threading..
I decided to use QTimer, but am thinking about using QThread. Which one should I use?
This is my QTimer code, which doesn't seem to work:
Clock_Application::Clock_Application(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::Clock_Application)
{
ui->setupUi(this);
QTimer *timer_Stopwatch = new QTimer(this);
connect(timer_Stopwatch, SIGNAL(timeout()), this, SLOT(timer_Start()));
timer_Stopwatch->start(1000);
}
Edit: a simple fix to this solution is using the QCoreApplication::processEvents() function, after every iteration of the loop, as that allows the widget application to process necessary events. This is a quick fix however, and using the QTimer library is a better way to fix it.
As you suggest yourself, performing your processing in a separate thread would be an option and I would surely look into doing that. I don't think that the QTimer as you show it will help.
A quick fix however would be to manually call QCoreApplication::processEvents() at the end of each loop. This will make sure that all the pending events get processed by Qt, keeping your UI responsive.
Your timer is not working because it only exists in the constructor. You should declare it out the constructor in the header file as private for example.
Related
I am currently starting on QTCreator. I have been asked to use QTimers in a particular context which is this:
We have an open window,
One or more QTimers are triggered and make things appear on the screen every x msec.
When we press "Escape" the window should close and everything should be reset to 0.
But here is the problem, the timers are defined in a static way:
QTimer::singleShot(500, this, SLOT(foo());
When I call this->close() (which closes my window), the timers do not stop and continue. I tried several solutions: browse all the QTimers contained in my object, obviously there are none since they are defined in static. Instead of declaring them in static I've tried to create each time a new QTimer object like that:
QTimer *timer= new QTimer(this);
timer->setSingleShot(true);
timer->setInterval(2000);
timer->setParent(this);
timer->start();
And then call timer->stop() later, but I think it's very brutal when you have multiple Timers in the same code.
Is there a way to stop the timers when this->close is called, knowing that the timers are defined as a static one ?
Assuming you are using,
QWindow *qw = new QWindow();
QTimer *timer= new QTimer();
To solve the issue you need to connect destroyed() signal of QWindow to timer's slot stop()
So as soon as window is destroyed all registered timers will be stopped without explicit stop call. make sure you connect all timer instances. Code snippet as following,
QObject::connect(&qw, SIGNAL(destroyed()), timer, SLOT(stop()))
QObject::connect(&qw, SIGNAL(destroyed()), timer2, SLOT(stop()))
QObject::connect(&qw, SIGNAL(destroyed()), timer3, SLOT(stop()))
PS:
QTimer *timer= new QTimer(this); // here you are setting parent as 'this' already
timer->setSingleShot(true);
timer->setInterval(2000);
timer->setParent(this); // remove this, no need to set parent again.
timer->start();
I have a quite lengthy foreach loop in a QDialog. It basically looks like this:
foreach (xxx, xxx) {
... doSomeStuff ...
QApplication::processEvents();
if (m_cancelMapLoading) {
break;
}
}
m_cancelMapLoading is set to true by clicking a "Cancel" button. The QApplication::processEvents(); makes this possible.
This works quite fine, but if the dialog is closed as long as that foreach loop still runs, it continues running. I tried to set m_cancelMapLoading to true in each function closing the dialog, but this does not help.
I also tried to test not only for m_cancelMapLoading being true, but also for isVisible(). This actually stops the dialog, but it re-opens it at once without the GUI elements in it.
Unfortunately, QtConcurrent::run etc. can't be used for the function, because the data structures that are manipulated by the foreach loop are not thread safe.
Is there a convenient way to solve this?
You can use a QTimer and Qt's parent-child structure to your advantage here. QTimer with a timeout value of zero has a special meaning in Qt
As a special case, a QTimer with a timeout of 0 will time out as soon
as all the events in the window system's event queue have been
processed. This can be used to do heavy work while providing a snappy
user interface:
So you could do something like
void Dialog::beginDoingStuff()
{
m_timer = new QTimer(this);
connect(m_timer, SIGNAL(timeout()), this, SLOT(processData());
m_timer->start(0);
}
void Dialog::processData()
{
// Perform one cycle of your process here
}
This will perform the processData() function in the same thread as the rest of the dialog, and when the dialog is destroyed by being closed, the timer will be deleted (because it's parent is the dialog), meaning the processing will stop.
A good and quite easy way to unload your GUI from heavy processing is assigning it to another thread or QtConcurrent.
You could then either poll a "should-I-terminate-yet?" variable or terminate the thread manually when it is no longer needed.
I highly recommend a parallel processing since offers better control rather than doing a "DoEvents"-like queue emptying.
We actually managed to solve the problem by connecting the dialog's finished signal to the click slot of the cancel button. This actually stops the loop in all circumstances.
We also introduced starting the function by a QTimer (for a nicer implementation not blocking the function where it's started), but this does not stop the loop (perhaps because we don't destroy the dialog when it's closed).
