Icons Example

MainWindow Class Implementation

 MainWindow::MainWindow()
 {
     centralWidget = new QWidget;
     setCentralWidget(centralWidget);

     createPreviewGroupBox();
     createImagesGroupBox();
     createIconSizeGroupBox();

     createActions();
     createMenus();
     createContextMenu();

     QGridLayout *mainLayout = new QGridLayout;
     mainLayout->addWidget(previewGroupBox, 0, 0, 1, 2);
     mainLayout->addWidget(imagesGroupBox, 1, 0);
     mainLayout->addWidget(iconSizeGroupBox, 1, 1);
     centralWidget->setLayout(mainLayout);

     setWindowTitle(tr("Icons"));
     checkCurrentStyle();
     otherRadioButton->click();

     resize(minimumSizeHint());
 }

In the constructor we first create the main window's central widget and its child widgets, and put them in a grid layout. Then we create the menus with their associated entries and actions.

Before we resize the application window to a suitable size, we set the window title and determine the current style for the application. We also enable the icon size spin box by clicking the associated radio button, making the current value of the spin box the icon's initial size.

 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."));
 }

The about() slot displays a message box using the static QMessageBox::about() function. In this example it displays a simple box with information about the example.

The about() function looks for a suitable icon in four locations: It prefers its parent's icon if that exists. If it doesn't, the function tries the top-level widget containing parent, and if that fails, it tries the active window. As a last resort it uses the QMessageBox's Information icon.

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

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

In the changeStyle() slot we first check the slot's parameter. If it is false we immediately return, otherwise we find out which style to change to, i.e. which action that triggered the slot, using the QObject::sender() function.

This function returns the sender as a QObject pointer. Since we know that the sender is a QAction object, we can safely cast the QObject. We could have used a C-style cast or a C++ static_cast(), but as a defensive programming technique we use a qobject_cast(). The advantage is that if the object has the wrong type, a null pointer is returned. Crashes due to null pointers are much easier to diagnose than crashes due to unsafe casts.

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

     smallRadioButton->setText(tr("Small (%1 x %1)")
             .arg(style->pixelMetric(QStyle::PM_SmallIconSize)));
     largeRadioButton->setText(tr("Large (%1 x %1)")
             .arg(style->pixelMetric(QStyle::PM_LargeIconSize)));
     toolBarRadioButton->setText(tr("Toolbars (%1 x %1)")
             .arg(style->pixelMetric(QStyle::PM_ToolBarIconSize)));
     listViewRadioButton->setText(tr("List views (%1 x %1)")
             .arg(style->pixelMetric(QStyle::PM_ListViewIconSize)));
     iconViewRadioButton->setText(tr("Icon views (%1 x %1)")
             .arg(style->pixelMetric(QStyle::PM_IconViewIconSize)));
     tabBarRadioButton->setText(tr("Tab bars (%1 x %1)")
             .arg(style->pixelMetric(QStyle::PM_TabBarIconSize)));

     changeSize();
 }

Once we have the action, we extract the style name using QAction::data(). Then we create a QStyle object using the static QStyleFactory::create() function.

Although we can assume that the style is supported by the QStyleFactory: To be on the safe side, we use the Q_ASSERT() macro to check if the created style is valid before we use the QApplication::setStyle() function to set the application's GUI style to the new style. QApplication will automatically delete the style object when a new style is set or when the application exits.

The predefined icon size options provided in the application are style dependent, so we need to update the labels in the icon size group box and in the end call the changeSize() slot to update the icon's size.

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

     int extent;

     if (otherRadioButton->isChecked()) {
         extent = otherSpinBox->value();
     } else {
         QStyle::PixelMetric metric;

         if (smallRadioButton->isChecked()) {
             metric = QStyle::PM_SmallIconSize;
         } else if (largeRadioButton->isChecked()) {
             metric = QStyle::PM_LargeIconSize;
         } else if (toolBarRadioButton->isChecked()) {
             metric = QStyle::PM_ToolBarIconSize;
         } else if (listViewRadioButton->isChecked()) {
             metric = QStyle::PM_ListViewIconSize;
         } else if (iconViewRadioButton->isChecked()) {
             metric = QStyle::PM_IconViewIconSize;
         } else {
             metric = QStyle::PM_TabBarIconSize;
         }
         extent = QApplication::style()->pixelMetric(metric);
     }
     previewArea->setSize(QSize(extent, extent));
     otherSpinBox->setEnabled(otherRadioButton->isChecked());
 }

The changeSize() slot sets the size for the preview area's icon.

