I have a QTableView in a PyQt application, and I want to keep track of when the selection changes. I've tried connecting the signal to a slot as follows (using the advice on this page:
self.view.selectionModel().selectionChanged.connect(self.selChanged)
where the slot it is connected to is defined as:
def selChanged(self, selected, deselected):
print "Sel changed"
However, whenever I load the QMainWindow on which the QTableView resides, I get an immediate segmentation fault.
Am I doing something silly here?
I was having a similar problem and the fix was here:
PySide: Segfault(?) when using QItemSelectionModel with QListView
Namely, replace:
self.view.selectionModel().selectionChanged.connect(self.selChanged)
with two commands:
selectionModel = self.view.selectionModel()
selectionModel.selectionChanged.connect(self.selChanged)
Not sure why this works, frankly.
This has been fixed now, it turned out that I was using an old version of Qt on that machine - which seemed to cause the crash.
The moral of the story is: if your code is crashing for no sensible reason, check all of your dependencies (in this case Qt and PyQt) are up-to-date.
Related
I am using qt 4.8.6 and visual studio 2008 to develop a project and confused by clicked() and clicked(bool). While building a connection for an object which will emit a signal:
connect(sender, SIGNAL(clicked(bool)), receiver, SLOT(myslot()));
will trigger myslot(); and
connect(sender, SIGNAL(clicked()), receiver, SLOT(myslot()));
will not trigger it. However, I find other many examples about connect which all use clicked() not clicked(bool). Why cannot I use clicked()?
I look through Qt Assistant about:
void QAbstractButton::clicked ( bool checked = false ) [signal]
This signal is emitted when the button is activated (i.e. pressed down then released while the mouse cursor is inside the button), when the shortcut key is typed, or when click() or animateClick() is called. Notably, this signal is not emitted if you call setDown(), setChecked() or toggle().
If the button is checkable, checked is true if the button is checked,
or false if the button is unchecked.
I cannot find its reason. At the same time, what are the differences of the "checked" and "unchecked"?
By the way, I build a connect by pressing the left mouse button and drag the cursor. Another way is to rightclick the object, then the context menu will applear "go to slot", but my Qt Designer(4.8.6) will not. How to deal with it?
3 quesions hope to get help. Many thanks in advance.
I'm not sure I really understand the question(s), but the reason you can't connect to a clicked() signal is because there is no such thing... the function profile is clearly clicked(bool) (see docs).
Qt will only show a runtime error when it can't connect signal/slot (qWarning to stderr), it is not obvious at compile-time. Try examining the program output for warnings.
Edit: removed misleading information.
I encountered the same phenomenon too.
I think clicked(bool) and clicked(bool = false) is different.
The signal has different events.
I guess your "myslot()" may trigger the second event.
I have a problem with slots and signals. I created buttons and connected them to the clicked() slot. Then i decided to connect signals and slots manually and since then when I click the button it calls its function twice.
connect(ui->okButton, SIGNAL(clicked()), this, SLOT(on_okButton_clicked()));
void settingswindow::on_okButton_clicked()
{
qDebug() << "ok clicked";
this->close();
}
I was looking for the answer on google, but all i found was this: Where is the generated code of qt signals slots editor but my *.ui file looks like this: pastebin to the code. As you can see there's only one line with and nothing more. I can't find where the information about signals and slots is saved. Rebuild and clean options won't help.
This is not a bug in Qt. If you look at the generated code for your ui_*.h file, you'll notice that the last statement executed in the setupUi() function is a call to QMetaObject::connectSlotsByName().
Since your slot already conforms to the naming convention that this function is looking for, your slot is automatically connected to the signal.
By manually connecting the signal to the slot, in your settingswindow class, you effectively duplicate the connection.
As #Devopia mentioned, this is a documented feature.
I am subclassing QFileDialog to try to get some custom behavior. I would like to connect to signals emitted by components of the dialog, e.g. the textEdited signal when the file name line edit is manually edited. I understand that QFileDialog emits some signals itself, but these do not cover the cases I would like to respond to.
I have two ways about this I can think of, but don't know how to implement. One is to somehow attain a reference to the component to connect to it's signal. The other would be something with event filters, but the event source is the dialog itself, so I don't know how to determine where mouse clicks or key presses occur.
Are either of these methods feasible? Or another way?
Here is one option (your first suggestion):
dialog = QFileDialog()
layout = dialog.layout()
# for i in range(layout.rowCount()):
# for j in range(layout.columnCount()):
# try:
# print i,j
# print layout.itemAtPosition(i,j).widget()
# except:
# pass
line_edit = layout.itemAtPosition(2,1).widget()
line_edit.setText('Hello Stack Overflow')
dialog.exec_()
This gives you access to the QLineEdit in the dialog, which has a bunch of signals you can connect to.
I've also included the code I used to find this widget. I just iterated over the widgets in the layout of the dialog and found the indices of the one I was after. So if you need access to anything else in the dialog, you should be able to find it pretty easily!
