/*=========================================================================

  Program:   ParaView
  Module:    vtkUndoElement.h

  Copyright (c) Kitware, Inc.
  All rights reserved.
  See Copyright.txt or http://www.paraview.org/HTML/Copyright.html for details.

     This software is distributed WITHOUT ANY WARRANTY; without even
     the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
     PURPOSE.  See the above copyright notice for more information.

=========================================================================*/
/**
 * @class   vtkUndoElement
 * @brief   unit undo-redo-able operation.
 *
 * This is an abstract class that defines the API for an undo-redo-able operation.
 * One or more vtkUndoElement objects can define
 * a single undo-redo step. Every concrete implementation of this class
 * must know how to undo as well as redo the operation, and save and load the
 * state as an XML.
 *
 * vtkUndoElement, vtkUndoSet and vtkUndoStack form the undo/redo framework core.
 * @sa
 * vtkUndoStack vtkUndoSet
*/

#ifndef vtkUndoElement_h
#define vtkUndoElement_h

#include "vtkObject.h"
#include "vtkPVVTKExtensionsCoreModule.h" // needed for export macro
class vtkCollection;

class VTKPVVTKEXTENSIONSCORE_EXPORT vtkUndoElement : public vtkObject
{
public:
  vtkTypeMacro(vtkUndoElement, vtkObject);
  void PrintSelf(ostream& os, vtkIndent indent) VTK_OVERRIDE;

  /**
   * Undo the operation encapsulated by this element.
   * \return the status of the operation, 1 on success, 0 otherwise.
   */
  virtual int Undo() = 0;

  /**
   * Redo the operation encaspsulated by this element.
   * \return the status of the operation, 1 on success, 0 otherwise.
   */
  virtual int Redo() = 0;

  //@{
  /**
   * Returns if this undo element can be merged with other
   * undo elements.
   * When an undo element is added to a vtkUndoSet unsing AddElement,
   * an attempt is made to \c "merge" the element with the
   * most recently added undo element, if any, if both the undo elements
   * are mergeable.
   */
  vtkGetMacro(Mergeable, bool);
  //@}

  /**
   * Called on the older element in the UndoSet to merge with the
   * element being added if  both the elements are \c mergeable.
   * Returns if the merge was successful.
   * Default implementation doesn't do anything.
   */
  virtual bool Merge(vtkUndoElement* vtkNotUsed(new_element)) { return false; }

  // Set the working context if run inside a UndoSet context, so object
  // that are cross referenced can leave long enough to be associated
  // to another object. Otherwise the undo of a Delete will create the object
  // again but as no-one is holding a reference to that newly created object
  // it will be automatically deleted. Therefore, we provide a collection
  // that will hold a reference during an undoset so the object has a chance
  // to be attached to the ProxyManager or any other object.
  virtual void SetUndoSetWorkingContext(vtkCollection* workCTX)
  {
    this->UndoSetWorkingContext = workCTX;
  }

protected:
  vtkUndoElement();
  ~vtkUndoElement() override;

  //@{
  /**
   * Subclasses must set this flag to enable merging of consecutive elements
   * in an UndoSet.
   */
  bool Mergeable;
  vtkSetMacro(Mergeable, bool);
  vtkCollection* UndoSetWorkingContext;
  //@}

private:
  vtkUndoElement(const vtkUndoElement&) = delete;
  void operator=(const vtkUndoElement&) = delete;
};

#endif
