From NAMIC Wiki
Jump to: navigation, search
Home < Slicer3:EventBroker


The basic idea of the EventBroker is that currently we have a lot of this kind of code in GUIs:

node->AddObserver(vtkCommand::ModifiedEvent, callbackCommand)


The problems with this:

  • node 'owns' the observer, but the callbackCommand is opaque so it doesn't know anything about what will happen when the event is invoked
  • the GUI needs to explicitly remove the observer before it is destroyed
  • node is not introspectable; you cannot get a list of observers on the node
  • there's no easy way to know what side effects will happen for any Set call (either a priori or experimentally).
  • there's no way to collapse events or disable them

Goals for Solution

So the EventBroker would be a singleton, perhaps owned by the MRML Scene that would look something like:

broker->AddObservation(node, vtkCommand::ModifiedEvent, this, callbackCommand);

The broker would do the following:

  • add DeleteEvent observers to both node and this so it can remove the observer automatically if either side is destroyed
  • keep an introspectable list of all observers it knows about
  • have an option to keep a log of all event invocations for debugging and performance analysis
  • have an option to turn off all event invocations
  • have an option to queue all event invocations and invoke them later
  • have methods to collapse redundant events in the queue
  • perhaps have method to pass event invocations from a processing thread to the main GUI thread?
  • the callbackCommand could be avoided if the vtkObject had a virtual method like this:
 virtual void HandleEvent( vtkObject *caller, unsigned long event, void *clientData, void *callData );
  • vtkEventBroker can have a class static GetInstance() method returning pointer to the global broker (like the way vtkSlicerApplication is done).

Additional possible extensions:

  • rather than maintaining a distinct queue, the broker could queue events into the GUI event queue
  • the event queue could be protected by a mutex lock so that multiple threads can access the MRML scene in parallel but only the GUI thread talks to the display


Dependency Graphs

EventBroker code should have the option to put out log files that are compatible with graphviz .dot file format. This can be rendered with a variety of programs, or even pasted directly in the wiki:

<graphviz border='frame' format='svg'> digraph G {

vtkImageViewer -> vtkImageEllipsoidSource[ label = ModifiedEvent ]