10.1. QActions, QMenus, and QMenuBars

10.1. `QAction`s, `QMenu`s, and `QMenuBar`s
	Chapter 10. MainWindows and Actions

10.1. `QAction`s, `QMenu`s, and `QMenuBar`s

[ fromfile: menus.xml id: menus ]

QAction, derived from QObject, is a base class for user-selected actions.
The QWidget interface enables each widget to maintain a QList<QAction*>.

All QWidgets can have QActions.
Some widgets provide the list of QActions via a context menu, others via menu bar.

A QMenu is a QWidget that provides a particular kind of view for a collection of QActions.
A QMenuBar is a collection of menus, usually found in a QMainWindow.

When the parent of a QMenu is a QMenuBar, the QMenu appears as a pull-down menu with a familiar interface.
When its parent is not a QMenuBar it can pop up, like a dialog, in which case it is considered a context menu^[48].
A QMenu can have another QMenu as its parent, in which case it becomes a submenu.

To help the user make the right choice, each QAction can have the following:
- Text and/or icon that appears on a menu and/or button
- An accelerator, or a shortcut key
- A "What's this?" and a tool-tip
- A way to toggle the state of the action between visible/invisible, enabled/disabled, and checked/not checked
- changed(), hovered(), toggled(), and triggered() signals

The function QMainWindow::menuBar() returns a pointer to the QMenuBar which is a child of the QMainWindow.
The menuBar() function creates and returns a pointer to an empty QMenuBar child if the menu bar does not already exist.

Example 10.2. src/widgets/dialogs/messagebox/dialogs.cpp

[ . . . . ]
    
    /* Insert a menu into the menubar. */
    QMenu* menu = new QMenu(tr("&Questions"), this);

    QMainWindow::menuBar()->addMenu(menu);
    
    /* Add some choices to the menu. */
    menu->addAction(tr("&Ask question"),
                    this, SLOT(askQuestion()), tr("Alt+A"));
    menu->addAction(tr("Ask a &dumb question"),
                    this, SLOT(askDumbQuestion()), tr("Alt+D"));
}

Each call to QMenu::addAction(text, target, slot, shortcut) creates an unnamed QAction and adds it to the QMenu.
It then calls its hidden base class function, QWidget::addAction(QAction*), which adds the newly created QAction to the list of QActions of the QMenu for use in a context menu.

^[48] A context menu is usually activated by clicking the right mouse button or by pressing the menu button. It is called a context menu because the menu always depends on the context (which QWidget or item is currently selected or focused).


Chapter 10. MainWindows and Actions		10.1.1. `QAction`s, `QToolbar`s, and `QActionGroup`s