Provide general utilities to attach many handlers to be applied on events processing. Events can be of any type.
Handlers may or may not be applied to certain events at runtime based on rules provided as a predicate (function).
If an event doesn't match a handler's predicate, that handler is ignored, and the next one will be processed.
When a handler's predicate returns true, its callback will be called. That block will process the event, and return a modified copy with the changes.
The callback block will decide if subsequent handler's will be evaluated by calling the next handler explicitly (if
next method is not called, the handlers defined later won't be evaluated).
There are types of handlers like 'After' y 'Before' that call
next as part as their operation discharging the user of doing so.
The events are passed to handlers' callbacks wrapped in a 'context'. The context can also pass information among handlers in the
attributes field. And store an exception if it is thrown in a previous handler processing.
Types of handlers:
On Handler: the callback is executed before calling the remaining handlers.
nextis called after the callback.
After Handler: the predicate is evaluated after the remaining handlers return, and the callback is executed after.
nextis called before the callback
nextis not called by this handler, its callback is responsible for doing so if needed.
Chain Handler: this handler contains a list of handlers inside.
IMPORTANT: the order is NOT the order, it is the depth. Handlers are not linked, they are NESTED. The
next() method passes control to the next level.
Is really this execution: