Difference between revisions of "Slicer3:EventBroker"

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* there's no way to collapse events or disable them
 
* there's no way to collapse events or disable them
  
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== Goals for Solution ==
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== EventBroker Solution ==
  
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So the EventBroker would be a singleton, perhaps owned by the MRML Scene that would look something like:
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The EventBroker is a singleton, available as:
  
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  broker->AddObservation(node, vtkCommand::ModifiedEvent, this, callbackCommand);
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  vtkEventBroker *broker = vtkEventBroker::GetInstance();
  
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The broker would do the following:
+
with the broker, you can make a call like the following:
  
−
* add DeleteEvent observers to both node and this so it can remove the observer automatically if either side is destroyed
+
broker->AddObservation(node, vtkCommand::ModifiedEvent, this, callbackCommand);
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* keep an introspectable list of all observers it knows about
 
−
 
 
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* have an option to keep a log of all event invocations for debugging and performance analysis
 
−
 
 
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* have an option to turn off all event invocations
 
  
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* have an option to queue all event invocations and invoke them later
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where ''node'' is the subject of the observation.
  
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* have methods to collapse redundant events in the queue
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The broker does the following:
  
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* perhaps have method to pass event invocations from a processing thread to the main GUI thread?
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* adds DeleteEvent observers to both ''node'' and ''this'' so it can remove the observation automatically when either side is destroyed
  
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* the callbackCommand could be avoided if the vtkObject had a virtual method like this:
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* keeps an introspectable list of all observers it knows about
  
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  virtual void HandleEvent( vtkObject *caller, unsigned long event, void *clientData, void *callData );
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* has an option to keep a log of all event invocations for debugging and performance analysis
  
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* vtkEventBroker can have a class static GetInstance() method returning pointer to the global broker (like the way vtkSlicerApplication is done).
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* has an option to turn off all event invocations
  
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Additional possible extensions:
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* has an ''asynchronous'' option to queue all event invocations and invoke them later (off by default)
  
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* rather than maintaining a distinct queue, the broker could queue events into the GUI event queue
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* has option to collapse redundant events in the queue
  
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* 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
+
Future options include:
  
 
* add a timer to log the amount of time taken to process each event
 
* add a timer to log the amount of time taken to process each event

Revision as of 20:24, 4 April 2008

Home < 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

Future options include:

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

References

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?
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