I've got a general question on qt design.
Say, I created a custom classes, impelementing QAbstractTableModel and QTableView class. I've re-implemented event handlers in View, such as mousePressed, mouseRelease etc.
Still Qt's View manages to perform some of its default functionality: it still responds to mouse clicks and movements on cells by changing selection, thus it somehow fires selectionModel "built-in" signals, though I didn't ask it to. It still resizes columns, if I drag on cell borders etc. What's the mechanism that triggers those "buit-in, default slots" and how can I disable some parts of it? E.g. if I want to disable default behavior of selectionModel or default resize?
For the sake of comparison, in gtk+ there's a concept of default per-class signal handler, which is a function, by default connected to its signal and called prior or after your custom per-class or per-object signal handlers, depending on parameters you set. You can disable it from your custom slot, if you want to and thoroughly control behavior of e.g. resize or selection.
Is Qt opaque in this place and provides customization via its interface functions only? My question is particularly related to pyqt. Please ask for clarification, if I'm too vague.
The event handlers that you refer to are used to listen to events, not to filter them. You can't override any events in them, since they don't have a return value: there's no way to inform subsequent event processing that you don't wish it to run.
To filter events, you must reimplement the event method, and invoke the base class's implementation on events that you do not wish to filter.
In Qt, event handling is done per-object, and you can install external objects as event filters on any other object. An object receives the events in its event method. The QObject class implements this method and invokes the timerEvent method. The QWidget class reimplements this method and invokes the widget-specific xxxEvent methods. And so on. All of those classes still process some events internally. Those are the per-class handlers that you speak of.
Related
I have a suspicion that something is preventing my mouse event to be called in Qt. Therefore, I think it might help to add the event to a thread. Is there any way to do that? And if so how would the syntax look?
Qt standard mouse events come to QWidget objects. Those must exist in the main thread, always.
So no, you can't receive the normal mouse events in other threads.
However, you should perhaps install an event filter to your main window or to your qApp object, so you will see all the events. Look in the docs for how to use the event filter, but in short, you need to subclass QObject to override the eventFilter method, and then create instance of this class, and install that as event filter for another object.
Question/Issue
I tried reimplementing the event method in a custom delegate to handle clicks. The delegate is used to render table cells in a table view. However, I do not get any events for the delegate (the method is never called according to the debuger). Is there anything special I need to do so my delegate can track events (in particular mouse entering/exiting, clicks)?
Context
I would like to create my own data representation for table cells. The functionality should be close to a button, but slightly different. I read that the two options for implementing buttons in table are either setting a cell widget which supposedly has a high performance cost (I did not quite understand why) or using a delegate.
Since I want different behaviour than that of a button, and for the speed myth I decided to go with a delegate.
Mouse events are send to the QAbstractItemDelegate::editorEvent() method, even if they don't start editing of the item.
See: http://doc.qt.io/qt-5/qabstractitemdelegate.html#editorEvent
I'm writing a program that uses the Qt Graphics View framework. I have subclassed QGraphicsItem to a class that includes other QGraphicsItem (or other subclasses of it). This class is the parent of the included QGraphicsItem; the idea is to work with composite objects.
From the docs it seems to be a conflict in what I try to achieve:
Calling ignore() in mousePressEvent will make my object unmovable. I want to move it.
Calling accept() in mousePressEvent will prevent the event from being propagated to the child object. Some of the child objects should react to mouse events.
How can I make this work?
I think your interpretation of the documentation is incorrect.
Calling ignore() in mousePressEvent will make my object unmovable.
I don't believe that is true. To me it looks like calling ignore() is like the object saying "I have assessed this event. I have taken all actions I want to in response to this event. I have also decided it was not intended for me, so I will now pass it on to the next object underneath me". I can't find anything which suggests the ignore event will unset the QGraphicsItem::ItemIsMovable flag (which is what decides if the QGraphicsItem is movable or not).
I don't see why you couldn't make your object move and ignore() the event, but I would advise that this is not a sensible approach (in most instances: obviously you may have cause for it).
Calling accept() in mousePressEvent will prevent the event from being propagated to the child object.
I believe this is true, but the parent can still modify its children. My understanding is calling accept() is like the object saying "I have assessed this event. I have taken all actions I want to in response to this event (which may include modifying my children). I have also decided that the event was intended for me, so I will not be passing the event on".
