Icons Example

class IconPreviewArea : public QWidget
{
    Q_OBJECT

public:
    explicit IconPreviewArea(QWidget *parent = nullptr);

    void setIcon(const QIcon &icon);
    void setSize(const QSize &size);

    static QVector<QIcon::Mode> iconModes();
    static QVector<QIcon::State> iconStates();
    static QStringList iconModeNames();
    static QStringList iconStateNames();

private:
    QLabel *createHeaderLabel(const QString &text);
    QLabel *createPixmapLabel();
    void updatePixmapLabels();

    enum { NumModes = 4, NumStates = 2 };

    QIcon icon;
    QSize size;
    QLabel *stateLabels[NumStates];
    QLabel *modeLabels[NumModes];
    QLabel *pixmapLabels[NumModes][NumStates];
};

QVector<QIcon::Mode> IconPreviewArea::iconModes()
{
    static const QVector<QIcon::Mode> result = {QIcon::Normal, QIcon::Active, QIcon::Disabled, QIcon::Selected};
    return result;
}

QVector<QIcon::State> IconPreviewArea::iconStates()
{
    static const QVector<QIcon::State> result = {QIcon::Off, QIcon::On};
    return result;
}

QStringList IconPreviewArea::iconModeNames()
{
    static const QStringList result = {tr("Normal"), tr("Active"), tr("Disabled"), tr("Selected")};
    return result;
}

QStringList IconPreviewArea::iconStateNames()
{
    static const QStringList result = {tr("Off"), tr("On")};
    return result;
}

QVector<QIcon::Mode> IconPreviewArea::iconModes()
{
    static QVector<QIcon::Mode> result;
    if (result.isEmpty())
        result << QIcon::Normal << QIcon::Active << QIcon::Disabled << QIcon::Selected;
    return result;
}

IconPreviewArea::IconPreviewArea(QWidget *parent)
    : QWidget(parent)
{
    QGridLayout *mainLayout = new QGridLayout(this);

    for (int row = 0; row < NumStates; ++row) {
        stateLabels[row] = createHeaderLabel(IconPreviewArea::iconStateNames().at(row));
        mainLayout->addWidget(stateLabels[row], row + 1, 0);
    }
    Q_ASSERT(NumStates == 2);

    for (int column = 0; column < NumModes; ++column) {
        modeLabels[column] = createHeaderLabel(IconPreviewArea::iconModeNames().at(column));
        mainLayout->addWidget(modeLabels[column], 0, column + 1);
    }
    Q_ASSERT(NumModes == 4);

    for (int column = 0; column < NumModes; ++column) {
        for (int row = 0; row < NumStates; ++row) {
            pixmapLabels[column][row] = createPixmapLabel();
            mainLayout->addWidget(pixmapLabels[column][row], row + 1, column + 1);
        }
    }
}

if (!condition)
   qFatal("ASSERT: "condition" in file ...");

qmake "CONFIG += debug" icons.pro

qmake "CONFIG += release" icons.pro

void IconPreviewArea::setIcon(const QIcon &icon)
{
    this->icon = icon;
    updatePixmapLabels();
}

void IconPreviewArea::setSize(const QSize &size)
{
    if (size != this->size) {
        this->size = size;
        updatePixmapLabels();
    }
}

QLabel *IconPreviewArea::createHeaderLabel(const QString &text)
{
    QLabel *label = new QLabel(tr("<b>%1</b>").arg(text));
    label->setAlignment(Qt::AlignCenter);
    return label;
}

