[vtk-developers] [VTK 0012044]: Order of called observers

[Mantis Bug Tracker]

The following issue has been SUBMITTED. 
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http://public.kitware.com/Bug/view.php?id=12044 
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Reported By:                Julien Finet
Assigned To:                
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Project:                    VTK
Issue ID:                   12044
Category:                   (No Category)
Reproducibility:            always
Severity:                   major
Priority:                   normal
Status:                     new
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Date Submitted:             2011-04-04 14:15 EDT
Last Modified:              2011-04-04 14:15 EDT
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Summary:                    Order of called observers
Description: 
Nightly build documentation of vtkObject::AddObserver() says: "When events are
invoked, the observers are called in the order they were added."

However the first observer of any given priority (default 0. included) is always
called last.
For observers added in the following order:
observer1 
observer2
observer3
observer4

The actual calling order is:
observer2
observer3
observer4
observer1  <- notice how the first observer is called last
 

Steps to Reproduce: 
void objectModified1(vtkObject *caller,unsigned long eid, void *clientData, void
*callData) {
  std::cout << "modified1" << std::endl;
}

void objectModified2(vtkObject *caller,unsigned long eid, void *clientData, void
*callData) {
  std::cout << "modified2" << std::endl;
}

void objectModified3(vtkObject *caller,unsigned long eid, void *clientData, void
*callData) {
  std::cout << "modified3" << std::endl;
}

void objectModified4(vtkObject *caller,unsigned long eid, void *clientData, void
*callData) {
  std::cout << "modified4" << std::endl;
}

int Test(int,char *[])
{
  vtkObject* obj = vtkObject::New();

  vtkCallbackCommand* callback1 = vtkCallbackCommand::New();
  callback1->SetCallback(objectModified1);
  obj->AddObserver(vtkCommand::ModifiedEvent, callback1);

  vtkCallbackCommand* callback2 = vtkCallbackCommand::New();
  callback2->SetCallback(objectModified2);
  obj->AddObserver(vtkCommand::ModifiedEvent, callback2);

  vtkCallbackCommand* callback3 = vtkCallbackCommand::New();
  callback3->SetCallback(objectModified3);
  obj->AddObserver(vtkCommand::ModifiedEvent, callback3);

  vtkCallbackCommand* callback4 = vtkCallbackCommand::New();
  callback4->SetCallback(objectModified4);
  obj->AddObserver(vtkCommand::ModifiedEvent, callback4);

  obj->Modified();

  callback4->Delete();
  callback3->Delete();
  callback2->Delete();
  callback1->Delete();

  obj->Delete();

  return 1;
}

Output:
modified2
modified3
modified4
modified1


Additional Information: 
Current code (and suggested fix):
//----------------------------------------------------------------------------
unsigned long vtkSubjectHelper::
AddObserver(unsigned long event, vtkCommand *cmd, float p)
{
  vtkObserver *elem;

  // initialize the new observer element
  elem = new vtkObserver;
  elem->Priority = p;
  elem->Next = NULL;
  elem->Event = event;
  elem->Command = cmd;
  cmd->Register(0);
  elem->Tag = this->Count;
  this->Count++;

  // now insert into the list
  // if no other elements in the list then this is Start
  if (!this->Start)
    {
    this->Start = elem;
    }
  else
    {
    // insert high priority first
    vtkObserver* prev = 0;
    vtkObserver* pos = this->Start;
    while(pos->Priority >= elem->Priority && pos->Next)
      {
      prev = pos;
      pos = pos->Next;
      }
    // pos is Start and elem should not be start
    if(pos->Priority > elem->Priority)   ////////////////////////// HERE IT
SHOULD BE: pos->Priority >= elem->Priority
      {
      pos->Next = elem;
      }
    else
      {
      if(prev)
        {
        prev->Next = elem;
        }
      elem->Next = pos;
      // check to see if the new element is the start
      if(pos == this->Start)
        {
        this->Start = elem;
        }
      }
    }
  return elem->Tag;
}
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Issue History 
Date Modified    Username       Field                    Change               
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2011-04-04 14:15 Julien Finet   New Issue                                    
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