In your parent QGraphicsItem, you might try to
MyObject::mousePressEvent ( QGraphicsSceneMouseEvent * event )
{
QGraphicsItem::mousePressEvent(event);
event->ignore();
}
This would allow normal processing of the mouse event (i.e. make your object moveable), but then ignoring it so that it is propagated.
The logic would need to be more robust, though, because there is a high risk of side effects if a parent and child respond to the same mouse event.
Send QCoreApplication::postEvent(child, mouseEvent) to child objects.
I need help to understand to use QEvents in QT, this is driving me crazy.
I am writting an application using custom events, but as in QApplication::postEvent function, it's necesary to specify the target object.
As I understand, it's possible to post events to Qt's event loop with
QApplication::postEvent(obj_target, QEvent myevent);
This means that I'm trying to catch "myevent" event in obj_target an do some stuff.
But I need to post events without specify a target object, as QMouseEvent or QKeyEvent do
I mean, when clicking in a QMainWindow with a lot of buttons, how is that I can click
any button and that button is pressed?
What is the target object when the click event is posted?
It's possible to register objects to "listen" for a specific event?
I'm really confused, it's possible to post an event without specifying a target object?
Thank you very much in advance
There is no trivial way to post events "globally", as Dan has said. All of the event dispatching of native events is done by private Qt implementation code.
The important distinction is:
There are native messages/events, delivered by the operating system, usually received by a window-specific event loop.
There are QEvents.
Internally, Qt keeps track of the top-level Widgets (windows, really), so when it receives an event from the OS, it knows which window it should go to - it can match it using the platform window id, for example.
QEvent delivery makes no sense without a receiving object, since sending an event to an object really only means that QObject::event(QEvent*) method is called on that object. It's impossible to call this method without having an object instance!
If you want to synthesize a global key press or mouse click event, then you have to figure out what object the event goes to. Namely:
Identify what top-level window (widget) the event should go to. You can enumerate top level widgets via qApp->topLevelWidgets().
Identify the child widget the event should go to. If it's a keyboard event, then sending the event to currently focused widget via qApp->focusWidget() is sufficient. You need to enumerate the child widgets to find the deepest one in the tree that overlaps the mouse coordinates.
Send the correct QEvent subclass to the widget you've just identified. Events delivered to top-level widgets will be routed to the correct child widget.
When sending mouse events, you also need to synthesize relevant enter and leave events, or you risk leaving the widgets in an invalid state. The application.cpp source file should give you some ideas there.
This doesn't give you access to native graphical items, such as menus on OS X.
Please tell us exactly what you're trying to do. Why do you want to post a broadcast event? Who receives it? Since your own QObject-derived classes will receive those broadcasts, I presume, it's easy enough to use signal-slot mechanism. You'd simply connect(...) those receiver classes to some global broadcaster QObject's signal(s).
For this purpose, I have a specific singleton class which I call GuiSignalHub. It regroups all the application-wide signals.
Objects that want to trigger an application-level action (such as opening context help) just connect their signal to the GuiSignalHub signal. Receivers just connect the GuiSignalHub to their slot.
I would like to know what the established procedure is for initializing the controls within a Qt custom dialog box. In the code I am writing, the dialog would present a QListView containing directories from an object passed (by reference) to the dialog class during construction. When the dialog is displayed, I obviously want the list to display the directories currently configured in the object.
Where should this be done though? Perhaps in the overridden showEvent() method?
Background: I used to do a lot of MFC programming back in the day, and would have done this sort of stuff in the OnCreate method, or some such, once the window object had been created.
Thankfully Qt doesn't require you to do any hooking to find the moment to create things (unless you want to). If you look over the Qt examples for dialogs, most do all the constructing in the constructor:
http://doc.qt.io/archives/qt-4.7/examples-dialogs.html
The tab dialog example--for instance--doesn't do "on-demand" initializing of tabs. Although you could wire something up via the currentChanged signal:
http://doc.qt.io/archives/qt-4.7/qtabwidget.html#currentChanged
Wizard-style dialogs have initializePage and cleanupPage methods:
http://doc.qt.io/archives/qt-4.7/qwizardpage.html#initializePage
http://doc.qt.io/archives/qt-4.7/qwizardpage.html#cleanupPage
But by and large, you can just use the constructor. I guess the main exception would be if find yourself allocating the dialog at a much earlier time from when you actually display it (via exec), and you don't want to bear the performance burden for some part of that until it's actually shown. Such cases should be rare and probably the easiest thing to do is just add your own function that you call (like finalizeCreationBeforeExec).