To determine the new size we first check if the spin box is enabled. If it is, we extract the extent of the new size from the box. If it's not, we search through the predefined size options, extract the QStyle::PixelMetric and use the QStyle::pixelMetric() function to determine the extent. Then we create a QSize object based on the extent, and use that object to set the size of the preview area's icon.

 void MainWindow::addImages()
 {
     QStringList fileNames = QFileDialog::getOpenFileNames(this,
                                     tr("Open Images"), "",
                                     tr("Images (*.png *.xpm *.jpg);;"
                                        "All Files (*)"));
     if (!fileNames.isEmpty()) {
         foreach (QString fileName, fileNames) {
             int row = imagesTable->rowCount();
             imagesTable->setRowCount(row + 1);

The first thing we do when the addImage() slot is called, is to show a file dialog to the user. The easiest way to create a file dialog is to use QFileDialog's static functions. Here we use the getOpenFileNames() function that will return one or more existing files selected by the user.

For each of the files the file dialog returns, we add a row to the table widget. The table widget is listing the images the user has loaded into the application.

             QString imageName = QFileInfo(fileName).baseName();
             QTableWidgetItem *item0 = new QTableWidgetItem(imageName);
             item0->setData(Qt::UserRole, fileName);
             item0->setFlags(item0->flags() & ~Qt::ItemIsEditable);

We retrieve the image name using the QFileInfo::baseName() function that returns the base name of the file without the path, and create the first table widget item in the row. Then we add the file's complete name to the item's data. Since an item can hold several information pieces, we need to assign the file name a role that will distinguish it from other data. This role can be Qt::UserRole or any value above it.

We also make sure that the item is not editable by removing the Qt::ItemIsEditable flag. Table items are editable by default.

             QTableWidgetItem *item1 = new QTableWidgetItem(tr("Normal"));
             QTableWidgetItem *item2 = new QTableWidgetItem(tr("Off"));

             if (guessModeStateAct->isChecked()) {
                 if (fileName.contains("_act")) {
                     item1->setText(tr("Active"));
                 } else if (fileName.contains("_dis")) {
                     item1->setText(tr("Disabled"));
                 } else if (fileName.contains("_sel")) {
                     item1->setText(tr("Selected"));
                 }

                 if (fileName.contains("_on"))
                     item2->setText(tr("On"));
             }

Then we create the second and third items in the row making the default mode Normal and the default state Off. But if the Guess Image Mode/State option is checked, and the file name contains "_act", "_dis", or "_sel", the modes are changed to Active, Disabled, or Selected. And if the file name contains "_on", the state is changed to On. The sample files in the example's images subdirectory respect this naming convension.

             imagesTable->setItem(row, 0, item0);
             imagesTable->setItem(row, 1, item1);
             imagesTable->setItem(row, 2, item2);
             imagesTable->openPersistentEditor(item1);
             imagesTable->openPersistentEditor(item2);

             item0->setCheckState(Qt::Checked);
         }
     }
 }

In the end we add the items to the associated row, and use the QTableWidget::openPersistentEditor() function to create comboboxes for the mode and state columns of the items.

Due to the connection between the table widget's itemChanged() signal and the changeIcon() slot, the new image is automatically converted into a pixmap and made part of the set of pixmaps available to the icon in the preview area. So, corresponding to this fact, we need to make sure that the new image's check box is enabled.

 void MainWindow::changeIcon()
 {
     QIcon icon;

     for (int row = 0; row < imagesTable->rowCount(); ++row) {
         QTableWidgetItem *item0 = imagesTable->item(row, 0);
         QTableWidgetItem *item1 = imagesTable->item(row, 1);
         QTableWidgetItem *item2 = imagesTable->item(row, 2);

         if (item0->checkState() == Qt::Checked) {
             QIcon::Mode mode;
             if (item1->text() == tr("Normal")) {
                 mode = QIcon::Normal;
             } else if (item1->text() == tr("Active")) {
                 mode = QIcon::Active;
             } else if (item1->text() == tr("Disabled")) {
                 mode = QIcon::Disabled;
             } else {
                 mode = QIcon::Selected;
             }

             QIcon::State state;
             if (item2->text() == tr("On")) {
                 state = QIcon::On;
             } else {
                 state = QIcon::Off;
             }

The changeIcon() slot is called when the user alters the set of images listed in the QTableWidget, to update the QIcon object rendered by the IconPreviewArea.

