How to position an added widget to a layout based on another widget in the same layout?

Question

In my GUI, I would like to add a QComboBox to a verticalLayout programmatically based on a a signal triggered by specific action. The following code works fine and the widget is added:

QComboBox* userOptions = new QComboBox();
ui->verticalLayout_13->addWidget(userOptions);

However, this way the widget is always added to the end of the layout.

My question is: how to position the added QComboBox to the verticalLayout in alignment to another widget in the same layout ? (i.e.: above the "Go" push button for example)

Do you really need to create a new QComboBox ? Because you could let it in the layout and set it visible or not whether you need it or not. — ymoreau, Apr 20 '16 at 14:54
@YMoreau: I just used the visibility property as you suggested. Thanks — McLan, Apr 20 '16 at 18:05

dtech · Accepted Answer · 2016-04-20T22:23:14.863

4

There doesn't seem to be a way to explicitly insert an item in a layout where you want it to be.

You have a few choices to achieve that the "hard" way:

use QLayout::takeAt(int index) to take all items after the index you want to insert at, insert your item, then insert back the taken items.
create a placeholder widget which you can use to reserve an index in the layout, then you don't insert the item in the layout, but in a layout nested inside the placeholder widget. Without an item, the placeholder widget takes no space, and expands to accommodate whatever is put into it.
implement your own QLayout subclass which supports inserting at a specific index. There are several functions you will have to implement.

EDIT: An omission on my end, as Kuba Ober noted, most of the concrete layout implementations support inserting at a specific index, for example QBoxLayout derived have insert methods which pass an index as a parameter.

edited Apr 20 '16 at 22:23

answered Apr 20 '16 at 14:34

dtech

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It looks like a lot of unnecessary "hard" work for what I thought it will be a nice and easy skill to learn. As far as my application is concern, I will stick to the first line in your answer (for now at least), and use visibility property as "Y Moreau" suggested. Thanks – McLan Apr 20 '16 at 18:05
You might want to look at the documentation of concrete layouts again. Yes, there's no way of doing it using only the `QLayout` interface, but all concrete layouts definitely support it. `QGridLayout` doesn't let you insert directly, but you can easily enough shuffle existing items to make room for the new one. – Kuba hasn't forgotten Monica Apr 20 '16 at 21:14
@KubaOber - yes, it is true, I haven't noticed before, mainly because I haven't used layouts all that much, and most of the time - in a static scenario. I am mostly using QML, and QML layouts don't have that at all. – dtech Apr 20 '16 at 22:18
In QML if you don't use layouts it's IIRC possible to rebind the positions to change the visual layout on the fly, too. – Kuba hasn't forgotten Monica Apr 21 '16 at 12:49

score 1 · Answer 2 · answered Apr 20 '16 at 21:13

First, iterate a layout to find the index of the reference item you're inserting relative to. Then use the concrete layout's specific widget insertion/addition functionality.

Since you presumably use a QBoxLayout, you'd use its insertWidget method to insert a widget.

// https://github.com/KubaO/stackoverflown/tree/master/questions/insert-widget-36746949
#include <QtWidgets>

namespace SO { enum InsertPosition { InsertBefore, InsertAfter }; }

bool insertWidget(QBoxLayout * layout, QWidget * reference, QWidget * widget,
                  SO::InsertPosition pos = SO::InsertBefore, int stretch = 0,
                  Qt::Alignment alignment = 0) {
   int index = -1;
   for (int i = 0; i < layout->count(); ++i)
      if (layout->itemAt(i)->widget() == reference) {
         index = i;
         break;
      }
   if (index < 0) return false;
   if (pos == SO::InsertAfter) index++;
   layout->insertWidget(index, widget, stretch, alignment);
   return true;
}

Similar functions can be easily devised for QFormLayout, QGridLayout and QStackedLayout.

And a test harness:

int main(int argc, char ** argv) {
   QApplication app{argc, argv};
   QWidget w;
   QVBoxLayout l{&w};
   QLabel first{"First"};
   QLabel second{"Second"};
   l.addWidget(&first);
   l.addWidget(&second);
   insertWidget(&l, &first, new QLabel{"Before First"}, SO::InsertBefore);
   insertWidget(&l, &second, new QLabel{"After Second"}, SO::InsertAfter);
   w.show();
   return app.exec();
}

How to position an added widget to a layout based on another widget in the same layout?

2 Answers2