source: trunk/doc/src/examples/undoframework.qdoc@ 1168

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/*!
    \example tools/undoframework 
    \title Undo Framework Example

    This example shows how to implement undo/redo functionality 
    with the Qt undo framework.

    \image undoframeworkexample.png The Undo Diagram Example

    In the Qt undo framework, all actions that the user performs are
    implemented in classes that inherit QUndoCommand. An undo command
    class knows how to both \l{QUndoCommand::}{redo()} - or just do
    the first time - and \l{QUndoCommand::}{undo()} an action. For
    each action the user performs, a command is placed on a
    QUndoStack.  Since the stack contains all commands executed
    (stacked in chronological order) on the document, it can roll the
    state of the document backwards and forwards by undoing and redoing
    its commands. See the \l{Overview of Qt's Undo Framework}{overview
    document} for a high-level introduction to the undo framework.

    The undo example implements a simple diagram application. It is
    possible to add and delete items, which are either box or
    rectangular shaped, and move the items by dragging them with the
    mouse. The undo stack is shown in a QUndoView, which is a list in
    which the commands are shown as list items. Undo and redo are
    available through the edit menu. The user can also select a command
    from the undo view.

    We use the \l{Graphics View Framework}{graphics view
    framework} to implement the diagram. We only treat the related
    code briefly as the framework has examples of its own (e.g., the
    \l{Diagram Scene Example}).

    The example consists of the following classes:

    \list 
        \o \c MainWindow is the main window and arranges the
              example's widgets. It creates the commands based 
              on user input and keeps them on the command stack.
        \o \c AddCommand adds an item to the scene.
        \o \c DeleteCommand deletes an item from the scene.
        \o \c MoveCommand when an item is moved the MoveCommand keeps record
              of the start and stop positions of the move, and it
              moves the item according to these when \c redo() and \c undo()
              is called.
        \o \c DiagramScene inherits QGraphicsScene and
              emits signals for the \c MoveComands when an item is moved.
        \o \c DiagramItem inherits QGraphicsPolygonItem and represents
              an item in the diagram.
    \endlist

    \section1 MainWindow Class Definition

    \snippet examples/tools/undoframework/mainwindow.h 0

    The \c MainWindow class maintains the undo stack, i.e., it creates
    \l{QUndoCommand}s and pushes and pops them from the stack when it
    receives the \c triggered() signal from \c undoAction and \c
    redoAction.
    
    \section1 MainWindow Class Implementation

    We will start with a look at the constructor:    

    \snippet examples/tools/undoframework/mainwindow.cpp 0

    In the constructor, we set up the DiagramScene and QGraphicsView. 

    Here is the \c createUndoView() function:    

    \snippet examples/tools/undoframework/mainwindow.cpp 1

    The QUndoView is a widget that display the text, which is set with
    the \l{QUndoCommand::}{setText()} function, for each QUndoCommand
    in the undo stack in a list.
    
    Here is the \c createActions() function:

    \snippet examples/tools/undoframework/mainwindow.cpp 2
    \codeline
    \snippet examples/tools/undoframework/mainwindow.cpp 3
    \dots
    \snippet examples/tools/undoframework/mainwindow.cpp 5

    The \c createActions() function sets up all the examples actions
    in the manner shown above. The
    \l{QUndoStack::}{createUndoAction()} and
    \l{QUndoStack::}{createRedoAction()} helps us crate actions that
    are disabled and enabled based on the state of the stack.  Also,
    the text of the action will be updated automatically based on the
    \l{QUndoCommand::}{text()} of the undo commands. For the other
    actions we have implemented slots in the \c MainWindow class.

    Here is the \c createMenus() function:

    \snippet examples/tools/undoframework/mainwindow.cpp 6

    \dots
    \snippet examples/tools/undoframework/mainwindow.cpp 7
    \dots
    \snippet examples/tools/undoframework/mainwindow.cpp 8

    We have to use the QMenu \c aboutToShow() and \c aboutToHide()
    signals since we only want \c deleteAction to be enabled when we
    have selected an item.     

    Here is the \c itemMoved() slot:

    \snippet examples/tools/undoframework/mainwindow.cpp 9

    We simply push a MoveCommand on the stack, which calls \c redo()
    on it.

    Here is the \c deleteItem() slot:

    \snippet examples/tools/undoframework/mainwindow.cpp 10

    An item must be selected to be deleted.  We need to check if it is
    selected as the \c deleteAction may be enabled even if an item is
    not selected. This can happen as we do not catch a signal or event
    when an item is selected.

    Here is the \c itemMenuAboutToShow() and itemMenuAboutToHide() slots: