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== Currently ==
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<big>'''Note:''' We are migrating this content to the slicer.org domain - <font color="orange">The newer page is [https://www.slicer.org/wiki/Slicer3:EventBroker  here]</font></big>
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 ApplicationLogic that would look something like:
 
 
 
broker->RegisterObserver(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?
 
 
 
 
 
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==
 
 
 
* [http://java.sun.com/products/jms/javadoc-102a/index.html Java Message Service (JMS) API]
 
* [http://en.wikipedia.org/wiki/Observer_pattern Wikipedia definition of Observer Pattern]
 
* [http://xlobject.sourceforge A C++ implementation]
 
* [http://sigslot.sourceforge.net/ Another C++ implementation]
 
* [http://doc.trolltech.com/4.3/signalsandslots.html The Qt implementation]
 

Latest revision as of 17:15, 10 July 2017

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Note: We are migrating this content to the slicer.org domain - The newer page is here