We first create a QIcon object, and then we run through the QTableWidget, which lists the images the user has loaded into the application.

             QString fileName = item0->data(Qt::UserRole).toString();
             QImage image(fileName);
             if (!image.isNull())
                 icon.addPixmap(QPixmap::fromImage(image), mode, state);
         }
     }

We also extract the image file's name using the QTableWidgetItem::data() function. This function takes a Qt::DataItemRole as an argument to retrieve the right data (remember that an item can hold several pieces of information) and returns it as a QVariant. Then we use the QVariant::toString() function to get the file name as a QString.

To create a pixmap from the file, we need to first create an image and then convert this image into a pixmap using QPixmap::fromImage(). Once we have the final pixmap, we add it, with its associated mode and state, to the QIcon's set of available pixmaps.

     previewArea->setIcon(icon);
 }

After running through the entire list of images, we change the icon of the preview area to the one we just created.

 void MainWindow::removeAllImages()
 {
     imagesTable->setRowCount(0);
     changeIcon();
 }

In the removeAllImages() slot, we simply set the table widget's row count to zero, automatically removing all the images the user has loaded into the application. Then we update the set of pixmaps available to the preview area's icon using the changeIcon() slot.

The createImagesGroupBox() function is implemented to simplify the constructor. The main purpose of the function is to create a QTableWidget that will keep track of the images the user has loaded into the application.

 void MainWindow::createImagesGroupBox()
 {
     imagesGroupBox = new QGroupBox(tr("Images"));

     imagesTable = new QTableWidget;
     imagesTable->setSelectionMode(QAbstractItemView::NoSelection);
     imagesTable->setItemDelegate(new ImageDelegate(this));

First we create a group box that will contain the table widget. Then we create a QTableWidget and customize it to suit our purposes.

We call QAbstractItemView::setSelectionMode() to prevent the user from selecting items.

The QAbstractItemView::setItemDelegate() call sets the item delegate for the table widget. We create a ImageDelegate that we make the item delegate for our view.

The QItemDelegate class can be used to provide an editor for an item view class that is subclassed from QAbstractItemView. Using a delegate for this purpose allows the editing mechanism to be customized and developed independently from the model and view.

In this example we derive ImageDelegate from QItemDelegate. QItemDelegate usually provides line editors, while our subclass ImageDelegate, provides comboboxes for the mode and state fields.

     QStringList labels;
     labels << tr("Image") << tr("Mode") << tr("State");

     imagesTable->horizontalHeader()->setDefaultSectionSize(90);
     imagesTable->setColumnCount(3);
     imagesTable->setHorizontalHeaderLabels(labels);
     imagesTable->horizontalHeader()->setResizeMode(0, QHeaderView::Stretch);
     imagesTable->horizontalHeader()->setResizeMode(1, QHeaderView::Fixed);
     imagesTable->horizontalHeader()->setResizeMode(2, QHeaderView::Fixed);
     imagesTable->verticalHeader()->hide();

Then we customize the QTableWidget's horizontal header, and hide the vertical header.

     connect(imagesTable, SIGNAL(itemChanged(QTableWidgetItem*)),
             this, SLOT(changeIcon()));

     QVBoxLayout *layout = new QVBoxLayout;
     layout->addWidget(imagesTable);
     imagesGroupBox->setLayout(layout);
 }

At the end, we connect the QTableWidget::itemChanged() signal to the changeIcon() slot to ensuret that the preview area is in sync with the image table.

The createIconSizeGroupBox() function is called from the constructor. It creates the widgets controlling the size of the preview area's icon.

 void MainWindow::createIconSizeGroupBox()
 {
     iconSizeGroupBox = new QGroupBox(tr("Icon Size"));

     smallRadioButton = new QRadioButton;
     largeRadioButton = new QRadioButton;
     toolBarRadioButton = new QRadioButton;
     listViewRadioButton = new QRadioButton;
     iconViewRadioButton = new QRadioButton;
     tabBarRadioButton = new QRadioButton;
     otherRadioButton = new QRadioButton(tr("Other:"));

     otherSpinBox = new IconSizeSpinBox;
     otherSpinBox->setRange(8, 128);
     otherSpinBox->setValue(64);

First we create a group box that will contain all the widgets; then we create the radio buttons and the spin box.

