What is the difference between hide,close and show of pushbutton or any widget in terms of memory?
Which is better if I don't want to use widget again?
First as said #Hayt, read the documentation.
For the actual answer:
hide() is the same as setVisible(false).
show() is the same as setVisible(true).
close() attempts to close the widget by triggering a QCloseEvent, if the event is accepted the result is:
The same as calling hide() if Qt::WA_DeleteOnClose attribute is not set on the widget which is the default.
The same as calling deleteLater() if Qt::WA_DeleteOnClose is set.
In term of memory, any of the 3 will not change anything (except for close() if you have set Qt::WA_DeleteOnClose). If you do not want to use the widget ever, the best is to delete it:
delete pointerToMyWidget;
or
pointerToMyWidget->deleteLater();
The second form is generally safer as the 1st one can be dangerous depending on where your write it. (e.g you delete it in a slot called by a signal emitted by the widget you delete).
According to Qt, you can read this :
CLOSE :
Closes this widget. Returns true if the widget was closed; otherwise
returns false.
First it sends the widget a QCloseEvent. The widget is hidden if it
accepts the close event. If it ignores the event, nothing happens. The
default implementation of QWidget::closeEvent() accepts the close
event.
If the widget has the Qt::WA_DeleteOnClose flag, the widget is also
deleted. A close events is delivered to the widget no matter if the
widget is visible or not.
The QApplication::lastWindowClosed() signal is emitted when the last
visible primary window (i.e. window with no parent) with the
Qt::WA_QuitOnClose attribute set is closed. By default this attribute
is set for all widgets except transient windows such as splash
screens, tool windows, and popup menus.
.
HIDE : Hides the widget. This function is equivalent to
setVisible(false).
Note: If you are working with QDialog or its subclasses and you invoke
the show() function after this function, the dialog will be displayed
in its original position.
.
SHOW : Shows the widget and its child widgets. This function is
equivalent to setVisible(true).
If you don't need to use your widget, call close(). You can manage the event to destroy your widget.
hide() only hides. It's only graphical, you can't see your widget but you don't destroy it.
But I think that the name fo the function are enough explicit to understand!
Related
I have a QTabWidget, and I defined showEvent for one of the child widget.
Now how can I know where the showEvent is from?
It could be:
Switched from other tabs
The current index of tabwidget was not changed, the whole window just become visible
Is that possible?
QShowEvent is very generic, so there's no direct way to get information about what triggered it. Depending on your needs either save current value of QTabWidget::currentIndex between show events or move your logic to QTabWidget::currentChanged slot.
I've made my first qt window. Now I'd like to make my first dialog, using qt. I have just finished creating the dialog, which is basically made of a QDialogButtonBox, and now I'd like to connect it to the window. I have two beginner's questions:
How can I retrieve how the dialog was closed (ok pressed or cancel pressed) from the window.cpp, which creates a new dialog, and then calls dialog->show() ?
Where and how to destroy the dialog pointer ?
If you use dialog->show() then I assume it's non-modal dialog.
If you have created QDialogButtonBox and connected its signals with accept() and reject() slots of your dialog as documentation shows, then your dialog will emit finished(int) and additionally accepted() or rejected() signals by which you can determine how it was closed.
If you need more customized behavior, then you can reimplement closeEvent(QCloseEvent *event) or create your own singnals.
If you need to delete your dialog you can use setAttribute(Qt::WA_DeleteOnClose, true);, which will delete instance on close.
you can use one flag, and signal and slot.
when put OK flag=1 , and when put cancel then flag=-1; and then use signal.
in in the window.h write code how to handle that flags with 1 simple slot.
for destroying the pointer you can use signal and slot in your Dialog and tell when user push
Ok, or Cancel , or exit (up- right (red cross)) go to slot in call the Destructer of dialog
and also you that you better set parent of dialog to window.
First Question:
When you want to show the dialog,just construct it,using myDialog *d = new myDialog(this)(the this pointer will make sure that you havn't to delete the pointer you created 'cause Qt will handle this if you specified the dialog's parent). And use d->exec() if you need a modal dialog, or d->show() to make it non-modal;
Second Question:
Once your specified the dialog's parent object, all u need is just use it and leave alone the memory managent,Qt will do this for you. Also you can use d->setAttribute(Qt::WA_DeleteOnClose,true) to make it destroy itself when it is closed.
Remember to link the QDialogButtonBox to your dialog's actions.
