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

• 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:

yew/src/agent.rs

Line 321 in 99220f5

Some(None) => warn!("The Id of the handler: {}, while present in the slab, is not associated with a callback.", id.raw_id()),

@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:

yew/yew-router/src/agent/mod.rs

Lines 140 to 142 in e4a609f

	for sub in &self.subscribers {
	self.link.respond(*sub, route.clone());
	}

yew-router's buttons have RouteAgentDispatcher:

yew/yew-router/src/components/router_button.rs

Lines 15 to 19 in e4a609f

	pub struct RouterButton<SW: Switch + Clone + 'static, STATE: RouterState = ()> {
	link: ComponentLink<Self>,
	router: RouteAgentDispatcher<STATE>,
	props: Props<SW>,
	}

That subscribe to the RouterAgent:

yew/yew-router/src/agent/dispatcher.rs

Line 22 in e4a609f

let dispatcher = RouteAgent::dispatcher();

yew/yew/src/agent.rs

Lines 97 to 113 in e4a609f

	/// This trait allows the creation of a dispatcher to an existing agent that will not send replies when messages are sent.
	pub trait Dispatched: Agent + Sized + 'static {
	/// Creates a dispatcher to the agent that will not send messages back.
	///
	/// # Note
	/// Dispatchers don't have `HandlerId`s and therefore `Agent::handle` will be supplied `None`
	/// for the `id` parameter, and `connected` and `disconnected` will not be called.
	///
	/// # Important
	/// Because the Agents using Context or Public reaches use the number of existing bridges to
	/// keep track of if the agent itself should exist, creating dispatchers will not guarantee that
	/// an Agent will exist to service requests sent from Dispatchers. You **must** keep at least one
	/// bridge around if you wish to use a dispatcher. If you are using agents in a write-only manner,
	/// then it is suggested that you create a bridge that handles no-op responses as high up in the
	/// component hierarchy as possible - oftentimes the root component for simplicity's sake.
	fn dispatcher() -> Dispatcher<Self>;
	}

yew/yew-router/src/agent/mod.rs

Lines 115 to 117 in e4a609f

	fn connected(&mut self, id: HandlerId) {
	self.subscribers.insert(id);
	}

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.

[lukechu10]

So if I understood this correctly, the solution is to just to remove the warning message?

[siku2]

This was addressed in #1846 and is also no longer relevant because of #1791