What is the problem with this QThread class?
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Of course Signals will not be lost. But, I need updated value in GUI thread, which ll be updated in Slot executed when the signal sent by helper.may be processEvents() is useful, but should use it again and again in the code to get updated values, before GUI thread returns to event loop.isn't it.So for a conclusion === SIGNAL/SLOT is the safest mechanism to implement than mutex in helper thread and accesing the value by main thread through helper Thread object.
[quote author="MuldeR" date="1384088661"]WIth Signals & Slots you don't need to worry. GUI thread (and any thread running an event loop) will continuously process events. So it's all event based! If GUI thread needs some info from "worker" thread before it can proceed with function XYZ, simply call function XYZ from the Slot that will be invoked by the "worker" thread (via signal) as soon as that info is present.Of course GUI thread can do other work in the meantime. Signals will never get lost! All pending signals are processed as soon as the GUI thread returns to the event loop. Nevertheless, you should avoid running lengthy (blocking) function calls in the GUI thread. This would "block" the GUI thread and your application window will appear to "freeze". Also no incoming signals (e.g. from "worker" thread) are processed while the GUI thread is in a "blocked" state!
(Though signals that arrive while the GUI thread is in "blocked" state are not lost. They are processed when the GUI thread returns to its event loop!)
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Note: QApplication::processEvents() may be used to process all pending signals within a "long" function, but be very careful with it! It can happen that processEvents() processes an event that calls a Slot and that Slot in turn may call other functions. It's not easy to understand what can happen and you can even end up in an infinite recursion.[/quote]
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I think your design to "actively" wait for the updated value is not good. Think of a different design! Instead of making a monolithic function that waits for the updated value, cut it down into several functions. Function (1) can start the "worker" thread. Function (2) can do some other work that is not dependent on result from "worker" thread. Function (3) does the stuff that depends on result from "worker" thread. Now function (2) might be invoked immediately after function (1), but function (3) will not be invoked before the "worker" thread sends the signal. You can invoke function (3) directly from the Slot that receives the signal. There is no need to actively "wait" here.
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In case you really need to wait for a signal in a "synchronous" way (but without making your application freeze!), you can still do this:
@QEventLoop loop;
connect(myThread, SIGNAL(someSignal()), &loop, SLOT(quit()));
loop.exec(); //<-- Event processing until thread is done@But you should ONLY use this as a "last resort" solution, as it can have some unexpected behavior!
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yes. i think I ve to change my design. and in slot i ve to update value, and when needed updated value, the GUI thread will processEvents(), then access the value.
but my aim to create a helper thread is-to copy files(time taking operation), and my GUI ll show only few (6) widgets having one of the properties as the new file name.till the file is not set to individual widgets,no small widgets will be displayed. hence I need updated value and then only i can show widgets. So any how i ve to wait, whether the image is set to widget/not. but i can set individual image to small widgets by GUI thread in a slot,without depending on helper thread to set image on small widget(which cause of freezing).
Thank you so much sir for sharing some valuable suggestion.Thanks a lot.
[quote author="MuldeR" date="1384091572"]I think your design to "actively" wait for the updated value is not good. Think of a different design! Instead of making a monolithic function that waits for the updated value, cut it down into several functions. Function (1) can start the "worker" thread. Function (2) can do some other work that is not dependent on result from "worker" thread. Function (3) does the stuff that depends on result from "worker" thread. Now function (2) might be invoked immediately after function (1), but function (3) will not be invoked before the "worker" thread sends the signal. You can invoke function (3) directly from the Slot that receives the signal. There is no need to actively "wait" here.__
In case you really need to wait for a signal in a "synchronous" way (but without making your application freeze!), you can still do this:
@QEventLoop loop;
connect(myThread, SIGNAL(someSignal()), &loop, SLOT(quit()));
loop.exec(); //<-- Event processing until thread is done@But you should ONLY use this as a "last resort" solution, as it can have some unexpected behavior![/quote]
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You don't have to call processEvents(). Especially don't call it in order to "access" some value, because there is absolutely no guarantee that the desired Signal will arrive during your processEvents() call. And also please do not call processEvents() in a loop or something. That's really bad design.
Instead: Simply do nothing at all! As soon as the desired Signal arrives, the Slot connected to that signal will be invoked - fully automatically. Then, whatever needs to be done with the "new" information can be done in the that Slot - or in some function that is now invoked from that Slot.
There really should be no need to "actively" wait for the signal...
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Is it possible for the main thread to process events while running some other codes?If not then how it is possible to get updated data while running.please tell me how to design it?please go through the previous reply, i ve edited my requirement in previous post.
may be I need to implement mutex in helper thread and access data from GUI thread,and wait for specific value and then proceed.
[quote author="MuldeR" date="1384093010"]You don't have to call processEvents(). Especially don't call it in order to "access" some value, because there is absolutely no guarantee that the desired Signal will arrive during your processEvents() call. And also please do not call processEvents() in a loop or something. That's really bad design.Instead: Simply do nothing at all! As soon as the desired Signal arrives, the Slot connected to that signal will be invoked - fully automatically. Then, whatever needs to be done with the "new" information can be done in the that Slot - or in some function that is now invoked from that Slot.
There really should be no need to "actively" wait for the signal...[/quote]
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No, if your GUI thread is doing something else, it obviously can NOT process signals at the same time, but it will do so, as soon as it returns to the event loop. Also, with a proper design, the GUI thread will spent about 99% of its time in the event loop, so signals should be caught up very quickly under normal circumstances....