The spin box is not a regular QSpinBox but an IconSizeSpinBox. The IconSizeSpinBox class inherits QSpinBox and reimplements two functions: QSpinBox::textFromValue() and QSpinBox::valueFromText(). The IconSizeSpinBox is designed to handle icon sizes, e.g., "32 x 32", instead of plain integer values.

     connect(smallRadioButton, SIGNAL(toggled(bool)),
             this, SLOT(changeSize(bool)));
     connect(largeRadioButton, SIGNAL(toggled(bool)),
             this, SLOT(changeSize(bool)));
     connect(toolBarRadioButton, SIGNAL(toggled(bool)),
             this, SLOT(changeSize(bool)));
     connect(listViewRadioButton, SIGNAL(toggled(bool)),
             this, SLOT(changeSize(bool)));
     connect(iconViewRadioButton, SIGNAL(toggled(bool)),
             this, SLOT(changeSize(bool)));
     connect(tabBarRadioButton, SIGNAL(toggled(bool)),
             this, SLOT(changeSize(bool)));
     connect(otherRadioButton, SIGNAL(toggled(bool)),
             this, SLOT(changeSize(bool)));
     connect(otherSpinBox, SIGNAL(valueChanged(int)), this, SLOT(changeSize()));

     QHBoxLayout *otherSizeLayout = new QHBoxLayout;
     otherSizeLayout->addWidget(otherRadioButton);
     otherSizeLayout->addWidget(otherSpinBox);
     otherSizeLayout->addStretch();

     QGridLayout *layout = new QGridLayout;
     layout->addWidget(smallRadioButton, 0, 0);
     layout->addWidget(largeRadioButton, 1, 0);
     layout->addWidget(toolBarRadioButton, 2, 0);
     layout->addWidget(listViewRadioButton, 0, 1);
     layout->addWidget(iconViewRadioButton, 1, 1);
     layout->addWidget(tabBarRadioButton, 2, 1);
     layout->addLayout(otherSizeLayout, 3, 0, 1, 2);
     layout->setRowStretch(4, 1);
     iconSizeGroupBox->setLayout(layout);
 }

Then we connect all of the radio buttons toggled() signals and the spin box's valueChanged() signal to the changeSize() slot to make sure that the size of the preview area's icon is updated whenever the user changes the icon size. In the end we put the widgets in a layout that we install on the group box.

 void MainWindow::createActions()
 {
     addImagesAct = new QAction(tr("&Add Images..."), this);
     addImagesAct->setShortcut(tr("Ctrl+A"));
     connect(addImagesAct, SIGNAL(triggered()), this, SLOT(addImages()));

     removeAllImagesAct = new QAction(tr("&Remove All Images"), this);
     removeAllImagesAct->setShortcut(tr("Ctrl+R"));
     connect(removeAllImagesAct, SIGNAL(triggered()),
             this, SLOT(removeAllImages()));

     exitAct = new QAction(tr("&Quit"), this);
     exitAct->setShortcuts(QKeySequence::Quit);
     connect(exitAct, SIGNAL(triggered()), this, SLOT(close()));

     styleActionGroup = new QActionGroup(this);
     foreach (QString styleName, QStyleFactory::keys()) {
         QAction *action = new QAction(styleActionGroup);
         action->setText(tr("%1 Style").arg(styleName));
         action->setData(styleName);
         action->setCheckable(true);
         connect(action, SIGNAL(triggered(bool)), this, SLOT(changeStyle(bool)));
     }

     guessModeStateAct = new QAction(tr("&Guess Image Mode/State"), this);
     guessModeStateAct->setCheckable(true);
     guessModeStateAct->setChecked(true);

     aboutAct = new QAction(tr("&About"), this);
     connect(aboutAct, SIGNAL(triggered()), this, SLOT(about()));

     aboutQtAct = new QAction(tr("About &Qt"), this);
     connect(aboutQtAct, SIGNAL(triggered()), qApp, SLOT(aboutQt()));
 }

In the createActions() function we create and customize all the actions needed to implement the functionality associated with the menu entries in the application.

In particular we create the styleActionGroup based on the currently available GUI styles using QStyleFactory. QStyleFactory::keys() returns a list of valid keys, typically including "windows", "motif", "cde", and "plastique". Depending on the platform, "windowsxp" and "macintosh" may be available.

