source: src/Actions/Action.hpp@ 2a374e

/*
 * Action.h
 *
 *  Created on: Dec 8, 2009
 *      Author: crueger
 */

#ifndef ACTION_H_
#define ACTION_H_

#include <string>
#include <boost/shared_ptr.hpp>

// forward declaration

class ActionState;
class ActionSequence;

/**
 * @file
 * <H1> Action Howto </H1>
 *
 * <H2> Introduction </H2>
 *
 * Actions are used in object oriented design as a replacement for callback functions.
 * In most ways Actions can be used in the same way that callbacks were used in non
 * OO-Systems, but can contain support for several extra mechanism such as undo/redo
 * or progress indicators.
 *
 * The main purpose of an action class is to contain small procedures, that can be repeatedly
 * called. These procedures can also be stored, passed around, so that the execution of an
 * action can happen quite far away from the place of creation. For a detailed description of
 * the Action pattern see GOF:1996.
 *
 * <H3> How to use an action </H3>
 *
 * The process of using an action is as easy as calling the call() method of the action. The
 * action will then do whatever it is supposed to do. If it is an action that can be undone, it
 * will also register itself in the history to make itself available for undo. To undo the last
 * action, you can either use the undoLast() method inside the ActionHistory class or call the
 * UndoAction also provided by the ActionHistory. If an action was undone it will be available for
 * redo, using the redoLast() method of the ActionHistory or the RedoAction also provided by this
 * class. To check whether undo/redo is available at any moment you can use the hasUndo() or
 * hasRedo() method respectively.
 *
 * Actions can be set to be active or inactive. If an action is set to inactive it is signaling, that
 * some condition necessary for this action to be executed is not currently met. For example the
 * UndoAction will set itself to inactive, when there is no action at that time that can be undone.
 * Using call() on an inactive Action results in a no-op. You can query the state of an action using
 * the isActive() method.
 *
 * The undo capabilities of actions come in three types as signaled by two boolean flags (one
 * combination of these flags is left empty as can be seen later).
 * <ul>
 * <li/> The first flag indicates if the undo mechanism for this action should be considered at all, i.e.
 *   if the state of the application changes in a way that needs to be reverted. Actions that should
 *   consider the undo mechanism are for example adding a molecule, moving atoms, changing
 *   the name of a molecule etc. Changing the View-Area on the other hand should be an action that
 *   does not consider the undo mechanism. This flag can be queried using the shouldUndo() method.
 *
 * <li/> The second flag indicates whether the changes can be undo for this action. If this flag is true
 *   the action will be made available for undo using the ActionHistory class and the actions of this
 *   class. If this flag is false while the shoudlUndo() flag is true this means that this action
 *   changes the state of the application changes in a way that cannot be undone, but might cause
 *   the undo of previous actions to fail. In this case the whole History is cleared, as to keep
 *   the state of the application intact by avoiding dangerous undos. This flag can be queried
 *   using the canUndo() method.
 *</ul>
 *
 * Each action has a name, that can be used to identify it throughout the run of the application.
 * This name can be retrieved using the getName() method. Most actions also register themselves with
 * a global structure, called the ActionRegistry. Actions that register themselves need to have a
 * unique name for the whole application. If the name is known these actions can be retrieved from
 * the registry by their name and then be used as normal.
 *
 * <H2> Building your own actions </H2>
 *
 * Building actions is fairly easy. Simply derive from the abstract Action base class and implement
 * the virtual methods. The main code that needs to be executed upon call() should be implemented in
 * the performCall() method. You should also indicate whether the action supports undo by implementing
 * the shouldUndo() and canUndo() methods to return the appropriate flags.
 *
 * The constructor of your derived class also needs to call the Base constructor, passing it the
 * name of the Action and a flag indicating whether this action should be made available in the
 * registry. WARNING: Do not use the virtual getName() method of the derived action to provide the
 * constructor with the name, even if you overloaded this method to return a constant. Doing this
 * will most likely not do what you think it does (see: http://www.parashift.com/c++-faq-lite/strange-inheritance.html#faq-23.5
