Cannot automatically resize a QScrollArea

Question

My environment:

Windows 10, 64-bit
Microsoft Visual Studio 2015
Qt 5.6.2
Qt Creator 4.8.1

I have a dialog which contains a scroll area. The scroll area is initially empty:

When the user clicks the check box, widgets are added to the scroll area:

As you can see from the above, only part of the scroll area is shown. I would like to automatically enlarge it (as well as the dialog which contains it), so the result would look something like this:

What is missing from my code?

I have reduced the problem to a minimal example program (from which the above pictures were taken). Here is the code:

scroll_area.pro:

QT += widgets
TEMPLATE = app
SOURCES += main.cpp my_dialog.cpp
HEADERS += my_dialog.h my_widget.h
TARGET = ScrollArea

main.cpp:

#include "my_dialog.h"
#include <QApplication>

int main(int argc, char* argv[])
{
    QApplication app(argc, argv);
    MyDialog myDialog;
    myDialog.show();
    return app.exec();
}

my_dialog.h:

#ifndef MY_DIALOG_H
#define MY_DIALOG_H

#include <QBoxLayout>
#include <QDialog>
#include <QScrollArea>

class MyDialog : public QDialog
{
    Q_OBJECT

public:
    MyDialog();

private:
    QScrollArea* m_scrollArea;
    QHBoxLayout* m_hLayout;

private slots:
    void changeScrollArea(int newState);
};

#endif // MY_DIALOG_H

my_dialog.cpp:

#include "my_dialog.h"
#include "my_widget.h"
#include <QCheckBox>
#include <QDialogButtonBox>

MyDialog::MyDialog() :
    QDialog(nullptr), m_scrollArea(new QScrollArea(this)), m_hLayout(new QHBoxLayout())
{
    setWindowTitle("My Dialog");

    QVBoxLayout* vlayout = new QVBoxLayout(this);

    // Set up the scroll area.
    m_scrollArea->setBackgroundRole(QPalette::ColorRole::Light);
    m_scrollArea->setHorizontalScrollBarPolicy(Qt::ScrollBarAsNeeded);
    vlayout->addWidget(m_scrollArea);

    // Set up the scroll contents.
    QWidget* scrollContents = new QWidget(this);
    scrollContents->setLayout(m_hLayout);
    m_scrollArea->setWidget(scrollContents);

    // Set up the check box.
    QCheckBox* checkbox = new QCheckBox("Check to add widgets", this);
    connect(checkbox, SIGNAL(stateChanged(int)), this, SLOT(changeScrollArea(int)));
    vlayout->addWidget(checkbox);
}

void MyDialog::changeScrollArea(int newState)
{
    // Set up the horizontal layout.
    m_hLayout->deleteLater();
    m_hLayout = new QHBoxLayout();

    // Set up the scroll contents.
    QWidget* scrollContents = new QWidget(this);
    scrollContents->setLayout(m_hLayout);
    if (newState == Qt::Checked)
    {
        m_hLayout->addWidget(new MyWidget(scrollContents));
        m_hLayout->addWidget(new MyWidget(scrollContents));
    }
    m_scrollArea->setWidget(scrollContents);
}

my_widget.h

#ifndef MY_WIDGET_H
#define MY_WIDGET_H

#include <QGridLayout>
#include <QPushButton>

class MyWidget : public QWidget
{
    Q_OBJECT

public:
    MyWidget(QWidget* parent) : QWidget(parent)
    {
        QGridLayout* gridLayout = new QGridLayout(this);

        for (int row = 0; row < 5; ++row)
        {
            for (int column = 0; column < 1; ++column)
            {
                QPushButton* button = new QPushButton("FOO");
                gridLayout->addWidget(button, row, column);
            }
        }
    }
};

#endif // MY_WIDGET_H

eyllanesc eyllanesc · Accepted Answer · 2019-01-24T17:39:08

QScrollArea will not change the size according to its content since its task is to establish a widget that can have a large size within a viewport, so the solution is to change the size using code:

mydialog.h

#ifndef MYDIALOG_H
#define MYDIALOG_H

#include <QDialog>

class QScrollArea;
class QHBoxLayout;

class MyDialog : public QDialog
{
    Q_OBJECT
public:
    MyDialog();
private:
    QScrollArea* m_scrollArea;
    QHBoxLayout* m_hLayout;
private slots:
    void changeScrollArea(int newState);
    void adjust();
};
#endif // MYDIALOG_H

mydialog.cpp

#include "mydialog.h"
#include "mywidget.h"

#include <QCheckBox>
#include <QHBoxLayout>
#include <QScrollArea>
#include <QScrollBar>
#include <QTimer>

MyDialog::MyDialog() :
    QDialog(nullptr), m_scrollArea(new QScrollArea), m_hLayout(new QHBoxLayout())
{
    setWindowTitle("My Dialog");
    QVBoxLayout* vlayout = new QVBoxLayout(this);
    // Set up the scroll area.
    m_scrollArea->setBackgroundRole(QPalette::ColorRole::Light);
    m_scrollArea->setHorizontalScrollBarPolicy(Qt::ScrollBarAsNeeded);
    m_scrollArea->setWidgetResizable(true);
    vlayout->addWidget(m_scrollArea);
    // Set up the scroll contents.
    QWidget* scrollContents = new QWidget;
    scrollContents->setLayout(m_hLayout);
    m_scrollArea->setWidget(scrollContents);
    // Set up the check box.
    QCheckBox* checkbox = new QCheckBox("Check to add widgets");
    connect(checkbox, &QCheckBox::stateChanged, this, &MyDialog::changeScrollArea);
    vlayout->addWidget(checkbox);
}
void MyDialog::changeScrollArea(int newState)
{
    m_hLayout->deleteLater();
    // Set up the scroll contents.
    QWidget* scrollContents = new QWidget;
    // Set up the horizontal layout.
    m_hLayout = new QHBoxLayout(scrollContents);
    if (newState == Qt::Checked)
    {
        m_hLayout->addWidget(new MyWidget(scrollContents));
        m_hLayout->addWidget(new MyWidget(scrollContents));
    }
    m_scrollArea->setWidget(scrollContents);
    m_scrollArea->resize(m_scrollArea->minimumSizeHint());
    resize(sizeHint());
    QTimer::singleShot(0, this, &MyDialog::adjust);
}

void MyDialog::adjust()
{
    const QSize max_size(1000, 1000);
    const int step = 5;
    while (m_scrollArea->verticalScrollBar()->isVisible() && height() < max_size.height())
        resize(width(), height() + step);
    while (m_scrollArea->horizontalScrollBar()->isVisible() && width() < max_size.width())
        resize(width()+step, height());
}

Cannot automatically resize a QScrollArea

1 Answers