View Full Version : Signal/slot responsiveness

21st May 2010, 20:12
Testing QClipboard class, i wrote:

clipboard = QApplication::clipboard();
connect(clipboard, SIGNAL(changed(QClipboard::Mode)), this, SLOT(clipboardChanged()));

After an initial proper behavior (10-15 times clipboardChanged slot called after every data change :)), app stops working (clipboardChanged not called anymore :().
Any suggestions are greatly appreciated.

21st May 2010, 20:31
Have you tried dataChanged instead ?

21st May 2010, 20:39
Of course, but had same behavior. The app is an extremely simple clipboard manager that runs in background. After a while (20-30 mins - sorry, can't be more specific, different behavior every time...) app stops working.

21st May 2010, 22:29
The whole application "freeze" or only that slot isn't executed after some time?

In the first case it "smells" like a memory leak, look for resources that allocate 'temporary' memory on the heap, every time that slot is called or with the part of your app that emit the signal.

And in the second look at your variables, they might go out of scope

21st May 2010, 22:51
The second case.
The signal
void QClipboard::changed(QClipboard::Mode mode) is emitted by the system, and no variable is involved in the signal/slot process.

21st May 2010, 22:56
You shouldn't depend on this signal happening when the application is in the background, particularly on MacOS.

If you click on your application window (If it doesn't have one, create one for this test), does it start to work again?

21st May 2010, 23:09
void QClipboard::dataChanged ()
On Mac OS X and with Qt version 4.3 or higher, clipboard changes made by other applications will only be detected when the application is activated.
This is the reason why i used changed() instead of dataChanged(). However, clicking on the application window (activating it) doesn't solve the problem.

21st May 2010, 23:14
See that "clipboard" or "this" variables don't go out of scope. ("this" is the main window?).
I think that if you describe where/how do you create those variables, we will be able to give better advices.

21st May 2010, 23:52
'this' is a C++ keyword. It's not likely to go out of scope :-)

'clipboard' is a pointer to the application global clipboard. Even if it goes out of scope, the original pointer assigned to it will not, so for the purpose of the connect call, it will remain valid for the lifetime of the QApplication object.

22nd May 2010, 00:15
You are right about clipboard, sorry, my bad :(

But "this" is a pointer to the class instance and if the class instance gets destroyed... this can be a possible cause of that behavior, and i don't see any other...