implement recv_raw for Connection wrappers into mavlink-core

[roby2014]

(depends on #284)

So, my goal is to have some sort of way to fetch raw messages from the connection wrappers (AsyncMavConnection, MavConnection) directly, without having to do the extra-step of providing a type and converting between and vice-versa.

At the moment, recv will always use the generic M.

Another thing I was planning was to somehow allow connections without knowing the message type M, but this looks a bit harder to implement (and probably impossible because dialect must be known at compile-time?).

I've already implemented for AsyncMavConnection and its protocols (udp, tcp, direct-serial, file):

async fn recv_raw(&self) -> Result<MAVLinkRawMessage, crate::error::MessageReadError>;

I've also implemented a small example (mavlink-dump-raw-async), which already shows how this can be used with async.

I want to ask if this is a good idea and if possible, to open discussion on this topic here.

[roby2014]

Well 1 possible idea I have is to implement raw connectors, e.g: AsyncMavRawConnection and MavRawConnection instead of adding recv_raw (or keep recv_raw just for API design). This would not care about dialect and just tries to parse a MAVLink message. Obviously, the message check (crc calculation, deserialization, etc), would have to be done manually by the user).

@patrickelectric (and other maintainers) what do you think?

[pv42]

So I had the idea of doing something like that:

pub trait MavRawConnection {
    fn recv_raw(&self) -> Result<MAVLinkVXMessageRaw>
    ...
}

and then have

pub trait MavConnection<M: Message> : MavRawConnection  {
    fn recv(&self) -> Result<(MavHeader, M), crate::error::MessageReadError> {
        M::parse(self.recv_raw())
    }
   ...
}

The issue is that MAVLinkVXMessageRaw does not exist and it probably would have to be a trait object.

[roby2014]

So I had the idea of doing something like that:

pub trait MavRawConnection {
    fn recv_raw(&self) -> Result<MAVLinkVXMessageRaw>
    ...
}

and then have

pub trait MavConnection<M: Message> : MavRawConnection  {
    fn recv(&self) -> Result<(MavHeader, M), crate::error::MessageReadError> {
        M::parse(self.recv_raw())
    }
   ...
}

The issue is that MAVLinkVXMessageRaw does not exist and it probably would have to be a trait object.

I still think you'll need a dialect that way since functions like read_v1_raw_message_async, etc... Depend on the generic M

P.S: My title might be misleading. My goal is to have Connection wrappers that do not depend on a dialect so you can just receive RAW data