Support lazily loading messages

[marcaddeo]

I have a middleware that uses the Messages extractor, but if a condition is not met the messages are never actually used by the middleware. Since the extractor is calling Message::load, the messages are removed by my middleware and never able to be used by further handlers/middleware.

It would be nice if there were a second extractor that did not automatically load the messages, leaving it up to the handler/middleware to do so manually. For that to work, the Messages::load method would need to be made public.

Is this something you'd consider adding or merging if I were to put together a PR?

[maxcountryman]

I think it would be nice to support this as a configuration option if possible--a second extractor could also work but it would be a shame to introduce a separate extractor just to allow manual loading.

[marcaddeo]

For my use case, I’d be using both the normal automatic extractor and the lazy extractor. I use the normal Messages extractor in my handlers, but I also need to access the unloaded messages in my response middleware.

[marcaddeo]

Another thought is removing the messages from the message store at a later stage in the lifecycle, i.e. when it’s turned into an iterator instead of in Messages::load. I’m not sure how feasible that solution would be, a lazy extractor was my first attempt at solving this without changing too much code in axum-messages.

[maxcountryman]

I have a middleware that uses the Messages extractor, but if a condition is not met the messages are never actually used by the middleware.

Couldn't you also work around this by loading the Messages extractor only if your condition is met? Specifically I mean by taking the request (as an extractor) and loading from its extensions manually.

It might even be possible to wrap this all in an extractor of your own that wraps Messages and the conditional logic. That would move all this logic out of your handler.

All this is say I think a better approach is to leverage Axum itself a little more as opposed to change this crate.

[marcaddeo]

I'm using map_response_with_state which doesn't support extractors that use FromRequest, only ones that use FromRequestParts. I suspect the Request extractor is a FromRequest, but I can't seem to actually confirm it from the docs..

I also just realized I can use the Extension<Messages> extractor to get the unloaded messages, so shouldn't need a custom extractor at all.

However, either way once I get the unloaded messages I can't load them due to Messages::load being a private method.

Can you share a concrete example of what you're trying to do?

Does that help explain enough? I can link to my repository I'm working on, but obviously it's got more code than just what's needed to show what I'm trying to do 😅

[maxcountryman]

You don't need to call load directly, you just need to call from_request_parts; remember, FromRequestParts is already implemented for Messages, so once you have the request and state, you can invoke it yourself and cause the messages to be loaded.

Without seeing a minimal code example, it's admittedly difficult to help but I suspect you could implement your own extractor and barring that then you could do this in your handler with the request and state. I'm not sure exactly what you're doing with map_response_with_state, but if you need Messages there then again wrapping the extractor is an option (use from_request_parts).

Regardless of approach, my opinion is this is a job for the Axum API.

[marcaddeo]

You don't need to call load directly, you just need to call from_request_parts; remember, FromRequestParts is already implemented for Messages, so once you have the request and state, you can invoke it yourself and cause the messages to be loaded.

Without seeing a minimal code example, it's admittedly difficult to help but I suspect you could implement your own extractor and barring that then you could do this in your handler with the request and state. I'm not sure exactly what you're doing with map_response_with_state, but if you need Messages there then again wrapping the extractor is an option (use from_request_parts).

Regardless of approach, my opinion is this is a job for the Axum API.

I don't have access to the Request in the map_response_with_state middleware, so I can't call Messages::from_request_parts. I'll put together an example.

[maxcountryman]

You probably want to use from_fn in that case.

[marcaddeo]

Just attempted with from_fn, but don't think that will work because running next.run(request) consumes the request to produce the response and I need to check the response to determine if I need to consume the Messages.

Here's a minimal example, hopefully that helps.