 * if you want to know why this wont work)
 *
 * <H3> Interfacing your Action with the Undo mechanism </H3>
 *
 * The performX() methods need to comply to a simple standard to allow for undo and redo. The first
 * convention in this standard concerns the return type. All methods that handle calling, undoing
 * or redoing return an object of Action::state_ptr. This is a smart pointer to a State object, that
 * can be used to store state information that is needed by your action for later redo. A rename
 * Action for example would need to store which object has been renamed and what the old name was.
 * A move Action on the other hand would need to store the object that has been moved as well as the
 * old position. If your Action does not need to store any kind of information for redo you can
 * simply return Action::success and skip the rest of this paragraph. If your action has been
 * abborted you can return Action::failure, which indicates to the history mechanism that this
 * action should not be stored.
 *
 * If your Action needs any kind of information to undo its execution, you need to store this
 * information in the state that is returned by the performCall() method. Since no assumptions
 * can be made on the type or amount of information the ActionState base class is left empty.
 * To use this class you need to derive a YourActionState class from the ActionState base class
 * adding your data fields and accessor functions. Upon undo the ActionState object produced
 * by the corresponding performCall() is then passed to the performUndo() method which should
 * typecast the ActionState to the appropriate sub class, undo all the changes and produce
 * a State object that can be used to redo the action if neccessary. This new state object is
 * then used if the redo mechanism is invoked and passed to the performRedo() function, which
 * again produces a State that can be used for performUndo().
 *
 * <H3> Outline of the implementation of Actions </H3>
 *
 * To sum up the actions necessary to build actions here is a brief outline of things methioned
 * in the last paragraphs:
 *
 * <H4> Basics </H4>
 *
 * <ul>
 *  <li/> derive YourAction from Action
 *  <li/> pass name and flag for registry to the base constructor
 *  <li/> implement performCall(), performUndo(), performRedo()
 *  <li/> implement the functions that return the flags for the undo mechanism
 *  <li/> Derive YourActionState from ActionState as necessary
 * </ul>
 *
 * <H4> Implementing performX() methods </H4>
 *
 * <ul>
 *  <li/> performCall():
 *  <ul>
 *   <li/> do whatever is needed to make the action work
 *   <li/> if the action was abborted return Action::failure
 *   <li/> if the action needs to save a state return a custom state object
 *   <li/> otherwise return Action::success
 *  </ul>
 *  <li/> performUndo():
 *  <ul>
 *   <li/> typecast the ActionState pointer to a Pointer to YourActionState if necessary
 *   <li/> undo the action using the information from the state
 *   <li/> produce a new state that can be used for redoing and return it
 *  </ul>
 *  <li/> performRedo():
 *  <ul>
 *   <li/> take the ActionState produced by performUndo and typecast it to a pointer to YourActionState if necessary
 *   <li/> redo the undone action using the information from the state
 *   <li/> produce a new state that can be used by performUndo() and return it
 *  </ul>
 * </ul>
 *
 * <H2> Advanced techniques </H2>
 *
 * <H3> Predefined Actions </H3>
 *
 * To make construction of actions easy there are some predefined actions. Namely these are
 * the MethodAction and the ErrorAction.
 *
 * The method action can be used to turn any function with empty arguments and return type void
 * into an action (also works for functors with those types). Simply pass the constructor for the
 * MethodAction a name to use for this action, the function to call inside the performCall()
 * method and a flag indicating if this action should be made retrievable inside the registry
 * (default is true). MethodActions always report themselves as changing the state of the
 * application but cannot be undone. i.e. calling MethodActions will always cause the ActionHistory
 * to be cleared.
 *
 * ErrorActions can be used to produce a short message using the Log() << Verbose() mechanism of
 * the molecuilder. Simply pass the constructor a name for the action, the message to show upon
 * calling this action and the flag for the registry (default is again true). Error action
 * report that they do not change the state of the application and are therefore not considered
 * for undo.
 *
 * <H3> Sequences of Actions and MakroActions </H3>
 *
 * <H4> Building sequences of Actions </H4>
 *
 * Actions can be chained to sequences using the ActionSequence class. Once an ActionSequence is
 * constructed it will be initially empty. Any Actions can then be added to the sequence using the
 * addAction() method of the ActionSequence class. The last added action can be removed using the
 * removeLastAction() method. If the construction of the sequence is done, you can use the
 * callAll() method. Each action called this way will register itself with the History to allow
 * separate undo of all actions in the sequence.
 *
 * <H4> Building larger Actions from simple ones </H4>
 *
 * Using the pre-defined class MakroAction it is possible to construct bigger actions from a sequence
 * of smaller ones. For this you first have to build a sequence of the actions using the ActionSequence
 * as described above. Then you can construct a MakroAction passing it a name, the sequence to use
 * and as usual a flag for the registry. You can then simply call the complete action-sequence through
 * this makro action using the normal interface. Other than with the direct use of the action sequence
 * only the complete MakroAction is registered inside the history, i.e. the complete sequence can be
 * undone at once. Also there are a few caveats you have to take care of when using the MakroAction:
 * <ul>
 *  <li/> All Actions as well as the sequence should exclusively belong to the MakroAction. This
 *        especially means, that the destruction of these objects should be handled by the MakroAction.
 *  <li/> none of the Actions inside the MakroAction should be registered with the registry, since the
 *        registry also assumes sole ownership of the actions.
 *  <li/> Do not remove or add actions from the sequence once the MakroAction has been constructed, since this
 *        might brake important assumptions for the undo/redo mechanism
 * </ul>
 *
 * <H3> Special kinds of Actions </H3>
 *
 * To make the usage of Actions more versatile there are two special kinds of actions defined,
 * that contain special mechanisms. These are defined inside the class Process, for actions that
 * take some time and indicate their own progress, and in the class Calculations for actions that
 * have a retrievable result.
 *
 * <H4> Processes </H4>
 *
 * Processes are Actions that might take some time and therefore contain special mechanisms
 * to indicate their progress to the user. If you want to implement a process you can follow the
 * guidelines for implementing actions. In addition to the normal Action constructor parameters,
 * you also need to define the number of steps the process takes to finish (use 0 if that number is
 * not known upon construction). At the beginning of your process you then simply call start() to
 * indicate that the process is taking up its work. You might also want to set the number of steps it
 * needs to finish, if it has changed since the last invocation/construction. You can use the
 * setMaxSteps() method for this. Then after each finished step of calulation simply call step(),
 * to let the indicators know that it should update itself. If the number of steps is not known
 * at the time of calculation, you should make sure the maxSteps field is set to 0, either through
 * the constructor or by using setMaxSteps(0). Indicators are required to handle both processes that
 * know the number of steps needed as well as processes that cannot predict when they will be finished.
 * Once your calculation is done call stop() to let every indicator know that the process is done with
 * the work and to let the user know.
 *
 * Indicators that want to know about processes need to implement the Observer class with all the
 * methods defined there. They can then globally sign on to all processes using the static
 * Process::AddObserver() method and remove themselves using the Process::RemoveObserver()
 * methods. When a process starts it will take care that the notification for this process
 * is invoked at the right time. Indicators should not try to observe a single process, but rather
 * be ready to observe the status of any kind of process using the methods described here.
 *
 * <H4> Calculations </H4>
 *
 * Calculations are special Actions that also return a result when called. Calculations are
 * always derived from Process, so that the progress of a calculation can be shown. Also
 * Calculations should not contain side-effects and not consider the undo mechanism.
 * When a Calculation is called using the Action mechanism this will cause it to calculate
 * the result and make it available using the getResult() method. Another way to have a Calculation
 * produce a result is by using the function-call operator. When this operator is used, the Calculation
 * will try to return a previously calculated and cached result and only do any actuall calculations
 * when no such result is available. You can delete the cached result using the reset() method.
 */

