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

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 $node ModifiedEvent {puts "$node was modified"}

Future options include:

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

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

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:

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?