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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{The 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:

    \snippet examples/tools/undoframework/mainwindow.cpp 11

    \codeline

    \snippet examples/tools/undoframework/mainwindow.cpp 12

    We implement \c itemMenuAboutToShow() and \c itemMenuAboutToHide()

    to get a dynamic item menu. These slots are connected to the 

    \l{QMenu::}{aboutToShow()} and \l{QMenu::}{aboutToHide()} signals.

    We need this to disable or enable the \c deleteAction. 

    Here is the \c addBox() slot:

    \snippet examples/tools/undoframework/mainwindow.cpp 13

    The \c addBox() function creates an AddCommand and pushes it on

    the undo stack.

    Here is the \c addTriangle() sot:

    \snippet examples/tools/undoframework/mainwindow.cpp 14

    The \c addTriangle() function creates an AddCommand and pushes it

    on the undo stack.    

    Here is the implementation of \c about():

    \snippet examples/tools/undoframework/mainwindow.cpp 15

    The about slot is triggered by the \c aboutAction and displays an

    about box for the example.

    \section1 AddCommand Class Definition

    \snippet examples/tools/undoframework/commands.h 2

    The \c AddCommand class adds DiagramItem graphics items to the

    DiagramScene.  

    \section1 AddCommand Class Implementation

    We start with the constructor:

    \snippet examples/tools/undoframework/commands.cpp 7

    We first create the DiagramItem to add to the DiagramScene.  The

    \l{QUndoCommand::}{setText()} function let us set a QString that

    describes the command. We use this to get custom messages in the

    QUndoView and in the menu of the main window.

    \snippet examples/tools/undoframework/commands.cpp 8

    \c undo() removes the item from the scene. We need to update the

    scene as ...(ask Andreas)

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

    We set the position of the item as we do not do this in the

    constructor.

    \section1 DeleteCommand Class Definition

    \snippet examples/tools/undoframework/commands.h 1

    The DeleteCommand class implements the functionality to remove an

    item from the scene.

    \section1 DeleteCommand Class Implementation

    \snippet examples/tools/undoframework/commands.cpp 4

    We know that there must be one selected item as it is not possible

    to create a DeleteCommand unless the item to be deleted is

    selected and that only one item can be selected at any time. 

    The item must be unselected if it is inserted back into the 

    scene.

    \snippet examples/tools/undoframework/commands.cpp 5

    The item is simply reinserted into the scene.

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

    The item is removed from the scene.

    \section1 MoveCommand Class Definition

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

    The \l{QUndoCommand::}{mergeWith()} is reimplemented to make

    consecutive moves of an item one MoveCommand, i.e, the item will

    be moved back to the start position of the first move.

    \section1 MoveCommand Class Implementation

    The constructor of MoveCommand looks like this:

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

    We save both the old and new positions for undo and redo

    respectively.

    \snippet examples/tools/undoframework/commands.cpp 2

    We simply set the items old position and update the scene.    

    \snippet examples/tools/undoframework/commands.cpp 3

    We set the item to its new position.

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

    Whenever a MoveCommand is created, this function is called to

    check if it should be merged with the previous command. It is the

    previous command object that is kept on the stack. The function

    returns true if the command is merged; otherwise false.

    We first check whether it is the same item that has been moved

    twice, in which case we merge the commands. We update the position

    of the item so that it will take the last position in the move

    sequence when undone. 

    \section1 DiagramScene Class Definition

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

    The DiagramScene implements the functionality to move a

    DiagramItem with the mouse. It emits a signal when a move is

    completed. This is caught by the \c MainWindow, which makes

    MoveCommands. We do not examine the implementation of DiagramScene

    as it only deals with graphics framework issues.

    \section1 The \c main() Function

    The \c main() function of the program looks like this:

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

    We draw a grid in the background of the DiagramScene, so we use a

    resource file. The rest of the function creates the \c MainWindow and

    shows it as a top level window.

*/