Module enthought.​tvtk.​messenger

Implements a simple, robust, safe, Messenger class that allows one to register callbacks for a signal/slot (or event/handler) kind of messaging system. One can basically register a callback function/method to be called when an object sends a particular event. The Messenger class is Borg. So it is easy to instantiate and use. This module is also reload-safe, so if the module is reloaded the callback information is not lost. Method callbacks do not have a reference counting problem since weak references are used.

The main functionality of this module is provided by three functions, connect, disconnect and send.

Here is example usage with VTK:

>>> import messenger, vtk
>>> def cb(obj, evt):
...  print obj.__class__.__name__, evt
...
>>> o = vtk.vtkProperty()
>>> o.AddObserver('ModifiedEvent', messenger.send)
1
>>> messenger.connect(o, 'ModifiedEvent', cb)
>>>
>>> o.SetRepresentation(1)
vtkOpenGLProperty ModifiedEvent
>>> messenger.connect(o, 'AnyEvent', cb)
>>> o.SetRepresentation(2)
vtkOpenGLProperty ModifiedEvent
vtkOpenGLProperty ModifiedEvent
>>>
>>> messenger.send(o, 'foo')
vtkOpenGLProperty foo
>>> messenger.disconnect(o, 'AnyEvent')
>>> messenger.send(o, 'foo')
>>>

This approach is necessary if you don't want to be bitten by reference cycles. If you have a Python object holding a reference to a VTK object and pass a method of the object to the AddObserver call, you will get a reference cycle that cannot be collected by the garbage collector. Using this messenger module gets around the problem.

Also note that adding a connection for 'AnyEvent' will trigger a callback no matter what event was generated. The code above also shows how disconnection works.