I am writing a program in Qt that looks like this:
The main window is my class Window : QWidget, it has a QGridLayout containing four other widgets (Input_Menu : QWidget and Output_Menu : QWidget, and then two Canvas : QWidget)
I would like to trigger certain events when the user strikes a key. The problem is, the Window sometimes loses focus (it goes, say to Input_Menu, or maybe a button in Input_Menu...)
I have tried the following solutions, but they seem unsatisfactory (and dirty):
Give Window the focus whenever it loses it.
Tell each widget who could have the focus to trigger Window's KeyPressEvent function (or a clone of it) whenever it receives a keyboard event.
Ideally, I would like that if a widget receives an event (say a keyboard event) and doesn't know what to do with it, it should automatically call its parent's event handler. I would have hoped this to be a default feature of Qt but it doesn't look like it. On the other hand I am really confused about the whole focus thing, I don't really get what's going on. Can someone explain this to me: I have included a std::cout << "key pressed" << std::endl; in my Window::KeyPressEvent function. When I first run my program, it seems the focus is on the top QComboBox in Input_Menu: if I hit the Up/Down keys, I navigate in that box and no "key pressed" is showed in my console. If I hit most letters, nothing happens. But if I hit Left/Right keys, I do get a "key pressed" in my console!?
Thanks a lot in advance for your insights.
You can install an event filter on QApplication to filter the relevant QEvent::KeyPress events globally. From the Qt documentation:
It is also possible to filter all events for the entire application,
by installing an event filter on the QApplication or QCoreApplication
object. Such global event filters are called before the
object-specific filters. This is very powerful, but it also slows down
event delivery of every single event in the entire application; the
other techniques discussed should generally be used instead.
Besides the performance considerations, remember to check if your window currently has the focus before you filter the key event, or you might break popup dialogs or input into other windows.
Actually, I found that for keys that are modifiers (such as Shift, Control), Qt supports finding out whether they are pressed.
Eg : if(QApplication::keyboardModifiers() == Qt::ShiftModifier) ...
This is good enough.
Related
I am trying to fix a bug in a Qt app which I did not write. The window changes the background color of the entire window to red and puts up some buttons, dialog boxes, etc. When the escape key is pushed, the boxes and buttons go away, leaving an empty red screen. The Cancel button does the right thing in returning to the previous window. I think I need to somehow be notified of when the escape key is pushed, and then call the same function as the cancel pushbutton does. Hopefully, I can limit the scope of this special action to when the problem window is up. I am an experienced programmer but a complete Qt newbie. This app is purely C++. To my knowledge, it does not appear to use any QML. I am still searching through the Qt online documentation, but any suggestions / examples are appreciated.
This depends a lot on your specific Qt version and setup of your application. That said, I'll take a shot at helping. In the class where you're trying to intercept the escape key press, assuming it's inheriting QObject, simply override the virtual function eventFilter. Inside your overridden instance of eventFilter, check that the QEvent type is a QEvent::KeyPress, and then check whether the key of that KeyPress is the escape key, and handle as needed.
Be sure that you pass the event out of your function, else you'll see your overridden function eat all events. If you explicitly want the event to be "eaten", simply do not return it from your function. For specifics and examples, check out documentation of QObject::eventFilter().
Suppose I've a Qt app based on QWidget, in which I'd like to obtain a "feedback" everytime I press a QPushButton, I rotate a QDial, I press a mouse button. My question is: how can I set a slot that informs me everytime something happened inside my app built for a touchscreen, in which I need to know when user does something on it.
MousePressEvent partially solves my problem; so when user touches the screen, MousePressEvent warns me about it. Problem is related to widgets as QPushButtons or QDials.
At the moment, I've already arranged my widgets with a lot of subwidgets; number of pushbutton is about 300. How can obtain triggering signal when one of them is pressed without re-edit every single button?
You can use an "event filter" that is installed on QApplication instance. This filter will receive all the events in your application, afterwards you can check event type and object type to pick the events you are interested in. See my answer to another question here for details.