We create one action for each key, and adds the action to the action group. Also, for each action, we call QAction::setData() with the style name. We will retrieve it later using QAction::data().

 void MainWindow::createMenus()
 {
     fileMenu = menuBar()->addMenu(tr("&File"));
     fileMenu->addAction(addImagesAct);
     fileMenu->addAction(removeAllImagesAct);
     fileMenu->addSeparator();
     fileMenu->addAction(exitAct);

     viewMenu = menuBar()->addMenu(tr("&View"));
     foreach (QAction *action, styleActionGroup->actions())
         viewMenu->addAction(action);
     viewMenu->addSeparator();
     viewMenu->addAction(guessModeStateAct);

     menuBar()->addSeparator();

     helpMenu = menuBar()->addMenu(tr("&Help"));
     helpMenu->addAction(aboutAct);
     helpMenu->addAction(aboutQtAct);
 }

In the createMenu() function, we add the previously created actions to the File, View and Help menus.

The QMenu class provides a menu widget for use in menu bars, context menus, and other popup menus. We put each menu in the application's menu bar, which we retrieve using QMainWindow::menuBar().

 void MainWindow::createContextMenu()
 {
     imagesTable->setContextMenuPolicy(Qt::ActionsContextMenu);
     imagesTable->addAction(addImagesAct);
     imagesTable->addAction(removeAllImagesAct);
 }

QWidgets have a contextMenuPolicy property that controls how the widget should behave when the user requests a context menu (e.g., by right-clicking). We set the QTableWidget's context menu policy to Qt::ActionsContextMenu, meaning that the QActions associated with the widget should appear in its context menu.

Then we add the Add Image and Remove All Images actions to the table widget. They will then appear in the table widget's context menu.

 void MainWindow::checkCurrentStyle()
 {
     foreach (QAction *action, styleActionGroup->actions()) {
         QString styleName = action->data().toString();
         QStyle *candidate = QStyleFactory::create(styleName);
         Q_ASSERT(candidate);
         if (candidate->metaObject()->className()
                 == QApplication::style()->metaObject()->className()) {
             action->trigger();
             return;
         }
         delete candidate;
     }
 }

In the checkCurrentStyle() function we go through the group of style actions, looking for the current GUI style.

For each action, we first extract the style name using QAction::data(). Since this is only a QStyleFactory key (e.g., "macintosh"), we cannot compare it directly to the current style's class name. We need to create a QStyle object using the static QStyleFactory::create() function and compare the class name of the created QStyle object with that of the current style. As soon as we are done with a QStyle candidate, we delete it.

For all QObject subclasses that use the Q_OBJECT macro, the class name of an object is available through its meta-object.

We can assume that the style is supported by QStyleFactory, but to be on the safe side we use the Q_ASSERT() macro to make sure that QStyleFactory::create() returned a valid pointer.

IconSizeSpinBox Class Definition

 class IconSizeSpinBox : public QSpinBox
 {
     Q_OBJECT

 public:
     IconSizeSpinBox(QWidget *parent = 0);

     int valueFromText(const QString &text) const;
     QString textFromValue(int value) const;
 };

The IconSizeSpinBox class is a subclass of QSpinBox. A plain QSpinBox can only handle integers. But since we want to display the spin box's values in a more sophisticated way, we need to subclass QSpinBox and reimplement the QSpinBox::textFromValue() and QSpinBox::valueFromText() functions.

IconSizeSpinBox Class Implementation

 IconSizeSpinBox::IconSizeSpinBox(QWidget *parent)
     : QSpinBox(parent)
 {
 }

The constructor is trivial.

 QString IconSizeSpinBox::textFromValue(int value) const
 {
     return tr("%1 x %1").arg(value);
 }

QSpinBox::textFromValue() is used by the spin box whenever it needs to display a value. The default implementation returns a base 10 representation of the value parameter.

Our reimplementation returns a QString of the form "32 x 32".

 int IconSizeSpinBox::valueFromText(const QString &text) const
 {
     QRegExp regExp(tr("(\\d+)(\\s*[xx]\\s*\\d+)?"));

     if (regExp.exactMatch(text)) {
         return regExp.cap(1).toInt();
     } else {
         return 0;
     }
 }

The QSpinBox::valueFromText() function is used by the spin box whenever it needs to interpret text typed in by the user. Since we reimplement the textFromValue() function we also need to reimplement the valueFromText() function to interpret the parameter text and return the associated int value.

We parse the text using a regular expression (a QRegExp). We define an expression that matches one or several digits, optionally followed by whitespace, an "x" or the times symbol, whitespace and one or several digits again.

The first digits of the regular expression are captured using parentheses. This enables us to use the QRegExp::cap() or QRegExp::capturedTexts() functions to extract the matched characters. If the first and second numbers of the spin box value differ (e.g., "16 x 24"), we use the first number.