In my application, I have one tableview of items, and a side-panel "preview":ing the latest selected item.
I want clicking on an item to change the selection, and double-clicking to cause a "run"-action to be performed. More specifically, I want the "run"-action (including key-navigation and pressing enter) to be bound to the "activation" of the item in the table-row.
My problem is; single-clicks does not only change the selection, but fires the "activated" signal on the item. I would like to tweak it such that:
Navigation Keys, Single Mouse Click: Selection-change, update preview-panel
Enter Key, Double Mouse Click: Activate/run/open action triggered.
Is there a nice clean way to do it, or are overriding the onclick/doubleclick events my best option? Or is there some other tabular list-widget better suiting my needs?
I would connect the slot for the preview action to the currentChanged() signal of the table view's selectionModel(). This covers single clicks and key navigation.
Then there's two options for the double clicks and Enter key presses:
Subclass your tableview, override doubleClickEvent() and keyPressEvent() and fire your custom signal in there, with maybe the model index or something else as an argument. Then just connect your run method to your own signal as you have full control over when it is fired.
If you don't want to subclass, you can use the installEventFilter() mechanism.
Either I'm getting your approach wrong or I'm too tired, but if you want to trigger a run event you should avoid the activated signal completely. Set the signal slot mechanism so that your double click and Enter key press event trigger your run() function, and then the single click/nav buttons should trigger the 'activated' slot which will return your index in the tableview.
I'm pretty certain Qt wants you to be explicit about which signal points to which slot or it'll ignore it or point to a default.
I have a problem:
I'm creating a widget which displays current date's day number. It's like a button, but it's not derived from QPushButton class. Just from QWidget. So I've reimplemented enterEvent(), leaveEvent(), mousePressEvent(), mouseReleaseEvent(). I do call update() inside these methods and widget has realistic button behavior (paintEvent() is reimplemented also).
But when I change system date and hover that widget with other window, my widget doesn't get paintEvent() and the old date is displayed. Only when I place mouse over it, widget repaints it's contents.
I guess there is a feature (like buffering) which paints old contents on hovering with other window to avoid unnecessary recalculations. But I need to disable it. Tried to set many attributes (the Qt::WidgetAttribute enum). But it doesn't work.
I think you should find a way to detect that the system time has changed and call update() when that happens. Any other method (like detecting the "hovering" of a window or waiting for a mouse event) will cause the update to occur too late.
In my app have a window splitted by a QSplitter, and I need to remove an widget.
How can I do that? I can't find useful methods
It's not clear to me if you want to preserve the widget and put it somewhere else, or if you want to destroy the widget.
Destroying the widget: If you can
get a pointer to the widget, you can
simply delete it. The splitter will
safely be notified that its child is
being deleted and will remove it
from itself.
Preserving the widget: If you grab
the pointer to the widget, you can
simply set its parent to some other
widget and add it to another
widget's layout and it will show up
there. This is safe because the
QSplitter will be notified that one
of its children is being reparented.
If you want to set the parent to NULL (cjhuitt's answer) be aware that you are now responsible for cleaning up that memory because the widget no longer has a parent.
Many things in Qt cannot be "traditionally" removed. Instead call hide() on it and destruct it. From QSplitter documentation:
When you hide() a child its space will
be distributed among the other
children. It will be reinstated when
you show() it again.
I like Tuminoid's answer. But if you absolutely need it removed, try getting the widget you want to remove, and calling setParent( NULL ) on that widget. That's my best guess.
If you hold a pointer to the widget, then just delete it, or use deleteLater() if you want to be on the safe side.
If you do not have the widget pointer, use QSplitter::widget(int index) function. Then, you can use invoke its deleteLater() slot.
If you do not have the widget index, but you still know the widget objectName(), then QObject::findChild() is your only way to get the widget pointer.
I ran into the same problem. In Qt 4.8 to temporally hide one of the widget of a QSplitter I simply hide it. However it is not enough, as the splitter handle is still available to move. But the handle can be accessed and hidden as well:
frameA->setVisible(conditionA);
frameB->setVisible(conditionB);
if ( !(conditionA && conditionB) ) // if only 1 frame is visible
{
splitter->handle(0)->setVisible(false);
}
Another easy way to prevent the child widget from getting deleted is to use QSplitter.takeWidget(child). This is also the recommended way of removing the widget from a splitter. (Qt Documentation)