OnPageChange behavior for same URL path RFC

[dillonkearns]

The OnPageChange Msg allows users to listen for page change events since elm-pages wraps the Browser.application and handles routing.

What should happen in these two cases?

1. You navigate to a URL with the same path as your current location, but it has a different fragment (example: start at URL /docs/getting-started and click a link to /docs/getting-started#cli)
2. You navigate to the exact same URL (for example, you're at /docs and you click a link to /docs)

It seems like the user should have some way to know about this event. So the obvious solution is to just fire OnPageChange in both cases. Because otherwise the user would simply have no way to handle those events.

init and Cmds for these cases

What about init? In cases (1) and (2), should init be triggered, causing the page's Model to be started from scratch, and firing off any user Cmd's for the page? This would mean that clicking to a URL fragment would cause a new init, for example if you click a link in the table of contents to take you to a new section. That feels a little strange in that case.

One possible direction to explore is to give the user a way to call init in Shared.elm.

onPageChange : Url -> Url -> model -> ( model, Cmd msg ) -> ( model, Cmd msg )
onPageChange previousUrl newUrl currentModel ( initModel, initCmd ) =
    ( initModel, initCmd )

Then you could define custom logic for how to handle those situations in your app. You could even trigger an event on page change.

That's pretty low-level, though, and it would be ideal to just "do the right thing" if there's a way to get intuitive behavior without exposing low-level details like that.

[MarcoDaniels]

I like the idea of having this event on every page navigation even when it is a hash link /url#something as it is gives the ability to trigger some custom event (can be animation, tracking, etc.).

I just feel like the name OnPageChange might be misleading if it gets triggered when the user is on page /docs and clicks /docs as technically the page didn't really change but it might have reloaded/refreshed.

Not sure exactly what name I would give though, but maybe something like onNavigation, onPageLoad? I also don't believe it to be a dealbreaker.