source: src/Patterns/Observer.cpp@ f8879d

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

#include "Observer.hpp"


#include <iostream>

using namespace std;

/****************** Static stuff for the observer mechanism ************/

// All infrastructure for the observer-pattern is bundled at a central place
// this is more efficient if many objects can be observed (inherit from observable)
// but only few are actually coupled with observers. E.g. TMV has over 500.000 Atoms,
// which might become observable. Handling Observerable infrastructure in each of
// these would use memory for each atom. By handling Observer-infrastructure
// here we only need memory for objects that actually are observed.
// See [Gamma et al, 1995] p. 297

map<Observable*, int> Observable::depth;
multimap<Observable*,Observer*> Observable::callTable;
set<Observable*> Observable::busyObservables;

// The two functions start_observer_internal and finish_observer_internal
// have to be used together at all time. Never use these functions directly
// START_OBSERVER and FINISH_OBSERVER also construct a bogus while(0) loop
// thus producing compiler-errors whenever only one is used

void Observable::start_observer_internal(Observable *publisher){
  // increase the count for this observable by one
  // if no entry for this observable is found, an new one is created
  // by the STL and initialized to 0 (see STL documentation)
  depth[publisher]++;
}

void Observable::finish_observer_internal(Observable *publisher){
  // decrease the count for this observable
  // if zero is reached all observed blocks are done and we can
  // start to notify our observers
  if(--(depth[publisher])){}
  else{
    publisher->notifyAll();
    // this item is done, so we don't have to keep the count with us
    // save some memory by erasing it
    depth.erase(publisher);
  }
}

/************* Notification mechanism for observables **************/

typedef multimap<Observable*,Observer*>::iterator callIterator;
typedef pair<callIterator, callIterator> iteratorRange;

void Observable::notifyAll() {
  // we are busy notifying others right now
  // add ourselves to the list of busy subjects to enable circle detection
  busyObservables.insert(this);
  // see if anyone has signed up for observation
  // and call all observers
  if(callTable.count(this)) {
     iteratorRange callerRange = callTable.equal_range(this);
     callIterator iter;
     for(iter=callerRange.first;iter!=callerRange.second;iter++){
       (*iter).second->update(this);
     }
  }
  // done with notification, we can leave the set of busy subjects
  busyObservables.erase(this);
}

// this handles passing on updates from sub-Observables
void Observable::update(Observable *publisher) {
  // circle detection
  if(busyObservables.find(this)!=busyObservables.end()) {
    // somehow a circle was introduced... we were busy notifying our
    // observers, but still we are called by one of our sub-Observables
    // we cannot be sure observation will still work at this point
    cerr << "Circle detected in observation-graph." << endl;
    cerr << "Observation-graph always needs to be a DAG to work correctly!" << endl;
    cerr << "Please check your observation code and fix this!" << endl;
    return;
  }
  else {
    // see if we are in the process of changing ourselves
    // if we are changing ourselves at the same time our sub-observables change
    // we do not need to publish all the changes at each time we are called
    if(depth.find(this)==depth.end()) {
      notifyAll();
    }
  }
}

// methods to sign-on and off
void Observable::signOn(Observer *target) {
  bool res = false;
  iteratorRange callerRange = callTable.equal_range(this);
  callIterator iter;
  for(iter=callerRange.first;iter!=callerRange.second;iter++){
    res |= ((*iter).second == target);
  }
  if(!res)
    callTable.insert(pair<Observable*,Observer*>(this,target));
}

void Observable::signOff(Observer *target) {
  iteratorRange callerRange = callTable.equal_range(this);
  callIterator iter;
  for(iter=callerRange.first;iter!=callerRange.second;iter++) {
    if((*iter).second == target)
      callTable.erase(iter);
  }
}

// when an sub-observerable dies we usually don't need to do anything
void Observable::subjectKilled(Observable *publisher){
}

Observable::Observable()
{}

// when an observable is deleted, we let all our observers know
Observable::~Observable()
{
  // delete all entries for this observable
  iteratorRange callerRange = callTable.equal_range(this);
  callIterator iter;
  for(iter=callerRange.first;iter!=callerRange.second;iter++){
    (*iter).second->subjectKilled(this);
  }
  callTable.erase(callerRange.first,callerRange.second);
}

Observer::Observer()
{}

Observer::~Observer()
{}