Agent v2

[jstarry]

Problem

The current Agent trait API includes a way to handle "messages" and "input" and it's not immediately clear when to use each of these. Messages are unidirectional and primarily intended for internal agent usage. Input messages are for bidirectional communication and have a corresponding output message type and are intended for external client usage.

Suggested Changes

• Rename "input" to "message" and allow agents to implement handlers for arbitrary number of message input/output pairs
• Rename "message" to "event" and remove external access from AgentLink
• Remove the "respond" method from AgentLink

[frostu8]

In addition to the leading suggested changes, I think we should split the Agent trait into two variants like AgentCast and AgentCall, so that agent logic is better compartmentalized. After all, why should you worry about defining the Input or Ouput types when you're only going to use an Event type?

We could use a blanket trait to implement a single Agent class so an agent can still be both AgentCast and AgentCall.

/// An agent that consumes a value.
pub trait AgentCast<IN> {
    fn create(link: AgentLink<Self>) -> Self;
    // ...other general agent functions, if needed.

    fn cast(&mut self, input: IN);
}

/// An agent that consumes a value and returns a new value.
pub trait AgentCall<IN> {
    type Output;

    fn create(link: AgentLink<Self>) -> Self;
    // ...other general agent functions, if needed.
    fn call(&mut self, input: IN) -> Self::Output;
    // this might look closer to this when it is implemented:
    // fn call(&mut self, input: IN)
}

/// An agent.
#[doc(hidden)]
pub trait Agent {
    fn create(link: AgentLink<Self>) -> Self;
    // ...other general agent functions, if needed.
}

// these blanket implementations abstract away the hassle of having to
// implement two traits from the programmer, so they can focus more on
// the implementation

impl<T, IN> Agent for T
where T: AgentCast<IN> {
    fn create(link: AgentLink<Self>) -> Self {
        <Self as AgentCast<IN>>::create(link)
    }
    // ...other general agent functions, if needed.
}

impl<T, IN> Agent for T
where T: AgentCall<IN> {
    fn create(link: AgentLink<Self>) -> Self {
        <Self as AgentCall<IN>>::create(link)
    }
    // ...other general agent functions, if needed.
}

I'm not too knowledgeable about the internals yet, so excuse my assumptions.

[mc1098]

Now that the new Component trait has landed #1961 - with the removal of ComponentLink its buddy AgentLink feels somewhat out of sync. We should consider harmonizing the two as somewhat discussed in #2050.

[ranile]

Irrelevant since agents aren't implemented by Yew. The improvement will be in gloo.

See #2326