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/*!
    \example tools/undoframework
    \title Undo Framework Example
    \ingroup examples-widgets-tools

    \brief 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
        \li \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.
        \li \c AddCommand adds an item to the scene.
        \li \c DeleteCommand deletes an item from the scene.
        \li \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.
        \li \c DiagramScene inherits QGraphicsScene and
              emits signals for the \c MoveComands when an item is moved.
        \li \c DiagramItem inherits QGraphicsPolygonItem and represents
              an item in the diagram.
    \endlist

    \section1 MainWindow Class Definition

    \snippet 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 tools/undoframework/mainwindow.cpp 0

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

    Here is the \c createUndoView() function:

    \snippet 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 tools/undoframework/mainwindow.cpp 2
    \codeline
    \snippet tools/undoframework/mainwindow.cpp 3
    \dots
    \snippet 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()} methods help us create 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 tools/undoframework/mainwindow.cpp 6

    \dots
    \snippet tools/undoframework/mainwindow.cpp 7
    \dots
    \snippet 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 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 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 tools/undoframework/mainwindow.cpp 11
    \codeline
    \snippet 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 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 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 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 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 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 tools/undoframework/commands.cpp 8

    \c undo() removes the item from the scene.

    \snippet 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 tools/undoframework/commands.h 1

    The DeleteCommand class implements the functionality to remove an
    item from the scene.

    \section1 DeleteCommand Class Implementation

    \snippet 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 tools/undoframework/commands.cpp 5

    The item is simply reinserted into the scene.

    \snippet tools/undoframework/commands.cpp 6

    The item is removed from the scene.

    \section1 MoveCommand Class Definition

    \snippet 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 tools/undoframework/commands.cpp 0

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

    \snippet tools/undoframework/commands.cpp 2

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

    \snippet tools/undoframework/commands.cpp 3

    We set the item to its new position.

    \snippet 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 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 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.
*/
