Slicer3:EventBroker

Currently

The basic idea of the EventBroker is to avoid the following kind of code that shows up in GUIs and other classes that depend on events generated by other classes:

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 (all data and methods are private)

• 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

EventBroker Solution

The EventBroker is a singleton, available as:

vtkEventBroker *broker = vtkEventBroker::GetInstance();

with the broker, you can make a call like the following:

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

where node is the subject of the observation.

The broker does the following:

• adds DeleteEvent observers to both node and this so it can remove the observation automatically when either side is destroyed

• keeps an introspectable list of all observers it knows about

• has an option to keep a log of all event invocations for debugging and performance analysis

• has an option to turn off all event invocations

• has an asynchronous option to queue all event invocations and invoke them later (off by default)

• has option to collapse redundant events in the queue

The broker is also available for use from tcl. For convenience in slicer there's a global variable for the broker so you can use a code snippet like:

$::slicer3::Broker AddObservation $node ModifiedEvent {puts "the node was modified"}

Another nice feature is that you can use a numerical value for the event argument, which works around the limitation in VTK's native wrapping of event types. So, if you look in the header for a widget or object and know the custom event number, you can use it as an argument:

$::slicer3::Broker AddObservation $node 1 {puts "got an AnyEvent notification from the node"}

Future options include:

• add a timer to log the amount of time taken to process each event

• specify that some observations must be handled synchronously (e.g. for progress events that shouldn't be collapsed)

See the file Libs/MRML/vtkEventBroker.h for more information on logging options and introspection features.

References

The following reference provided insight on requirements and design precedents.

• 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 if the appropriate extension were installed:

Simple Dependency

<graphviz border='frame' format='svg'>
digraph G {
 vtkImageViewer -> vtkImageEllipsoidSource[ label = ModifiedEvent ];}
</graphviz>

Subset of Slicer Dependencies

<graphviz border='frame' format='svg'>
strict digraph G {
 vtkMRMLModelDisplayNode -> vtkMRMLModelDisplayNode [ label = "ModifiedEvent" ]
; vtkMRMLScalarVolumeDisplayNode -> vtkMRMLScalarVolumeDisplayNode [ label = "ModifiedEvent" ]
; vtkMRMLLabelMapVolumeDisplayNode -> vtkMRMLLabelMapVolumeDisplayNode [ label = "ModifiedEvent" ]
; vtkMRMLDiffusionWeightedVolumeDisplayNode -> vtkMRMLDiffusionWeightedVolumeDisplayNode [ label = "ModifiedEvent" ]
; vtkMRMLDiffusionTensorVolumeDisplayNode -> vtkMRMLDiffusionTensorVolumeDisplayNode [ label = "ModifiedEvent" ]
; vtkMRMLFiberBundleLineDisplayNode -> vtkMRMLFiberBundleLineDisplayNode [ label = "ModifiedEvent" ]
; vtkMRMLFiberBundleTubeDisplayNode -> vtkMRMLFiberBundleTubeDisplayNode [ label = "ModifiedEvent" ]
; vtkMRMLFiberBundleGlyphDisplayNode -> vtkMRMLFiberBundleGlyphDisplayNode [ label = "ModifiedEvent" ]
; vtkMRMLCameraNode -> vtkOpenGLCamera [ label = "ModifiedEvent" ]
; vtkMRMLUnstructuredGridDisplayNode -> vtkMRMLUnstructuredGridDisplayNode [ label = "ModifiedEvent" ]
; vtkMRMLCameraNode -> vtkOpenGLCamera [ label = "ModifiedEvent" ]
; vtkMRMLModelDisplayNode -> vtkMRMLModelDisplayNode [ label = "ModifiedEvent" ]
; vtkMRMLCameraNode -> vtkOpenGLCamera [ label = "ModifiedEvent" ]
; vtkMRMLModelNode -> vtkPolyData [ label = "ModifiedEvent" ]
; vtkMRMLModelNode -> vtkPolyData [ label = "ModifiedEvent" ]
; vtkMRMLModelDisplayNode -> vtkMRMLModelDisplayNode [ label = "ModifiedEvent" ]
; vtkMRMLModelDisplayNode -> vtkImageData [ label = "ModifiedEvent" ]
; vtkMRMLModelNode -> vtkPolyData [ label = "ModifiedEvent" ]
; vtkMRMLModelDisplayNode -> vtkMRMLModelDisplayNode [ label = "ModifiedEvent" ]
; vtkMRMLModelDisplayNode -> vtkImageData [ label = "ModifiedEvent" ]
; vtkMRMLModelNode -> vtkPolyData [ label = "ModifiedEvent" ]
; vtkMRMLModelDisplayNode -> vtkMRMLModelDisplayNode [ label = "ModifiedEvent" ]
; vtkMRMLModelDisplayNode -> vtkImageData [ label = "ModifiedEvent" ]
;}
</graphviz>

Issues

• how to work with the vtkObserverManager?
• Discussion of Jan 27 2012:

 [...] the problem wasn't that the Event Broker was used instead of going through vtkObserverManager, but that the vtkObject::AddObserver() was used with the callback command of the observer manager (that was later deleted which made the client data of the callback dirty when the event was fired).

We should eventually review the entire "observation" mechanism, there are currently 3 ways of observing vtkObjects:
 - via the vtkObserverManager API (please everybody use that one for now)
 - via the EventBroker directly 
 - via vtkObject::AddObserver directly

All three methods are valid but inconsistent with each other. In addition, it is still not user friendly to observe MRML nodes (using static functions as callback... how to synchronize a MRML node with a VTK widget in a displayable manager ? how to observe with Python...).

• Bugs related to observers:
  • Volume Rendering (1572, 1744)
    • -> Use AddObserver with vtkObserverManager callback that is deleted before an observed event is fired, crash when trying to dereference the deleted observer.
  • Python (1656)
    • -> PyObject of the observer is deleted before an observed event is fired. crash in the callback
• Current design notes
  • For logics, vtk[SetAnd]ObserveMRMLNode[Events]Macro can only observe MRML nodes (see vtkMRMLAbstractLogic::MRMLNodesCallback)
  • For QObjects listening to vtkObjects, destruction must unlink observations (modules must call setMRMLScene(0) in destructor? )