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Questions tagged [signals-slots]

Signals and slots is a mechanism for implementing the observer pattern.

Signals and slots is a mechanism for implementing the observer pattern. It has many implementations in various languages:

See also:

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PyQt: Connecting a signal to a slot to start a background operation

I have the following code that performs a background operation (scan_value) while updating a progress bar in the ui (progress). scan_value iterates over some value in obj, emitting a signal (value_changed) each time that the value is changed. For…
Hernan
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If a Qt signal is emitted in the forest and noone is around to hear it, does it make a sound?

Sorry for the humorous phrasing of the question. I couldn't resist ;-) If I emit a signal that's not connected to a slot anywhere in the code, is there still a performance cost associated with that "emission". I'm not familiar with the mechanism via…
Alan Turing
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Deletion of objects send by signals, Ownership of objects in signals, Qt

Here, my signal declaration: signals: void mySignal(MyClass *); And how I'm using it: MyClass *myObject=new myClass(); emit mySignal(myObject); Here comes my problem: Who is responsible for deletion of myObject: Sender code, what if it…
metdos
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Using Qt signals and slots vs calling a method directly

Lets say I have a main window with a slider and a widget inside that window with a method called setValue(int). I'd like to call this method every time the value of the slider has changed. Is there any practical difference between the two following…
jaho
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Define pyqt4 signals with a list as argument

According to http://pyqt.sourceforge.net/Docs/PyQt4/new_style_signals_slots.html I can define a pyqt4-signal with takes an integer argument by mysignal = pyqtSignal(int). How can I define a signal which takes an integer and a list of strings or…
student
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Qt Model/View/Controller Example

I am just getting started in Qt, and trying to get a simplified, working example of the model-view-controller design pattern. So far, I have been able to use signals and slots to connect basic widgets like push buttons to a QLabel, and have the view…
nairware
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PyQt4: combine textChanged and editingFinished for QLineEdit

Is there a way to combine textChanged and editingFinished for QLineEdit? The problem is that editingFinished is emitted even if I only move the cursor away from QLineEdit without any changes. Whereas I want to emit a signal only when any changes…
Ekaterina Mishina
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QT + How to call slot from custom C++ code running in a different thread

I am new to QT and I am doing some learning. I would like to trigger a slot that modify a GUI widget from a C++ thread(Currently a Qthread). Unfortunatly I get a: ASSERTION failed at: Q_ASSERT(qApp && qApp->thread() == QThread::currentThread());…
xcimo
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Connect signals to slots with constant values

To connect signals to slots, as far as I know, the parameters of the signal need to match the parameters of the slot. So for example: connect(dockWidget->titleBarWidget(), SIGNAL(closeButtonClicked()), ui->sideControls,…
Leif Andersen
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How to set value of a widget without triggering valueChanged callback in PySide?

I want to update a widget's value but in order to prevent infinite loops, I need to prevent calling the callback function of this widget's .valueChanged signal. Following example works entirely as expected: Qt = PySide class MainWindow(Actor,…
ceremcem
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Event loops and signal-slot processing when using multithreading in Qt

I've been having some problems with using QThreads which made me explore different combinations before I've found the right one. However I still don't fully understand what is really happening in the four cases shown below when it comes to event…
Moomin
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PySide: Easier way of updating GUI from another thread

I have a PySide (Qt) GUI which spawns multiple threads. The threads sometimes need to update the GUI. I have solved this in the following way: class Signaller(QtCore.QObject) : my_signal = QtCore.Signal(QListWidgetItem, QIcon) signaller =…
Petter
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Is it valid to define a pure virtual signal in C++/Qt?

I am making an abstract-base-class and was thinking I might want a pure virtual signal. But when I compiled I get a warning for the pure virtual signals I have defined: ../FILE1.h:27: Warning: Signals cannot be declared virtual ../FILE1.h:28:…
Trevor Boyd Smith
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Sending signal from static class method in Qt

I am trying to code a static callback function that is called frequently from another static function within the same class. My callback function needs to emit a signal but for some reason it simply fails to do so. I have put it under a debugger and…
user99545
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Qt issue passing arguments to slot

I can't seem to pass an argument to a slot. If I don't pass an argument, the function rolls through fine. If I pass an argument (integer), I get the errors "No such name type" and "No such slot" when I compile. In my header, I declare: private…
user375566
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