Both CMS and explorer have event notification and observation systems (with little differences between the APIs). These two systems should be merged in the Runtime.
In the meantime, the API should be improved:

• The event arguments should be a Map<String, Object> instead of Object[].
• The ObservationManager should provide a Map of data alongside the Event to the observe() method (as it's done in the workflow with the "transient vars") which can be used by one Observer to pass data along to the following ones.