__Why not simply:
@void MyMainWindow::init()
{
doSomeInitializationStuffFirst();m_thread = new MyWorkerThread(/*some input data*/); connect(m_thread, SIGNAL(newInfoAvail(Foo)), this, SLOT(processNewInfo(Foo))); m_thread.start(); doOtherStuffHereThatDoesNotNeedTheThreadResult();}
//SLOT
void MyMainWindow::processNewInfo(Foo bar)
{
//following function can update the GUI
doWhateverNeedsToBeDoneWithTheNewInfo(bar);
}@NOTE: After init() function exists, your GUI thread will simply keep on processing events until a Signal form the worker thread arrives - in which case the Slot will be invoked for further processing.
No processEvents() required anywhere...
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thank you very much sir. I ll redesign according to my requirement and yur suggestions.
__Why not simply:
@void MyMainWindow::init()
{
doSomeInitializationStuffFirst();m_thread = new MyWorkerThread(/*some input data*/); connect(m_thread, SIGNAL(newInfoAvail(Foo)), this, SLOT(processNewInfo(Foo))); m_thread.start(); doOtherStuffHereThatDoesNotNeedTheThreadResult();}
void MyMainWindow::processNewInfo(Foo bar)
{
doWhateverNeedsToBeDoneWithTheNewInfo(bar);
}@No processEvents() required anywhere...[/quote]
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[quote]Is it possible for the main thread to process events while running some other codes?If not then how it is possible to get updated data while running.please tell me how to design it?please go through the previous reply, i ve edited my requirement in previous post.
may be I need to implement mutex in helper thread and access data from GUI thread,and wait for specific value and then proceed.[/quote]I don't see how this should be a problem ???
Example:
@void MyMainWindow::init()
{
QStringList filesToBeCopied = generateFileList();m_thread = new MyFileCopyThread(filesToBeCopied); connect(m_thread, SIGNAL(copyStarted(QString)), this, SLOT(copyStarted(QString))); connect(m_thread, SIGNAL(copyCompleted(QString)), this, SLOT(copyCompleted(QString))); m_thread.start(); doOtherStuffHereThatDoesNotNeedTheThreadResult();}
//SLOT
void MyMainWindow::copyStarted(QString name)
{
//Just an example. You can do whatever needs to be done!
myStatusLabel->setText(tr("Now Copying: %1").arg(name));
}//SLOT
void MyMainWindow::copyCompleted(QString name)
{
//Just an example. You can do whatever needs to be done!
myStatusLabel->setText(tr("Copy complete: %1").arg(name));
}@NOTE: After init() function exists, your GUI thread will simply keep on processing events until a Signal form the worker thread arrives – in which case the Slot will be invoked for further processing.
No processEvents() required anywhere…
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Sir tell me one thing. is there any problem if i access the HelperThread object data through my main thread with ReadWritemutex in helper thread function. Is there any problem here?
[quote author="MuldeR" date="1384094383"][quote]Is it possible for the main thread to process events while running some other codes?If not then how it is possible to get updated data while running.please tell me how to design it?please go through the previous reply, i ve edited my requirement in previous post.
may be I need to implement mutex in helper thread and access data from GUI thread,and wait for specific value and then proceed.[/quote]I don't see how this should be a problem ???
Example:
@void MyMainWindow::init()
{
QStringList filesToBeCopied = generateFileList();m_thread = new MyFileCopyThread(filesToBeCopied); connect(m_thread, SIGNAL(copyStarted(QString)), this, SLOT(copyStarted(QString))); connect(m_thread, SIGNAL(copyCompleted(QString)), this, SLOT(copyCompleted(QString))); m_thread.start(); doOtherStuffHereThatDoesNotNeedTheThreadResult();}
//SLOT
void MyMainWindow::copyStarted(QString name)
{
//Just an example. You can do whatever needs to be done!
MyStatusLabel->setText(tr("Now Copying: %1").arg(name));
}//SLOT
void MyMainWindow::copyCompleted(QString name)
{
//Just an example. You can do whatever needs to be done!
MyStatusLabel->setText(tr("Copy complete: %1").arg(name));
}@NOTE: After init() function exists, your GUI thread will simply keep on processing events until a Signal form the worker thread arrives – in which case the Slot will be invoked for further processing.
No processEvents() required anywhere…[/quote]
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[quote author="blue_sky" date="1384095586"]Sir tell me one thing. is there any problem if i access the HelperThread object data through my main thread with ReadWritemutex in helper thread function. Is there any problem here?[/quote]
Not quite sure what you mean?
But you can add a getter and setter function to your Worker thread like this:
@Foo MyWorkerThread::getData(void) const
{
QReadLocker readLock(&m_readWriteLock);
//real assignment operator below, NOT pointer assignment!
Foo data = m_data;
return data;
}MyWorkerThread::setData(const Foo &data)
{
QWriteLocker writeLock(&m_readWriteLock);
//real assignment operator below, NOT pointer assignment!
m_data = data;
}@Now make sure that all access to m_data is done via getData() and setData(). This applies to the GUI thread and the WorkerThread itself!
BUT: If communication between GUI thread and WorkerThread is done solely with Signals & Slots and queued connections, there is no need for such getter/setter functions. Just pass data "by value" in the Signal.