Observers are documented in the Observers section of the manual. The issues to be aware of when debugging observers are:
Most observers inherit smtk::common::Observers, which is templated on the signature of functions to be called when an event occurs.
The Observers class has a second template parameter (a boolean) that turns on debug printouts when true. The default is false.
Observers are forced to run on the main (GUI) thread in ParaView by the pqSMTKCallObserversOnMainThreadBehavior class. This means that observers are called asynchronously at some point after the operation has completed and potentially after other SMTK calls have been made. Because operation results may hold shared pointers to components and resources (thus keeping them alive), destructors may not be called when you expect.
An observer may remove itself from an Observers instance while it is being evaluated. The actual erasure will occur after all observers for the event have been invoked. When this occurs, it is noted in the debug printouts.
What the above means is that if you are having issues with observers of, for example, resource manager events, you will want to change smtk/resource/Observer.h from
typedef smtk::common::Observers<Observer> Observers
typedef smtk::common::Observers<Observer, /* debug */ true> Observers
and rebuild+install SMTK. Then, information about the lifecycle of each observer in the resource-manager event will be reported (its addition, invocation, and removal).