How to use signal and slots for handling events i.e when a pushButton is clicked some number should be displayed in QLineEdit.Just like in caluculator application when a number is clicked the corresponding is displayed .
connect a widget's signal to other's widget slot.
Signal = Emitted when the button (for instance) is pressed.
Slot = called when a certain singal is fired, and this signal is connected to this slot.
If You use QtCreator here is the procedure:
right click on the pushbutton. Select go to slot.
Select clicked()
QtCreator will open C++ editor with new empty member function defined that implements
your slot. This function will be called whenever someone clicks this button.
Now in this function you can call method of another widget ot modify its contants.
This book is quite good for starters:
http://www.amazon.com/Book-Qt-Art-Building-Applications/dp/1593271476/ref=sr_1_8?ie=UTF8&qid=1300905855&sr=8-8
Good luck.
Related
I have copied several buttons in design form. As result all buttons got the same slot for clicked(). How to create and assign new personal slots for buttons?
UPD
Trying to use Signals & Slots editor. It was empty in the beginning, and I added new one:
But if I right click on button btnInfoCB->go to slot.. clicked() , it will point me to the same old method.
Qt creator will create slots for you. From designer, Right click on any push button and select 'Go to slots' option. select Clicked() signal press OK. Qt creator will navigate you to the created slot. Qt automatically connects your button clicked signal to the created slots.
You can do the same for all push buttons you created
Qt has an auto-connect feature that connects the slots named like this: on_UIELEMENTNAME_SIGNALNAME(SIGNAL_PARAMS) with the corresponding signal of the ui element, see the documentation here
So to fix this you have 2 options:
Option 1: rename the slots to have all the names corresponding with the names from .ui file:
private slots:
void on_MyFirstBtn_clicked();
void on_MySecondBtn_clicked();
//and so on
Or use option 2: manually connect
private slots:
//notice the prefix "on_" and the suffix "_clicked()" not part of the function name
void MyFirstBtnClicked();
void MySecondBtnClicked();
And in the constructor of the class (i named it MyWidget) add the connect calls:
connect(ui->MyFirstBtn, &QPushButton::clicked, this, &MyWidget::MyFirstBtnClicked);
connect(ui->MySecondBtn, &QPushButton::clicked, this, &MyWidget::MySecondBtnClicked);
LE: I don't recommend using the UI Designer for connecting slots that are defined in code because of maintainability reasons. That feature in Designer is very good if you need to connect already defined signals and slots, like check/unched to show/hide and other similar predefined functionality, but for your defined slots i don't like it.
New slot for an existing QPushButton /w auto connection
You have to change the objectName of QPushButton first. Then right click and create a new slot connection e. g. clicked(). Qt Creator will then create a new slot with the new objectName e. g. void on_newObjectName_clicked() { }. The old slot is no longer used then and you will get a new fresh function.
Manually connect /w signals and slots editor
If you like to connect it through your editor, you first have to click at the QWidget (e. g. MainWindow) and select change signals/slots from the menu. There you have to add a new function e. g. myNewSlot(), which is then available in the drop-down in the signals and slots editor as shown in your screenshot. If you use this type of connection you need to declare the slot in your .h in private slots and in the .cpp file. The object name may still link to an auto connection, so you probably need to change the objectName as well or remove the auto connected slot.
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 am new to Qt and understands the concept of signal and slots. However I am not able to implement it.
My objective is:
Form1 has a button Config. So when I click Config it should open another form Form2( without closing Form1) and send a string strData to Form2.
In Form2 I set some value in the string strData. Then I click Ok button in Form2, Form2 should close and return back the string to Form1.
When the call returns back to Form1, it should continue from where it emitted the signal to invoke Form2.
Any help is highly appreciated.
You can't do this using signals/slots; the signal is emitted, and all of the connected slots are executed, and then the code continues from where the signal is emitted and eventually returns to the event loop. That's when your second form is actually shown and the user can respond to it, but by then, your code is long past where the signal was emitted.
What I believe you're looking for is the QDialog::exec method; use it in place of the signal. The basic pattern of the code is:
// This is the response to click on Config...
Form2Dialog form2;
form2.setSomeStringValue (some_value);
if (form2.exec() == QDialog::Accepted)
{
QString some_new_value = form2.newValue();
}
The Form2Dialog is a subclass of QDialog where you've added your own setSomeStringValue and newValue methods. (What you actually name these is up to you.)
The important thing is that the exec method blocks and doesn't return until the user selects OK or Cancel on the dialog, or closes it using the "close" button in the title bar (if there is one).
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've been having some trouble with create click of buttons in PyQT.
when i create click of button as below , this picture can't save
cv.SetImageROI(image, (pt1[0],pt1[1],pt2[0] - pt1[0],int((pt2[1] - pt1[1]) * 1)))
if self.Button.click():
cv.SaveImage('example.jpg', image)
cv.ResetImageROI(image)
The problem in your code is that you are performing a programmatic click on the button calling QPushButton.click on the line if self.Button.click():, what you need to do is to connect the signal QPushButton.clicked to a proper slot on your code. Singals and Slots is the way Qt handle some significant events that may occur on the object. Here I'll drop you an example, hope it helps:
import PyQt4.QtGui as gui
#handler for the signal aka slot
def onClick(checked):
print checked #<- only used if the button is checkeable
print 'clicked'
app = gui.QApplication([])
button = gui.QPushButton()
button.setText('Click me!')
#connect the signal 'clicked' to the slot 'onClick'
button.clicked.connect(onClick)
#perform a programmatic click
button.click()
button.show()
app.exec_()
Note: To understand the underlying behavior read the docs of Qt/PyQt.