Thanks for all help :-)
I am writing a mathematical simulator in Qt5 and when I click a button, some heavy calculation needs to be done. During it, I want to display a Please, wait window, eventually with a progress bar. Preferably without using threads.
Problem is, during calculation the application hangs, events are not handled, and what should be a please, wait window is merely a window frame without any contents (looks transparent).
PC: AMD X2 L325, 2GB RAM, Radeon E4690
OS: Debian 6 (Squeeze, Old stable)
gcc 4.4.5, Qt 5.2.0
Any help would be appreciated!
While you could call QApplication::processEvents, it's not the best approach.
If you're avoiding threads because they're new to you, then I'm sure you'll find it much easier once you've done it once. Qt makes it easy.
All you need to do is create a class derived from QObject which will do the calculations. Next, create a thread and call the object's moveToThread function. The thread is then controlled with a few signals and slots. That's all there is to it.
Rather than repeat the content here, I suggest you read this article, which explains it in detail and provides clean and concise example code.
From the new object that processes your calculations, it can then emit a signal to a slot on the main thread, which will update the progress on your "Please Wait" window.
Note that Widgets and all rendering must be done on the main thread.
I prefer use Qt Concurrent. It is not well documented, but very handy, much better then using QThread directly.
I usually do such things like that I have separate QObject for logic, in there some slot which calls heavy calculation like that (see run documentation):
void MyCalculationClass::startCalculating() {
QtConcurent::run(this, &MyCalculationClass::doHeavyCalculation);
}
Where method doHeavyCalculation emits signals with progress and calculation results.
Usually this method operates on its own data not modified by other threads/methods, so if synchronization is needed, it is trivial. Signal-Slot mechanism is handling most of the multithreading issues (note that default connections detects that signals is emitted from other threads and queues slot calls in proper thread, it is done automatically by default).
i have such problem and i solve it like this, i make calculation in another thread( i use std::thread instad of QThread) before i start my calculation i just start timer
QTimer *timer = new QTimer(this);
connect(timer, SIGNAL(timeout()), this, SLOT(repaint()));
timer->start(1);
and then
std::thread t(&Widget::calcBackgound, this);
t.detach();
You can put the heavy and time consuming part of your code in run() member function of another class which inherits from QThread. After that create an instance of the new class and simply call its start() function.
Your new class can have a signal which emits the current progress value. you can then connect this signal to the slot of a progress bar.
I'm building some code where I'm running a while loop and, within the loop, am trying to change the contents of a couple of textboxes with QLineEdit's setText(). However, merely calling setText within the loop does not work; the textboxes only change their actual value once the code has run through, instead of at each iteration.
I have little experience with C++ or Qt, but the project I'm working on must use them. Any help?
EDIT: I'm guessing this must be something simple that I simply am having troubles because of my lack of familiarity/knowledge, but if more information is needed I'll gladly provide it!
The problem is that QT needs control to return to the UI thread's event loop in order to update the QLineEdit's visual appearance. The quick and dirty way to run the event loop is to add QCoreApplication::processEvents() after each call to setText(). The proper way to fix it is to move the blocking process that sets the value of the text box into another thread, expose an updateText(QString text) signal, connect it to the TextBox's setText(const QString & text) slot and emit that signal whenever you want the text to be updated.
Please see my answer to a similar question for more detail: unexplained delay after QProgressBar finish loading
You might also want to check out some of the documentation on QThreads and the Qt signal slot system: http://harmattan-dev.nokia.com/docs/library/html/qt4/threads-qobject.html
In my case, calling only repaint() or processEvents() won't do the job.
Within your function loop, call both QCoreApplication::processEvents(); and repaint();:
for (i;...)
{
//do your calculations
//...
QCoreApplication::processEvents();
repaint();
}
Calling ui->mywidget->update() didn't make any different as well.
(Tested for Qt4.8.3 on Kubuntu 12.10 and Qt5.0.1 on Windows XP)
I have got a problem with my QProgressBar and I hope someone got an idea...
I have created a progress dialog with a QProgressBar on my own. I set minimum and maximum steps to 0 so that the progress indicates my program is busy (the animation thing...).
I show() this progress dialog and activated the Qt::WindowModal for this dialog.
The problem: I use this dialog while copying files but the progress bar stops and no animation anymore to indicate my program is still busy. I use the windows function 'SHFileOperation' to copy one directory with a lot of file to a destination. This, of course, produces a lot of load on the system but at least the progress should continue moving.
Any help is appreciated!
Thanks in advance,
BearHead
The problem is that the SHFileOperation call will block the main event loop. Therefore, no events will be processed preventing the QProgressBar from being updated.
To fix this you could perform the copy action in a separate thread. The easiest way to go about this is using Qt Concurrent, for example as follows:
QFuture<void> future = QtConcurrent::run(SHFileOperation, ...);
QFutureWatcher<void> watcher;
connect(&watcher, SIGNAL(finished()), dialog, SLOT(close()));
Assuming dialog is a pointer to your progress dialog.
Btw, why do you use SHFileOperation instead of the operations provided by QDir and QFile?