QLabel *IconPreviewArea::createPixmapLabel()
{
    QLabel *label = new QLabel;
    label->setEnabled(false);
    label->setAlignment(Qt::AlignCenter);
    label->setFrameShape(QFrame::Box);
    label->setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);
    label->setBackgroundRole(QPalette::Base);
    label->setAutoFillBackground(true);
    label->setMinimumSize(132, 132);
    return label;
}

void IconPreviewArea::updatePixmapLabels()
{
    QWindow *window = nullptr;
    if (const QWidget *nativeParent = nativeParentWidget())
        window = nativeParent->windowHandle();
    for (int column = 0; column < NumModes; ++column) {
        for (int row = 0; row < NumStates; ++row) {
            const QPixmap pixmap =
                icon.pixmap(window, size, IconPreviewArea::iconModes().at(column),
                            IconPreviewArea::iconStates().at(row));
            QLabel *pixmapLabel = pixmapLabels[column][row];
            pixmapLabel->setPixmap(pixmap);
            pixmapLabel->setEnabled(!pixmap.isNull());
            QString toolTip;
            if (!pixmap.isNull()) {
                const QSize actualSize = icon.actualSize(size);
                toolTip =
                    tr("Size: %1x%2\nActual size: %3x%4\nDevice pixel ratio: %5")
                        .arg(size.width()).arg(size.height())
                        .arg(actualSize.width()).arg(actualSize.height())
                        .arg(pixmap.devicePixelRatioF());
            }
            pixmapLabel->setToolTip(toolTip);
        }
    }
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = nullptr);

    void loadImages(const QStringList &fileNames);

    void show();

private slots:
    void about();
    void changeStyle(bool checked);
    void changeSize(int, bool);
    void triggerChangeSize();
    void changeIcon();
    void addSampleImages();
    void addOtherImages();
    void removeAllImages();
    void useHighDpiPixmapsChanged(int checkState);
    void screenChanged();

private:
    QWidget *createImagesGroupBox();
    QWidget *createIconSizeGroupBox();
    QWidget *createHighDpiIconSizeGroupBox();
    void createActions();
    void createContextMenu();
    void checkCurrentStyle();
    void addImages(const QString &directory);

    IconPreviewArea *previewArea;

    QTableWidget *imagesTable;

    QButtonGroup *sizeButtonGroup;
    IconSizeSpinBox *otherSpinBox;

    QLabel *devicePixelRatioLabel;
    QLabel *screenNameLabel;

    QAction *addOtherImagesAct;
    QAction *addSampleImagesAct;
    QAction *removeAllImagesAct;
    QAction *guessModeStateAct;
    QAction *nativeFileDialogAct;
    QActionGroup *styleActionGroup;
};

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
{
    QWidget *centralWidget = new QWidget(this);
    setCentralWidget(centralWidget);

    createActions();

    QGridLayout *mainLayout = new QGridLayout(centralWidget);

    QGroupBox *previewGroupBox = new QGroupBox(tr("Preview"));
    previewArea = new IconPreviewArea(previewGroupBox);
    QVBoxLayout *previewLayout = new QVBoxLayout(previewGroupBox);
    previewLayout->addWidget(previewArea);

    mainLayout->addWidget(previewGroupBox, 0, 0, 1, 2);
    mainLayout->addWidget(createImagesGroupBox(), 1, 0);
    QVBoxLayout *vBox = new QVBoxLayout;
    vBox->addWidget(createIconSizeGroupBox());
    vBox->addWidget(createHighDpiIconSizeGroupBox());
    vBox->addItem(new QSpacerItem(0, 0, QSizePolicy::Ignored, QSizePolicy::MinimumExpanding));
    mainLayout->addLayout(vBox, 1, 1);
    createContextMenu();

    setWindowTitle(tr("Icons"));
    checkCurrentStyle();
    sizeButtonGroup->button(OtherSize)->click();
}

void MainWindow::about()
{
    QMessageBox::about(this, tr("About Icons"),
            tr("The <b>Icons</b> example illustrates how Qt renders an icon in "
               "different modes (active, normal, disabled, and selected) and "
               "states (on and off) based on a set of images."));
}

void MainWindow::changeStyle(bool checked)
{
    if (!checked)
        return;

    const QAction *action = qobject_cast<QAction *>(sender());