I'm implementing a basic shape drawing tool using a custom subclass of Qt's QGraphicsScene and several QGraphicsItem. Now there are several situations where I don't want any "global" actions to be executed:
For example, while dragging items around, the user should not be allowed to create a new file or to undo the last action (by pressing Ctrl-Z for example) since this would lead to several problems that would have to be handled separately (if the user is currently drawing an edge between two nodes, what should happen if he presses Ctrl-Z with the last recorded action being the creation of the first node?)
I noticed that several commercial applications like Microsoft Word and Adobe Photoshop just seem to ignore any usual keyboard shortcuts while being in such an "intermediate" state. Furthermore, when dragging items out of the viewport, these tools display a "forbidden" cursor and do not allow any mouse press events to reach the outer window (like a right click on the toolbar, for example).
How should I implement this in my case, when using QGraphicsScene? I already tried to add the following override:
void MyGraphicsScene::keyPressEvent(QKeyEvent* keyEvent)
{
keyEvent->accept();
}
But any pressed keys were still delivered to the main window. In addition to that, I'm not sure if filtering just keyboard events is safe enough, since there might be other input events that could trigger forbidden actions.
Is there any generic approach to this problem that I could use in my software?
My team is developing an UI for an apparatus with touch screen and we would like it to emit a sound (from a buzzer) each time the user correctly presses a button (so using the release event). Notice that I don't want to play the sound after each click on the interface, but only when the click is over a button.
We use many types of button, sometimes QPushButton and most of the times customized buttons derived from QAbstractButton. In most cases these buttons get an objectName.
So I supposed in order to do that, I would have to catch the MouseButtonRelease event and since I'm already working with a subclass of QApplication to handle excetions, I decided to do this in the notify function.
I tried, then, some methods to recognized when the MouseButtonRelease was related to a button but none of them were successfull. The best one, verifying the receiver's objectName was still not good enought not only because not all buttons had an objectName (which, of course, can be handled), but specially because not always the event was caught for buttons with names set. In other words, sometimes I would click in a button and it recognizes the event and sometimes I would click in the same button and the event is not recognized.
I did some research and another method I found was to set an event filter in the MainWindow, but not all widgets have the MainWindow as their parent which means I would have to Ctrl+c / Ctrl+V the same code time after time when I obviously want something more localized (i.e. in only one spot).
So why it happens that the notify not always handles the events? And how could I do this? Any suggestions are appreciated specially one that is less heavier then handling the events globally.
As info, the other two ways I tried to catch the events with similar or even worst results inside notify were with receiver->inherits("...") and qobject_cast< QAbstractButton* >(receiver).
I have an issue with associating a keypress to a QAction. I'm mapping Enter to an action like this:
myAction->setShortcut(Qt::Key_Return);
The problem is that the QAction is also triggered when I type a value in a QSpinBox and then press Enter. I was expecting the spinbox to consume the event (so it's not caught by the mainwindow) but it's not working like that.
I've seen that I can add a context to my shortcut, I tried all values but this does not solve my issue.
#Matthew It is indeed rejecting the event:
case Qt::Key_Enter:
case Qt::Key_Return:
d->edit->d_func()->control->clearUndo();
d->interpret(d->keyboardTracking ? AlwaysEmit : EmitIfChanged);
selectAll();
event->ignore();
emit editingFinished();
return;
I find this behavior quite strange.
From my knowledge the QSpinBox default behavior does wait until you press return to accept the value. Technically the value is set/accepted as soon as you change it.
If you're looking to change that behaviour, i.e. when you hit return on your QSpinBox you the value is actually set/accepted, then you will most likely have to handle the associated key press event yourself on the QSpinBox and mark it as accepted (i.e. event->accepted() ).
Otherwise, this sounds like it is working as originally designed although not as you want.
I have exactly the same problem, only in PyQt and with QLineEdit
It may be that QActions are always considered menu actions, or global, and have a higher priority than Widget Default keys.
My "solution" was to create a global list of my actions with important keys (Return, Tab, Backspace etc.) as shortcuts.
Then I hook into QLineEdit focusInEvent and focusOutEvent. Each time the event happens I loop over the actions-list and deactivate them all on focusIn and activate them again on focusOut. Then I give the event to the original focus event handlers.
I really hope there is a better solution.