Yew: Handler Id Warning

[tarkah]

Problem
I'm getting the following warning when switching between different routes using yew_router. I have 5 different components that I switch between when route changes using Router::render. One of the components has a table with 100 rows, this warning returns ~100 times when I switch off that route. Whereas one route is just an index page with static text, and it returns the warning 4 times when switched off that route.

WARN:yew::agent -- The Id of the handler: 0, while present in the slab, is not associated with a callback.

Steps To Reproduce
Steps to reproduce the behavior:

1. Use yew_router
2. Use Router::render to render different components, ex: below

There is nothing special about these components. IndexPage for example just renders static text in a <p> tag, yet still produces the above warning 4 times when route changes and different component is then rendered.

html! {
    <Router<AppRoute, ()>
        render = Router::render(move |switch: AppRoute| {
            match switch {
                AppRoute::Index => html!{<IndexPage />},
                AppRoute::Songs => html! {<SongsPage />},
                AppRoute::Artist(id) => html!{<ArtistPage artist_id=id />},
                AppRoute::Artists => html!{<ArtistsPage />},
                AppRoute::Queue => html!{<QueuePage />},
                AppRoute::NotFound(Permissive(None)) => html!{"Page not found"},
                AppRoute::NotFound(Permissive(Some(missed_route))) => html!{format!("Page '{}' not found", missed_route)},
                _ => html!{"Page not found"},
            }
        })
        redirect = Router::redirect(|route: Route| {
            AppRoute::NotFound(Permissive(Some(route.route)))
        })
    />
}

Environment:

• Yew version v0.11
• Yew-router version v0.8.1
• Rust version 1.40.0
• Target wasm32-unknown-unknown
• stdweb version
• OS: linux
• Browser: firefox
• Browser version 72.0.1

Questionnaire

• [x] I'm interested in fixing this myself but don't know where to start
• [ ] I would like to fix and I have a solution
• [ ] I don't have time to fix this right now, but maybe later

[nurebrahimkanakan]

---

[hgzimmerman]

Hi, as the maintainer of the Router, I have some input on this.
The Router makes use of agent Dispatchers, which are just like Bridges, but lack a callback used direct messages back to components.

The warning message you see is the result of a message being attempted to be sent back to a component from an agent, but there not existing a callback to facilitate that. What this warning message is effectively saying is "Hey you are using a dispatcher somewhere!", which I consider to be noise, and the warning designation to be a misnomer - everything is acting as expected when you see this message.

There are two options for how to handle this: Remove the logging entirely, or switch it to log::trace! instead. I'm in favor of outright removal, but I'd understand if switching it to trace/debug was thought to be more preferable.
The relevant line of code is: https://github.com/yewstack/yew/blob/99220f542fedd22f0a99fca7cb906bb69d41dea6/src/agent.rs#L321

@jstarry I'll defer to you the decision about how to handle the removal or alteration of the log line.

[jstarry]

@hgzimmerman why is the dispatcher trying to send a message to the component?

[totorigolo]

Hi! I am currently playing with yew-router, and had the same warnings. @jstarry, here are a few links to the code to answer your question:

When the route changes, RouteAgent informs all its subscribers:
https://github.com/yewstack/yew/blob/e4a609f6a0d8448030e661aba04629606cefb065/yew-router/src/agent/mod.rs#L140-L142

yew-router's buttons have RouteAgentDispatcher:
https://github.com/yewstack/yew/blob/e4a609f6a0d8448030e661aba04629606cefb065/yew-router/src/components/router_button.rs#L15-L19

That subscribe to the RouterAgent:
https://github.com/yewstack/yew/blob/e4a609f6a0d8448030e661aba04629606cefb065/yew-router/src/agent/dispatcher.rs#L22
https://github.com/yewstack/yew/blob/e4a609f6a0d8448030e661aba04629606cefb065/yew/src/agent.rs#L97-L113
https://github.com/yewstack/yew/blob/e4a609f6a0d8448030e661aba04629606cefb065/yew-router/src/agent/mod.rs#L115-L117

I guess that's parts of the how. It's late, the why will have to wait ^^
_(I hope GitHub will show previews for the links)_

[joprice]

Might be unrelated, but I saw this when I defined routes out of order, where an earlier definition was a prefix of the later one. This specific case is mentioned in the docs, but maybe the warning could mention it or link to the docs, or mention an error code.

[joprice]

Nevermind I see this in other cases now.