    QStyle *style = QStyleFactory::create(action->data().toString());
    Q_ASSERT(style);
    QApplication::setStyle(style);

    const QList<QAbstractButton*> buttons = sizeButtonGroup->buttons();
    for (QAbstractButton *button : buttons) {
        const QStyle::PixelMetric metric = static_cast<QStyle::PixelMetric>(sizeButtonGroup->id(button));
        const int value = style->pixelMetric(metric);
        switch (metric) {
        case QStyle::PM_SmallIconSize:
            button->setText(tr("Small (%1 x %1)").arg(value));
            break;
        case QStyle::PM_LargeIconSize:
            button->setText(tr("Large (%1 x %1)").arg(value));
            break;
        case QStyle::PM_ToolBarIconSize:
            button->setText(tr("Toolbars (%1 x %1)").arg(value));
            break;
        case QStyle::PM_ListViewIconSize:
            button->setText(tr("List views (%1 x %1)").arg(value));
            break;
        case QStyle::PM_IconViewIconSize:
            button->setText(tr("Icon views (%1 x %1)").arg(value));
            break;
        case QStyle::PM_TabBarIconSize:
            button->setText(tr("Tab bars (%1 x %1)").arg(value));
            break;
        default:
            break;
        }
    }

    triggerChangeSize();
}

void MainWindow::changeSize(int id, bool checked)
{
    if (!checked)
        return;

    const bool other = id == int(OtherSize);
    const int extent = other
        ? otherSpinBox->value()
        : QApplication::style()->pixelMetric(static_cast<QStyle::PixelMetric>(id));

    previewArea->setSize(QSize(extent, extent));
    otherSpinBox->setEnabled(other);
}

void MainWindow::triggerChangeSize()
{
    changeSize(sizeButtonGroup->checkedId(), true);
}

void MainWindow::addImages(const QString &directory)
{
    QFileDialog fileDialog(this, tr("Open Images"), directory);
    QStringList mimeTypeFilters;
    const QList<QByteArray> mimeTypes = QImageReader::supportedMimeTypes();
    for (const QByteArray &mimeTypeName : mimeTypes)
        mimeTypeFilters.append(mimeTypeName);
    mimeTypeFilters.sort();
    fileDialog.setMimeTypeFilters(mimeTypeFilters);
    fileDialog.selectMimeTypeFilter(QLatin1String("image/png"));
    fileDialog.setAcceptMode(QFileDialog::AcceptOpen);
    fileDialog.setFileMode(QFileDialog::ExistingFiles);
    if (!nativeFileDialogAct->isChecked())
        fileDialog.setOption(QFileDialog::DontUseNativeDialog);
    if (fileDialog.exec() == QDialog::Accepted)
        loadImages(fileDialog.selectedFiles());

        const QFileInfo fileInfo(fileName);
        const QString imageName = fileInfo.baseName();
        const QString fileName2x = fileInfo.absolutePath()
            + QLatin1Char('/') + imageName + QLatin1String("@2x.") + fileInfo.suffix();
        const QFileInfo fileInfo2x(fileName2x);
        const QImage image(fileName);
        const QString toolTip =
            tr("Directory: %1\nFile: %2\nFile@2x: %3\nSize: %4x%5")
               .arg(QDir::toNativeSeparators(fileInfo.absolutePath()), fileInfo.fileName())
               .arg(fileInfo2x.exists() ? fileInfo2x.fileName() : tr("<None>"))
               .arg(image.width()).arg(image.height());
        QTableWidgetItem *fileItem = new QTableWidgetItem(imageName);
        fileItem->setData(Qt::UserRole, fileName);
        fileItem->setIcon(QPixmap::fromImage(image));
        fileItem->setFlags((fileItem->flags() | Qt::ItemIsUserCheckable) & ~Qt::ItemIsEditable);
        fileItem->setToolTip(toolTip);