commands.cpp Example File

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 #include <QtWidgets>

 #include "commands.h"
 #include "diagramitem.h"

 MoveCommand::MoveCommand(DiagramItem *diagramItem, const QPointF &oldPos,
                  QUndoCommand *parent)
     : QUndoCommand(parent)
 {
     myDiagramItem = diagramItem;
     newPos = diagramItem->pos();
     myOldPos = oldPos;
 }

 bool MoveCommand::mergeWith(const QUndoCommand *command)
 {
     const MoveCommand *moveCommand = static_cast<const MoveCommand *>(command);
     DiagramItem *item = moveCommand->myDiagramItem;

     if (myDiagramItem != item)
     return false;

     newPos = item->pos();
     setText(QObject::tr("Move %1")
         .arg(createCommandString(myDiagramItem, newPos)));

     return true;
 }

 void MoveCommand::undo()
 {
     myDiagramItem->setPos(myOldPos);
     myDiagramItem->scene()->update();
     setText(QObject::tr("Move %1")
         .arg(createCommandString(myDiagramItem, newPos)));
 }

 void MoveCommand::redo()
 {
     myDiagramItem->setPos(newPos);
     setText(QObject::tr("Move %1")
         .arg(createCommandString(myDiagramItem, newPos)));
 }

 DeleteCommand::DeleteCommand(QGraphicsScene *scene, QUndoCommand *parent)
     : QUndoCommand(parent)
 {
     myGraphicsScene = scene;
     QList<QGraphicsItem *> list = myGraphicsScene->selectedItems();
     list.first()->setSelected(false);
     myDiagramItem = static_cast<DiagramItem *>(list.first());
     setText(QObject::tr("Delete %1")
         .arg(createCommandString(myDiagramItem, myDiagramItem->pos())));
 }

 void DeleteCommand::undo()
 {
     myGraphicsScene->addItem(myDiagramItem);
     myGraphicsScene->update();
 }

 void DeleteCommand::redo()
 {
     myGraphicsScene->removeItem(myDiagramItem);
 }

 AddCommand::AddCommand(DiagramItem::DiagramType addType,
                        QGraphicsScene *scene, QUndoCommand *parent)
     : QUndoCommand(parent)
 {
     static int itemCount = 0;

     myGraphicsScene = scene;
     myDiagramItem = new DiagramItem(addType);
     initialPosition = QPointF((itemCount * 15) % int(scene->width()),
                               (itemCount * 15) % int(scene->height()));
     scene->update();
     ++itemCount;
     setText(QObject::tr("Add %1")
         .arg(createCommandString(myDiagramItem, initialPosition)));
 }

 AddCommand::~AddCommand()
 {
     if (!myDiagramItem->scene())
         delete myDiagramItem;
 }

 void AddCommand::undo()
 {
     myGraphicsScene->removeItem(myDiagramItem);
     myGraphicsScene->update();
 }

 void AddCommand::redo()
 {
     myGraphicsScene->addItem(myDiagramItem);
     myDiagramItem->setPos(initialPosition);
     myGraphicsScene->clearSelection();
     myGraphicsScene->update();
 }

 QString createCommandString(DiagramItem *item, const QPointF &pos)
 {
     return QObject::tr("%1 at (%2, %3)")
         .arg(item->diagramType() == DiagramItem::Box ? "Box" : "Triangle")
         .arg(pos.x()).arg(pos.y());
 }