Slicer3:EventBroker

Currently

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)

Problems

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 );

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

References

• Java Message Service (JMS) API
• Wikipedia definition of Observer Pattern
• A C++ implementation
• Another C++ implementation
• The Qt implementation

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 ]

}

</graphviz>