Equeue implements generic event queues. An event system
consists of an event queue, a list of event handlers, and an
event source. The queue has FIFO semantics, i.e. the first event
is processed next, and new events are appended to its end. When
an event is delivered to an event handler, all handlers are tried
until a handler accepts the event (or the event is dropped if no
such handler can be found). See below how a handler can indicate
whether to accept or reject an event.
When the queue is empty, the event source is called once. The source can add events to the queue, in order to continue event processing. If the queue remains empty, the execution of the event system stops.
The module can be used in multi-threaded program provided no event system is shared by several threads, or if so, access to functions is serialized.
This is the type of an event system with events of type 'a
May be raised by event handlers to reject events
May be raised by event handlers to accept events while terminating themselves
run when the event source adds new events to the queue
but there are no event handlers to process them
Creates a new event system that has an event source, but is
otherwise empty. The argument of type
'a t -> unit is the
event source. The source can call
add_event to put new events
into the queue.
Adds a handler to the list of handlers of the system.
An event handler is called with the event system and the event as arguments. The handler can return in various ways:
Reject: The event is rejected by the handler. The other handlers are asked to process the event.
Terminate: The event is accepted, but the handler is terminated, i.e. will never be called again.
The handler can add new events and new event handlers. The latter will be activated when the next event is processed.
Running a system means that, unless the queue is empty, the events
at the time of the
run invocation and all later added events are
gone through. Each event is presented to the handlers until one
handler accepts the event. Events rejected by all handlers are
dropped silently. If there is no pending event the default event
source is called once. If there are still no events the system stops
and returns. If there are events to process but no handlers which
can do them all events are silently dropped, and the default event
source is called once.
Out_of_handlers is raised if there are events but no
handlers after the event source has been called. This is considered
as a programming error, and would cause infinite looping if not
Note that there is an implicit order among the handlers which is simply the order the handlers have been added to the system. This means that you can set a fallback handler which catches any unprocessed event by adding it last.
Note that the events are processed in the order they happen. There is no mechanism to assign priorities to events.
Handlers are allowed to raise arbitrary exceptions. Exceptions other
than Reject and Terminate are not caught, so the caller has to do this
if appropriate. It is possible to restart an event system by just