Menus Example

MainWindow Class Implementation

In the constructor, we start off by creating a regular QWidget and make it our main window's central widget. Note that the main window takes ownership of the widget pointer and deletes it at the appropriate time.

 MainWindow::MainWindow()
 {
     QWidget *widget = new QWidget;
     setCentralWidget(widget);

     QWidget *topFiller = new QWidget;
     topFiller->setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);

     infoLabel = new QLabel(tr("<i>Choose a menu option, or right-click to "
                               "invoke a context menu</i>"));
     infoLabel->setFrameStyle(QFrame::StyledPanel | QFrame::Sunken);
     infoLabel->setAlignment(Qt::AlignCenter);

     QWidget *bottomFiller = new QWidget;
     bottomFiller->setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);

     QVBoxLayout *layout = new QVBoxLayout;
     layout->setMargin(5);
     layout->addWidget(topFiller);
     layout->addWidget(infoLabel);
     layout->addWidget(bottomFiller);
     widget->setLayout(layout);

Then we create the information label as well as a top and bottom filler that we add to a layout which we install on the central widget. QMainWindow objects come with their own customized layout and setting a layout on a the actual main window, or creating a layout with a main window as a parent, is considered an error. You should always set your own layout on the central widget instead.

     createActions();
     createMenus();

     QString message = tr("A context menu is available by right-clicking");
     statusBar()->showMessage(message);

     setWindowTitle(tr("Menus"));
     setMinimumSize(160, 160);
     resize(480, 320);
 }

To create the actions and menus we call our two convenience functions: createActions() and createMenus(). We will get back to these shortly.

QMainWindow's statusBar() function returns the status bar for the main window (if the status bar does not exist, this function will create and return an empty status bar). We initialize the status bar and window title, resize the window to an appropriate size as well as ensure that the main window cannot be resized to a smaller size than the given one.

Now, let's take a closer look at the createActions() convenience function that creates the various actions:

 void MainWindow::createActions()
 {
     newAct = new QAction(tr("&New"), this);
     newAct->setShortcut(tr("Ctrl+N"));
     newAct->setStatusTip(tr("Create a new file"));
     connect(newAct, SIGNAL(triggered()), this, SLOT(newFile()));
     ...

A QAction object may contain an icon, a text, a shortcut, a status tip, a "What's This?" text, and a tooltip. Most of these can be set in the constructor, but they can also be set independently using the provided convenience functions.

In the createActions() function, we first create a newAct action. We make Ctrl+N its shortcut using the QAction::setShortcut() function, and we set its status tip using the QAction::setStatusTip() function (the status tip is displayed on all status bars provided by the action's top-level parent widget). We also connect its triggered() signal to the newFile() slot.

The rest of the actions are created in a similar manner. Please see the source code for details.

     alignmentGroup = new QActionGroup(this);
     alignmentGroup->addAction(leftAlignAct);
     alignmentGroup->addAction(rightAlignAct);
     alignmentGroup->addAction(justifyAct);
     alignmentGroup->addAction(centerAct);
     leftAlignAct->setChecked(true);
 }

Once we have created the Left Align, Right Align, Justify, and a Center actions, we can also create the previously mentioned action group.

Each action is added to the group using QActionGroup's addAction() function. Note that an action also can be added to a group by creating it with the group as its parent. Since an action group is exclusive by default, only one of the actions in the group is checked at any one time (this can be altered using the QActionGroup::setExclusive() function).

When all the actions are created, we use the createMenus() function to add the actions to the menus and to insert the menus into the menu bar:

 void MainWindow::createMenus()
 {
     fileMenu = menuBar()->addMenu(tr("&File"));
     fileMenu->addAction(newAct);
     fileMenu->addAction(openAct);
     fileMenu->addAction(saveAct);
     fileMenu->addAction(printAct);
     fileMenu->addSeparator();
     fileMenu->addAction(exitAct);

     editMenu = menuBar()->addMenu(tr("&Edit"));
     editMenu->addAction(undoAct);
     editMenu->addAction(redoAct);
     editMenu->addSeparator();
     editMenu->addAction(cutAct);
     editMenu->addAction(copyAct);
     editMenu->addAction(pasteAct);
     editMenu->addSeparator();

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

QMenuBar's addMenu() function appends a new QMenu with the given title, to the menu bar (note that the menu bar takes ownership of the menu). We use QWidget's addAction() function to add each action to the corresponding menu.

Alternatively, the QMenu class provides several addAction() convenience functions that create and add new actions from given texts and/or icons. You can also provide a member that will automatically connect to the new action's triggered() signal, and a shortcut represented by a QKeySequence instance.

The QMenu::addSeparator() function creates and returns a new separator action, i.e. an action for which QAction::isSeparator() returns true, and adds the new action to the menu's list of actions.

     formatMenu = editMenu->addMenu(tr("&Format"));
     formatMenu->addAction(boldAct);
     formatMenu->addAction(italicAct);
     formatMenu->addSeparator()->setText(tr("Alignment"));
     formatMenu->addAction(leftAlignAct);
     formatMenu->addAction(rightAlignAct);
     formatMenu->addAction(justifyAct);
     formatMenu->addAction(centerAct);
     formatMenu->addSeparator();
     formatMenu->addAction(setLineSpacingAct);
     formatMenu->addAction(setParagraphSpacingAct);
 }

Note the Format menu. First of all, it is added as a submenu to the Edit Menu using QMenu's addMenu() function. Secondly, take a look at the alignment actions: In the createActions() function we added the leftAlignAct, rightAlignAct, justifyAct and centerAct actions to an action group. Nevertheless, we must add each action to the menu separately while the action group does its magic behind the scene.

 void MainWindow::contextMenuEvent(QContextMenuEvent *event)
 {
     QMenu menu(this);
     menu.addAction(cutAct);
     menu.addAction(copyAct);
     menu.addAction(pasteAct);
     menu.exec(event->globalPos());
 }

To provide a custom context menu, we must reimplement QWidget's contextMenuEvent() function to receive the widget's context menu events (note that the default implementation simply ignores these events).

Whenever we receive such an event, we create a menu containing the Cut, Copy and Paste actions. Context menus can be executed either asynchronously using the popup() function or synchronously using the exec() function. In this example, we have chosen to show the menu using its exec() function. By passing the event's position as argument we ensure that the context menu appears at the expected position.