Downside to this method: If the layout changes in a future version of Qt, this will break. I suppose you could make the algorithm more robust by looking for widgets that are instances of QLineEdit, but there are always risks with hacky approaches like this!
I am working in Qt4.7, and I have a QListWidget in my dialog. I have a QString that needs to match the current text in the row of this widget (the individual rows are editable). Looking at the signals associated with QListWidget, there seem to be signals for when a different index is selected but none for when the text of a the currently selected row changes. I thought currentTextChanged(QString) would do it, but it didn't. I also thought to try to connect each individual row to something, but QListWidgetItem doesn't have any built-in signals. Does anyone know of a way to do this? Thanks!
At first it seems like QListWidget::itemChanged is the way to go, but soon you run into a problem: the signal is sent for everything - inserts, changing colors, checking boxes, and anything else that "changes" the item! Predelnik pointed that out in his answer. Some people have tried to put in flags and filter everywhere by intercepting various signals to find out if editing was the actual event. It gets very messy.
There is also QAbstractItemModel::dataChanged , which would seem like a good solution. It even has a parameter "const QVector& lstRoles" so you could scan for Qt::EditRole and see if it was really edited. Alas, there's a catch - it gets called for everything just like QListWidget::itemChanged and unfortunately, for QListWidget anyway, the roles parameter is always empty when it's called (I tried it). So much for that idea...
Fortunately, there's still hope... This solution does the trick! :
http://falsinsoft.blogspot.com/2013/11/qlistwidget-and-item-edit-event.html
He uses QAbstractItemDelegate::closeEditor, but I prefer using QAbstractItemDelegate::commitData.
So make a connect like so...
connect(ui.pLstItems->itemDelegate(), &QAbstractItemDelegate::commitData, this, &MyWidget::OnLstItemsCommitData);
Then implement the slot like this...
void MyWidget::OnLstItemsCommitData(QWidget* pLineEdit)
{
QString strNewText = reinterpret_cast<QLineEdit*>(pLineEdit)->text();
int nRow = ui.pLstItems->currentRow();
// do whatever you need here....
}
Now you have a slot that gets called only when the list item's text has been edited!
I guess you need to look into the following signal:
void QListWidget::itemChanged(QListWidgetItem * item)
But be careful because it's being sent every time some property of item changed, not only text. I remember when we ran into the problem once when we changed item colors and got tons of false positive slots called because of that. If you need more fine tuning I guess it's better to write model/view classes yourself and not rely on QListWidget.
I'm a beginner to Qt and am making (or at least trying to make) a basic calculator. If I understand correctly, when doing this:
connect(my_button_4, SIGNAL(clicked()), this, SLOT(writeNumberLbl("4")));
The "4" is not accessible (rather, only its type is) in writeNumberLbl. Basically, I would like so that when the button is clicked, the label sets its text to "4". However, I have the numbers 0 to 9, so I wanted to do:
connect(my_button_0, SIGNAL(clicked()), this, SLOT(writeNumberLbl("0")));
connect(my_button_1, SIGNAL(clicked()), this, SLOT(writeNumberLbl("1")));
...
connect(my_button_9, SIGNAL(clicked()), this, SLOT(writeNumberLbl("9")));
My writeNumberLbl function is:
void Calculator::preWriteVal(QChar val)
{
QString curVal = ui.lbl_output->text();
curVal += val;
ui.lbl_output->setText(curVal);
}
However, I can see that this will not work due to the parameter, 'val'. Could someone please point me in the right direction? Thank you. I did look to see if this question had already been answered and couldn't find anything. If it has, please provide me a link.
Also, is it possible, using Qt Designer 4, to connect a widget to a custom slot?
Thank you.
As far as I know,Qt's signal/slot system requires that the number of parameters of signal function is not less than that of slots function.
In your example,the signal function click() have zero parameters and slot function writeNumberLbl("4") have one parameter,this will not be accepted by the Qt's signal/slot system and if you run your program in debug mode of Qt creator,the qDebug will show you a message like 'Incompatiable signal/slot' blalbalba~. To solve this problem, just read the article given by Arnold Spence. It is quite clear.
There are a number of ways to tackle this problem and they are outlined very nicely here. Although that page is a bit old, I think it is still quite valid. I would recommend using a signal mapper.
For your second question, yes. You can connect signals and slots using Qt Designer by setting the designer in "Edit Signals/Slots" mode. Once in this mode, for example, you can drag a connection line from a button to the form. A dialog will open up allowing you to choose the signal and slot to connect. If you haven't already implemented a slot in code, you can specify the name of a slot and then add the code for it afterward.
The number of parameters in Slot can not exit those in Signal? and pressed() has none. You have two choices (three, counting the dumb one):
Use QSignalMapper. Its help is self-explanatory.
Connect all your buttons to single slot. In it, find out what button has been pressed. QObject::sender() function helps.
There are even more ways, but more complicated.