Delegates to processEvent(ApplicationEvent) to give sub-classes
a chance to deviate from the default. Unwraps the content of a
PayloadApplicationEvent if necessary to allow method declaration
to define any arbitrary event type. If a condition is defined, it is
evaluated prior to invoking the underlying method.
These arguments will be used to invoke the method handled by this instance. Can
return null to indicate that no suitable arguments could be resolved and
therefore the method should not be invoked at all for the specified event.