# Objective
Support systems that both read and write messages of the same type.
This is possible today with various workarounds: Either using
`ParamSet<(MessageReader<M>, MessageWriter<M>)>` or by manually storing
a `Local<MessageCursor<DebugMessage>>` to keep track of the reading
position. But those are both relatively complex for such a simple use
case.
The `ParamSet` workaround would get even more complex if we did #23339,
since it would be necessary to use a fallible system to handle the case
where the `Messages` resource is missing.
## Solution
Create a type that can both read and write messages. It needs to have a
`MessageCursor` and `ResMut<Messages<M>>`... which is exactly what
`MessageMutator<M>` has!
So, rather than creating a new type, simply add `write`, `write_batch`,
and `write_default` to `MessageMutator`.
Update the documentation to point users to use `MessageMutator` rather
than `ParamSet` or `MessageCursor`.
This renames the concept of `BufferedEvent` to `Message`, and updates
our APIs, comments, and documentation to refer to these types as
"messages" instead of "events". It also removes/updates anything that
considers messages to be "observable", "listenable", or "triggerable".
This is a followup to https://github.com/bevyengine/bevy/pull/20731,
which omitted the `BufferedEvent -> Message` rename for brevity.
See that post for rationale.
Chris Russell