Deferrer

A Deferrer is an object that allows a call to be submitted to the main queue. Mostly the coder does not need to worry about this, because there are ways to submit a call from almost everywhere:

• In all actor calls, there is the cx context available, which allows calls to be submitted directly to the runtime

• Each actor reference has access to a Deferrer kept in the actor's external data, which means that it is always possible to submit a call if you have an Actor or ActorOwn reference.

However in the case that you need to submit a call from a drop handler, and that drop handler does not have any actor references available, you may need to obtain a Deferrer and store it in the struct that will be dropped.

Note that a Deferrer takes zero bytes unless either the crate feature "multi-stakker" or "inline-deferrer" is enabled. This is because if there is only ever a single Stakker instance running in the whole process (or thread), we can optimise a Deferrer to use a global variable (or thread-local). Only in the case of needing multiple Stakker instances in a single thread (a much rarer case) does the Deferrer need to be a direct reference, in which case it consumes one usize. So you don't pay the cost unless you need it.