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| − | == Currently ==
| + | <big>'''Note:''' We are migrating this content to the slicer.org domain - <font color="orange">The newer page is [http://www.slicer.org/slicerWiki/index.php/Slicer3:EventBroker here]</font></big> |
| − | 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:
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| − | node->AddObserver(vtkCommand::ModifiedEvent, callbackCommand)
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| − | | |
| − | == Problems ==
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| − | The problems with this:
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| − | * node 'owns' the observer, but the callbackCommand is opaque so it doesn't know anything about what will happen when the event is invoked
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| − | * the GUI needs to explicitly remove the observer before it is destroyed
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| − | * node is not introspectable; you cannot get a list of observers on the node (all data and methods are private)
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| − | * there's no easy way to know what side effects will happen for any Set call (either a priori or experimentally).
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| − | * there's no way to collapse events or disable them
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| − | == EventBroker Solution ==
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| − | The EventBroker is a singleton, available as:
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| − | vtkEventBroker *broker = vtkEventBroker::GetInstance();
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| − | with the broker, you can make a call like the following:
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| − | broker->AddObservation(node, vtkCommand::ModifiedEvent, this, callbackCommand);
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| − | where ''node'' is the subject of the observation.
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| − | The broker does the following:
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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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| − | * keeps an introspectable list of all observers it knows about
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| − | * has an option to keep a log of all event invocations for debugging and performance analysis
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| − | * has an option to turn off all event invocations
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| − | * has an ''asynchronous'' option to queue all event invocations and invoke them later (off by default)
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| − | * has option to collapse redundant events in the queue
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| − | 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:
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| − | $::slicer3::Broker AddObservation $node ModifiedEvent {puts "the node was modified"}
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| − | 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:
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| − | $::slicer3::Broker AddObservation $node 1 {puts "got an AnyEvent notification from the node"}
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| − | Future options include:
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| − | * add a timer to log the amount of time taken to process each event
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| − | * specify that some observations must be handled synchronously (e.g. for progress events that shouldn't be collapsed)
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| − | See the file [http://www.na-mic.org/Slicer/Documentation/Slicer3/html/classvtkEventBroker.html Libs/MRML/vtkEventBroker.h] for more information on logging options and introspection features.
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| − | ==References==
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| − | The following reference provided insight on requirements and design precedents.
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| − | * [http://java.sun.com/products/jms/javadoc-102a/index.html Java Message Service (JMS) API]
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| − | * [http://en.wikipedia.org/wiki/Observer_pattern Wikipedia definition of Observer Pattern]
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| − | * [http://xlobject.sourceforge.net/ A C++ implementation]
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| − | * [http://sigslot.sourceforge.net/ Another C++ implementation]
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| − | * [http://doc.trolltech.com/4.3/signalsandslots.html The Qt implementation]
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| − | ==Dependency Graphs==
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| − | EventBroker code should have the option to put out log files that are compatible with [http://www.graphviz.org graphviz] .dot file format. This can be rendered with a variety of programs, or even pasted directly in the wiki: | |
| − | | |
| − | === Simple Dependency ===
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| − | <graphviz border='frame' format='svg'>
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| − | digraph G {
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| − | vtkImageViewer -> vtkImageEllipsoidSource[ label = ModifiedEvent ] | |
| − | ;}
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| − | </graphviz> | |
| − | | |
| − | === Subset of Slicer Dependencies ===
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| − | <graphviz border='frame' format='svg'>
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| − | strict digraph G {
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| − | vtkMRMLModelDisplayNode -> vtkMRMLModelDisplayNode [ label = "ModifiedEvent" ]
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| − | ; vtkMRMLScalarVolumeDisplayNode -> vtkMRMLScalarVolumeDisplayNode [ label = "ModifiedEvent" ]
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| − | ; vtkMRMLLabelMapVolumeDisplayNode -> vtkMRMLLabelMapVolumeDisplayNode [ label = "ModifiedEvent" ]
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| − | ; vtkMRMLDiffusionWeightedVolumeDisplayNode -> vtkMRMLDiffusionWeightedVolumeDisplayNode [ label = "ModifiedEvent" ]
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| − | ; vtkMRMLDiffusionTensorVolumeDisplayNode -> vtkMRMLDiffusionTensorVolumeDisplayNode [ label = "ModifiedEvent" ]
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| − | ; vtkMRMLFiberBundleLineDisplayNode -> vtkMRMLFiberBundleLineDisplayNode [ label = "ModifiedEvent" ]
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| − | ; vtkMRMLFiberBundleTubeDisplayNode -> vtkMRMLFiberBundleTubeDisplayNode [ label = "ModifiedEvent" ]
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| − | ; vtkMRMLFiberBundleGlyphDisplayNode -> vtkMRMLFiberBundleGlyphDisplayNode [ label = "ModifiedEvent" ]
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| − | ; vtkMRMLCameraNode -> vtkOpenGLCamera [ label = "ModifiedEvent" ]
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| − | ; vtkMRMLUnstructuredGridDisplayNode -> vtkMRMLUnstructuredGridDisplayNode [ label = "ModifiedEvent" ]
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| − | ; vtkMRMLCameraNode -> vtkOpenGLCamera [ label = "ModifiedEvent" ]
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| − | ; vtkMRMLModelDisplayNode -> vtkMRMLModelDisplayNode [ label = "ModifiedEvent" ]
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| − | ; vtkMRMLCameraNode -> vtkOpenGLCamera [ label = "ModifiedEvent" ]
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| − | ; vtkMRMLModelNode -> vtkPolyData [ label = "ModifiedEvent" ]
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| − | ; vtkMRMLModelNode -> vtkPolyData [ label = "ModifiedEvent" ]
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| − | ; vtkMRMLModelDisplayNode -> vtkMRMLModelDisplayNode [ label = "ModifiedEvent" ]
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| − | ; vtkMRMLModelDisplayNode -> vtkImageData [ label = "ModifiedEvent" ]
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| − | ; vtkMRMLModelNode -> vtkPolyData [ label = "ModifiedEvent" ]
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| − | ; vtkMRMLModelDisplayNode -> vtkMRMLModelDisplayNode [ label = "ModifiedEvent" ]
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| − | ; vtkMRMLModelDisplayNode -> vtkImageData [ label = "ModifiedEvent" ]
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| − | ; vtkMRMLModelNode -> vtkPolyData [ label = "ModifiedEvent" ]
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| − | ; vtkMRMLModelDisplayNode -> vtkMRMLModelDisplayNode [ label = "ModifiedEvent" ]
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| − | ; vtkMRMLModelDisplayNode -> vtkImageData [ label = "ModifiedEvent" ]
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| − | ;}
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| − | </graphviz> | |
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| − | == Issues ==
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| − | * how to work with the vtkObserverManager?
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