QScrollArea messing up with QGridLayout : QGridLayout hidden and no scroll

Question

I am trying to make a QGridLayout scrollable. It may contain several custom widgets, the number of widgets is not fixed. QGridLayout must be scrollable when there are more than x widgets, x being an arbitrary number.

The problem is, when I use QScrollArea, QScrollArea seems to hide the whole layout (only the background color of scroll area is shown). When I use QGridLayout alone, my view is - of course - not scrollable but everything works as it should do.

I am probably missing something, my guesses are:

I must fix somehow the size of the scrollable area, but I am not sure it's necessary or if I already have done it unconsciously
Parent Widgets or Child Widgets of the existing QScrollArea prevent QScrollArea by an unusual implementation to act normally.

Here's a piece of the involved code :

    QScrollArea *scrollArea = new QScrollArea;
    QWidget *resultsPage = new QWidget;
    booksGrid = new QGridLayout;
    booksGrid->setSizeConstraint(QLayout::SetMinAndMaxSize);
    resultsPage->setLayout(booksGrid);
    scrollArea->setBackgroundRole(QPalette::Dark);
    scrollArea->setWidget(resultsPage);
    mainWidget->addWidget(scrollArea);

Also, booksGrid is declared as a class attribute, mainWidget is a QStackedWidget.

Any help is welcomed, let me know if you need more information !

Does it change anything if you leave the booksGrid->setSizeConstraint(QLayout::SetMinAndMaxSize); out? — Scheff's Cat
Not at all, but this line was necessary anyway, take a look on my answer to resolve the problem ;) — SparkyRaccoon
I was somehow a little bit suspicious about what you actually described in your answer. You should have mentioned your custom widgets in your question (or I could've asked. In this case, I probably hadn't provided my answer.) However, problem solved. After 48 hours you may accept your own answer. — Scheff's Cat

Scheff's Cat Scheff's Cat · Accepted Answer · 2017-11-04T11:16:58

As I couldn't see anything suspicious in your code fragment, I made an MCVE to reproduce your issue:

#include <QtWidgets>

int main(int argc, char **argv)
{
  qDebug() << "Qt Version: " << QT_VERSION_STR;
  // main application
  QApplication app(argc, argv);
  // setup GUI
  QMainWindow qWin;
  QScrollArea qScrArea;
  QWidget qScrView;
  QGridLayout qGrid;
  enum { nCols = 4 };
#define MAKE_LABEL(I) \
  QLabel qLbl##I(QString::fromUtf8("Label "#I)); \
  qGrid.addWidget(&qLbl##I, I / nCols, I % nCols)
  MAKE_LABEL(0);  MAKE_LABEL(1);  MAKE_LABEL(2);  MAKE_LABEL(3);  MAKE_LABEL(4);
  MAKE_LABEL(5);  MAKE_LABEL(6);  MAKE_LABEL(7);  MAKE_LABEL(8);  MAKE_LABEL(9);
  MAKE_LABEL(10); MAKE_LABEL(11); MAKE_LABEL(12); MAKE_LABEL(13); MAKE_LABEL(14);
  MAKE_LABEL(15); MAKE_LABEL(16); MAKE_LABEL(17); MAKE_LABEL(18); MAKE_LABEL(19);
  MAKE_LABEL(20); MAKE_LABEL(21); MAKE_LABEL(22); MAKE_LABEL(23); MAKE_LABEL(24);
  MAKE_LABEL(25); MAKE_LABEL(26); MAKE_LABEL(27); MAKE_LABEL(28); MAKE_LABEL(29);
  MAKE_LABEL(30); MAKE_LABEL(31); MAKE_LABEL(32); MAKE_LABEL(33); MAKE_LABEL(34);
  MAKE_LABEL(35); MAKE_LABEL(36); MAKE_LABEL(37); MAKE_LABEL(38); MAKE_LABEL(39);
#undef MAKE_LABEL
  qScrView.setLayout(&qGrid);
  qScrArea.setWidget(&qScrView);
  qWin.setCentralWidget(&qScrArea);
  qWin.show();
  // run-time loop
  return app.exec();
}

Compiled and tested in VS2013, Qt 5.9.2 on Windows 10 (64 bit):

a) b) c)

The snapshots are taken after start (a), after resizing (b), and after scrolling (c).

For me, everything looks and works like expected.

You may compile and test the sample on your side as well. If it shows the same broken behavior like your application then something is wrong in your Qt version (otherwise something in your application).

QScrollArea messing up with QGridLayout : QGridLayout hidden and no scroll

2 Answers