When the user presses Enter, QSpinBox first calls QSpinBox::valueFromText() to interpret the text typed by the user, then QSpinBox::textFromValue() to present it in a canonical format (e.g., "16 x 16").

ImageDelegate Class Definition

 class ImageDelegate : public QItemDelegate
 {
     Q_OBJECT

 public:
     ImageDelegate(QObject *parent = 0);

The ImageDelegate class is a subclass of QItemDelegate. The QItemDelegate class provides display and editing facilities for data items from a model. A single QItemDelegate object is responsible for all items displayed in a item view (in our case, a QTableWidget).

A QItemDelegate can be used to provide an editor for an item view class that is subclassed from QAbstractItemView. Using a delegate for this purpose allows the editing mechanism to be customized and developed independently from the model and view.

     QWidget *createEditor(QWidget *parent, const QStyleOptionViewItem &option,
                           const QModelIndex &index) const;
     void setEditorData(QWidget *editor, const QModelIndex &index) const;
     void setModelData(QWidget *editor, QAbstractItemModel *model,
                       const QModelIndex &index) const;

The default implementation of QItemDelegate creates a QLineEdit. Since we want the editor to be a QComboBox, we need to subclass QItemDelegate and reimplement the QItemDelegate::createEditor(), QItemDelegate::setEditorData() and QItemDelegate::setModelData() functions.

 private slots:
     void emitCommitData();
 };

The emitCommitData() slot is used to emit the QImageDelegate::commitData() signal with the appropriate argument.

ImageDelegate Class Implementation

 ImageDelegate::ImageDelegate(QObject *parent)
     : QItemDelegate(parent)
 {
 }

The constructor is trivial.

 QWidget *ImageDelegate::createEditor(QWidget *parent,
                                      const QStyleOptionViewItem & /* option */,
                                      const QModelIndex &index) const
 {
     QComboBox *comboBox = new QComboBox(parent);
     if (index.column() == 1) {
         comboBox->addItem(tr("Normal"));
         comboBox->addItem(tr("Active"));
         comboBox->addItem(tr("Disabled"));
         comboBox->addItem(tr("Selected"));
     } else if (index.column() == 2) {
         comboBox->addItem(tr("Off"));
         comboBox->addItem(tr("On"));
     }

     connect(comboBox, SIGNAL(activated(int)), this, SLOT(emitCommitData()));

     return comboBox;
 }

The default QItemDelegate::createEditor() implementation returns the widget used to edit the item specified by the model and item index for editing. The parent widget and style option are used to control the appearance of the editor widget.

Our reimplementation create and populate a combobox instead of the default line edit. The contents of the combobox depends on the column in the table for which the editor is requested. Column 1 contains the QIcon modes, whereas column 2 contains the QIcon states.

In addition, we connect the combobox's activated() signal to the emitCommitData() slot to emit the QAbstractItemDelegate::commitData() signal whenever the user chooses an item using the combobox. This ensures that the rest of the application notices the change and updates itself.

 void ImageDelegate::setEditorData(QWidget *editor,
                                   const QModelIndex &index) const
 {
     QComboBox *comboBox = qobject_cast<QComboBox *>(editor);
     if (!comboBox)
         return;

     int pos = comboBox->findText(index.model()->data(index).toString(),
                                  Qt::MatchExactly);
     comboBox->setCurrentIndex(pos);
 }

The QItemDelegate::setEditorData() function is used by QTableWidget to transfer data from a QTableWidgetItem to the editor. The data is stored as a string; we use QComboBox::findText() to locate it in the combobox.

Delegates work in terms of models, not items. This makes it possible to use them with any item view class (e.g., QListView, QListWidget, QTreeView, etc.). The transition between model and items is done implicitly by QTableWidget; we don't need to worry about it.

 void ImageDelegate::setModelData(QWidget *editor, QAbstractItemModel *model,
                                  const QModelIndex &index) const
 {
     QComboBox *comboBox = qobject_cast<QComboBox *>(editor);
     if (!comboBox)
         return;

     model->setData(index, comboBox->currentText());
 }

The QItemDelegate::setEditorData() function is used by QTableWidget to transfer data back from the editor to the QTableWidgetItem.

 void ImageDelegate::emitCommitData()
 {
     emit commitData(qobject_cast<QWidget *>(sender()));
 }

The emitCommitData() slot simply emit the QAbstractItemDelegate::commitData() signal for the editor that triggered the slot. This signal must be emitted when the editor widget has completed editing the data, and wants to write it back into the model.