Creating events instead of signals/slots



  • I have multiple windows(screens) in my application.I need to activate windows with the respective events that I receive from the controller code.Now,I want to close the previous window while activating the other window.I had used signals and slots mechanism to close a particular window but I want it to be done using events.
    How to do this using events?
    Am using QtCreator with qt4.7 version to design my windows(screens).I read the documents regarding events in Qt Documentation.

    Thanks in Advance.



  • Hi,

    is there a specific reason to use events further than signal/slot?
    But events are also easy:

    Create a QEventSubclass and give it a unique type (>QEvent::User).

    @
    QApplication::postEvent(target, new MyEvent);
    @



  • Could you explain in a bit more detail why you want to use events instead of signal/slot?



  • Yes.I have to wait for an event to activate my window requested by the controller code (at the back end).By using close() when I run my application continuously the previous window are not closed.So the same is done using signal slot mechanism.All my windows are frameless, so I want to pass a event to close my current window and raise my next window when I get the event from the controller.

    Do I have an option to use Signal and slot connection by passing event to a particular window?



  • Hi,

    I think, close should do it. If close does not work, perhaps there is a different problem. Can you please provide some code example, where close does not work? Do you do something in the closeEvent of that widget?



  • @Geroif:Hi..Yes close does its job.The problem is that am using static to raise my other window.I want to activate my next window before I close my current window.Always the next window will be event dependent to change to the next screen.
    like I want to navigate between the screens initially from Idle screen to Dial screen.After I dial the number the number and press dial button I should show Progress Screen and finally Connect Screen after the call has been established.



  • Whether I could pass the events between two widgets/windows?Can you tell me how to do it?There is no mush details regarding QCloseEvent in the documentation.



  • In his first reply, Gerolf already told you how to use custom events. -It doesn't matter much if you want to post your own event or a Qt event.- A QCloseEvent is just a subclass of QEvent that does not add any special members or methods.

    I am not sure if posting your own QCloseEvent is going to work, since the spontaneous flag would be false. AFAIK, there is no public API to manipulate that flag, and I don't know if it would affect the handling of the event.

    Edit: it turns out it does matter what kind of event you post.



  • [quote author="Andre" date="1301564768"]In his first reply, Gerolf already told you how to use custom events. It doesn't matter much if you want to post your own event or a Qt event. A QCloseEvent is just a subclass of QEvent that does not add any special members or methods.
    [/quote]
    I thinnk it will not. QCloseEvent is also a reaction on the close() slot (if close is no reaction to a system close event, there is an if in the code for that) :-).

    Sending a QCloseEvent will NOT close the widget!

    @
    ... QWidget::event()
    {
    ...
    case QEvent::Close:
    closeEvent((QCloseEvent *)event);
    break;
    ...
    }

    void QWidget::closeEvent(QCloseEvent *event)
    {
    event->accept();
    }
    @

    [quote author="Andre" date="1301564768"]
    I am not sure if posting your own QCloseEvent is going to work, since the spontaneous flag would be false. AFAIK, there is no public API to manipulate that flag, and I don't know if it would affect the handling of the event.
    [/quote]

    It's totaly internal, I was trying to use it once... Did not work. Needed it for a virtual key board.



  • Do I have a solution to close a widget from another widget of different classes using events?How could I do that?



  • As you need the widget pointer for sending events, you can just call close() method also. And it does the job. So why bother about sending events? Dou you need it asynchronous? Use QmetaObject::invokeMethod(...) with the queued invokation.



  • It should be synchronous with the close of the current widget and raise of other widget.



  • Then call

    @
    widget2()->show();
    widget2()->raise();
    widget1()->close();
    @



  • I had already used it in my program and its working.My question is that can't we do the same by passing events between those widgets.in examples given in qt4,in mainwindows there is a sample program named application in which the QCloseEvent has been used but am not able take that and implement to my requirement.It throws a compile error as
    @/usr/include/qt4/QtGui/qwidget.h:84: error: forward declaration of ‘struct QCloseEvent’
    @

    Help me.



  • you should add the include for the QCloseEvent, it's only forward declared!

    @
    #include <QtGui/QCloseEvent>
    @

    It's staed in the error message...



  • Thank You.That is clear now.How could I pass the event between two different widgets.Suppose if I had implementted the QCloseEvent in MainWindow then how to call that from my SubWindow?



  • to forward events to a parent, you can just use event->ignore() which should then send the take care that it is also send to the parent (don't know if that also happens to a CloseEvent).



  • Well!I have multiple screens.I need to display a particular screen based on the event I pass from my first screen.With signal and slot,I need to write those lines of code in all the files.Suppose if I have some 20 screens then by passing an event from my first screen I should display 10th(or 15th whatever) or close that particular window by having all my control in my first screen.



  • Then connect your signals to a slot/some slots in the first screen.



  • @Volker:I had done that earlier.How well can I organise these so as to have the control to close my other windows?Just like as we have goto in C.



  • It's the same task as if you had events dropping in.

    EDIT:
    And as C++ is a superset of C, you have goto - if you really want to use this.



  • Can I traverse from 1st MainWindow to some 15th MainWindow that I have in my application?How well can I do this with signals?



  • What do you mean with "traverse"? Call each of them in sequence?

    Of course you can do this. Qt does it all the time. Slots are invoked in the order the connections are created, so that would also "traverse".



  • The GUI should respond depending upon the event received from the C running as back-end.



  • OK, here's what I would do, and then I think I'll quit responding to this thread.

    I would create a single QObject that is responsible for the communication with the C-based backend. This object receives the call-backs or events or whatever you call them from the backend, and emits corresponding signals for these events. The different windows connect to this object's signals to learn about the data they are interested in.



  • @Andre:Thank you for your response.Could you explain the same with a code snippet please.So that I can understand better.



  • [quote author="Revu" date="1301986907"]@Andre:Thank you for your response.Could you explain the same with a code snippet please.So that I can understand better.[/quote]

    Not without knowing what your back end looks like and how to communicate with it, no. And to be honest, I don't think I can or want to do that even if you provided the needed code and documentation. I think the concept should be clear though?



  • I have multiple screens for a outgoing call like Idle,Dial,Progress and Connect screen.My backend controller program is in C.I need to change from Idle to Dial.After I enter the number show progress screen and finally connect screen after the call is established.Based on the events I get from C,my screen will change.Its like with button click I will wait for an event and then display the screen.While showing the other screen,am closing the current(using signal and slot mechanism).So how could I do that with events.



  • Now you are just asking the same question all over again. Sorry, but you really have to do your actual work yourself. Many people have already chimed in to try to help you, but if you just keep repeating your question, we're getting nowhere.

    Perhaps a simple state machine is what you want? The different events from your backend could trigger state changes (read the documentation on QStateMachine before asking here how that works!) and your different states could determine what screen is visible.



  • Another solution (which I personally prefer) is to make a "Controller" object (derived from QObject) which encapsulates the non Qt interface (your C-backend) in a "facade design pattern":http://en.wikipedia.org/wiki/Facade_pattern . This goes to the direction -Andrea- Andre posted before the state machine.

    This controller object then sends signals which can be connected to anyone, who needs to know them... This is something we do often here with COM interfaces we have to use. It works well.

    Edit: my name is Andre


Log in to reply
 

Looks like your connection to Qt Forum was lost, please wait while we try to reconnect.