source: src/Patterns/Observer.hpp@ 5b0b98

/*
 * Observer.hpp
 *
 *  Created on: Jan 19, 2010
 *      Author: crueger
 */

#ifndef OBSERVER_HPP_
#define OBSERVER_HPP_

#include <map>
#include <set>

/**
 * Basic structure for the observer pattern
 *
 * Observers register themselves with the observables to be notified when something changes.
 * In the Observable code that changes, attributes should be started with OBSERVE;. This macro
 * locks the observer mechanism while changes are done. At the end of the scope in which the
 * macro was placed the lock is released. When the last lock is released all changes are
 * propagated to the observers.
 *
 * Each observerable can have sub-observables. When one of these sub-observables changes and
 * notifies its observers the observable that contains them will also notify its observers.
 * This passing on of updates is blocked, when the main-observable is in the process of
 * updating many of its internal sub-observables. This means the update is not passed, if
 * it is produced while the main-observable itself is within any Observation block.
 */

class Observable;

/**
 * An Observer is notified by all Observed objects, when anything changes.
 *
 * If a simple change is done to an Object the Obervable will call the update() method
 * of all signed on observers, passing itself as a parameter for identification. The
 * Observers should then react to the changes and update themselves accordingly.
 *
 * If an observed Object is destroyed it will call the subjectKilled() method
 * of all signed on Observers, again passing itself as a parameter for identification.
 * The Observers should handle the destruction of an observed Object gracefully, i.e.
 * set themselves inactive, display something else, etc. There is no need
 * to sign of from the dying object, since this will be handled by the Observable destructor.
 */
class Observer
{
  friend class Observable;
public:
  Observer();
  virtual ~Observer();

protected:
  /**
   * This method is called upon changes of the Observable
   */
  virtual void update(Observable *publisher)=0;

  /**
   * This method is called when the observed object is destroyed.
   */
  virtual void subjectKilled(Observable *publisher)=0;
};

/**
 * An Observable implements all neccessary method for being observed.
 *
 * That is, it provides methods for signing on and of from an
 * Observable that can be used by any observer. The actual
 * observer-mechanism is handled at a central static place
 * to avoid memory issues when many observable are around but only few
 * are actually observed.
 */
class Observable : public Observer {
public:
  Observable();
  virtual ~Observable();

  /**
   * Sign an Observer on to this Observable. The Observer will be notified
   * whenever something inside the Observable changes. The Observer can
   * assign itself a priority for the changes in the range of -20:+20.
   * The Observer with lower priority will be called before the others,
   * same as with Unix nice-levels. This can be used when an Object
   * contains other objects that observe it (derived values), and these objects have
   * to recalculate their states before the changes should be propageted to the
   * UI. A default priority of 0 should be fine in most cases, since there is
   * ussually no need to order the update sequence.
   */
  virtual void signOn(Observer *target, int priority=0);

  /**
   * Sign of a previously signed on Observer. After this no more
   * updates will be recieved from that observer.
   */
  virtual void signOff(Observer *target);

  /**
   * Ask an Observer if it is currently in a blocked state, i.e. if
   * Changes are in Progress, that are not yet published.
   */
  virtual bool isBlocked();

protected:
  virtual void update(Observable *publisher);
  virtual void subjectKilled(Observable *publisher);

  virtual void notifyAll();
protected:
// Observer mechanism is done from a static central place
  /**
   * Internal method.
   * Do not call directly. Use OBSERVE macro instead
   */
  static void start_observer_internal(Observable *publisher);
  /**
   * Internal method.
   * Do not call directly. Use OBSERVE macro instead
   */
  static void finish_observer_internal(Observable *publisher);

private:
  typedef std::multimap<int,Observer*> callees_t;
  static std::map<Observable*, int> depth;
  static std::map<Observable*,callees_t*> callTable;
  static std::set<Observable*> busyObservables;

  //! @cond
  // Structure for RAII-Style notification
protected:
  /**
   * This structure implements the Observer-mechanism RAII-Idiom.
   * It triggers certain functions on creation and destruction so that
   * Observer mechanisms can be linked to scope block.
   */
  class _Observable_protector {
  public:
    _Observable_protector(Observable *);
    ~_Observable_protector();
  private:
    Observable *protege;
  };
  //! @endcond
};

// extra macro is necessary to work with __LINE__
#define PASTE(a,b) PASTE_HELPER(a,b)
#define PASTE_HELPER(a,b) a ## b
#define OBSERVE Observable::_Observable_protector PASTE(_scope_obs_protector_,__LINE__)(this)
// deprecated macros from before RAII was used
//#define START_OBSERVER Observable::start_observer_internal(this);do{do{}while(0)
//#define FINISH_OBSERVER }while(0);Observable::finish_observer_internal(this)
//#define RETURN_OBSERVER( retval ) do{Observable::finish_observer_internal(this); return (retval);}while(0)
#endif /* OBSERVER_HPP_ */