/**
 * Base class for all actions.
 *
 * Actions describe something that has to be done.
 * Actions can be passed around, stored, performed and undone (Command-Pattern).
 */
class Action
{
friend class ActionSequence;
friend class ActionHistory;
public:

  /**
   * This type is used to store pointers to ActionStates while allowing multiple ownership
   */
  typedef boost::shared_ptr<ActionState> state_ptr;

  /**
   * Standard constructor of Action Base class
   *
   * All Actions need to have a name. The second flag indicates, whether the action should
   * be registered with the ActionRegistry. If the Action is registered the name of the
   * Action needs to be unique for all Actions that are registered.
   */
  Action(std::string _name,bool _doRegister=true);
  virtual ~Action();

  /**
   * This method is used to call an action. The basic operations for the Action
   * are carried out and if necessary/possible the Action is added to the History
   * to allow for undo of this action.
   *
   * If the call needs to undone you have to use the History, to avoid destroying
   * invariants used by the History.
   */
  void call();

  /**
   * This method provides a flag that indicates if an undo mechanism is implemented
   * for this Action. If this is true, and this action was called last, you can
   * use the History to undo this action.
   */
  virtual bool canUndo()=0;

  /**
   * This method provides a flag, that indicates if the Action changes the state of
   * the application in a way that needs to be undone for the History to work.
   *
   * If this is false the Action will not be added to the History upon calling. However
   * Actions called before this one will still be available for undo.
   */
  virtual bool shouldUndo()=0;

  /**
   * Indicates whether the Action can do it's work at the moment. If this
   * is false calling the action will result in a no-op.
   */
  virtual bool isActive();

  /**
   * Returns the name of the Action.
   */
  virtual const std::string getName();

protected:
  /**
   * This method is called by the History, when an undo is performed. It is
   * provided with the corresponding state produced by the performCall or
   * performRedo method and needs to provide a state that can be used for redo.
   */
  state_ptr undo(state_ptr);

  /**
   * This method is called by the Histor, when a redo is performed. It is
   * provided with the corresponding state produced by the undo method and
   * needs to produce a State that can then be used for another undo.
   */
  state_ptr redo(state_ptr);

  /**
   * This special state can be used to indicate that the Action was successfull
   * without providing a special state. Use this if your Action does not need
   * a speciallized state.
   */
  static state_ptr success;

  /**
   * This special state can be returned, to indicate that the action could not do it's
   * work, was abborted by the user etc. If you return this state make sure to transactionize
   * your Actions and unroll the complete transaction before this is returned.
   */
  static state_ptr failure;

private:
  /**
   * This is called internally when the call is being done. Implement this method to do the actuall
   * work of the Action. Implement this in your Derived classes. Needs to return a state that can be
   * used to undo the action.
   */
  virtual state_ptr performCall()=0;

  /**
   * This is called internally when the undo process is chosen. This Method should use the state
   * produced by the performCall method to return the state of the application to the state
   * it had before the Action.
   */
  virtual state_ptr performUndo(state_ptr)=0;

  /**
   * This is called internally when the redo process is chosen. This method shoudl use the state
   * produced by the performUndo method to return the application to the state it should have after
   * the action.
   *
   * Often this method can be implement to re-use the performCall method. However if user interaction
   * or further parameters are needed, those should be taken from the state and not query the user
   * again.
   */
  virtual state_ptr performRedo(state_ptr)=0;

  std::string name;
};

/**
 * This class can be used by actions to save the state.
 *
 * It is implementing a memento pattern. The base class is completely empty,
 * since no general state internals can be given. The Action performing
 * the Undo should downcast to the apropriate type.
 */
class ActionState{
public:
  ActionState(){}
  virtual ~ActionState(){}
};

#endif /* ACTION_H_ */