Document how to preserve beans with @RouteScope when migrating from v7/v8

[denis-anisimov]

Prerequisites:
we have ticket vaadin/spring#473 and vaadin/cdi#293 which are about not preserving UIScopes beans on refresh.

The tickets are in fact consequence of confusion.
But the main idea is still valid: the developer want to preserve on refresh the whole UI instance (instead of specific components) and expects that UIScoped beans are preserved.

@PreserveOnRefresh doesn't fit the expectations and there is a reason for this: it's attached not the a UI instance but rather for a specific route target/component. So it may not work preserving UI beans since one route target may be preserved and another may be not.

What we need is: a way (most likely not via an annotation since we don't recommend to use your own UI instances) to make UI to be preserved on refresh. It may be a (deployment) config parameter or something else.
The way how it works: the whole UI is preserved on refresh (the instance doesn't have to be preserved but all ui scoped beans should be preserved): the ui scope should stay the same when UI is refreshed.

[TatuLund]

This is related to #6287

[mkn1988]

Hi,
I'm currently running into this exact issue. Any idea yet, when you'll start working on it?
Greetings,
Max

[thigg]

One usecase is to store data per tab, for example having seperate login sessions for each tab.
In Vaadin 8 this was commonly done via the UI class.

https://vaadin.com/forum/thread/17810577/17812148 presents a hack, which has several problems.
For one, you need to have an additional root layout for all your views (which is incompatible with AppLayout, because it doesnt like to be a nested layout).
A way to store data which corresponds to a tab without binding it to a specific component would be nice.

[pleku]

We've decided that this is not worth it - it would be tricky to implement technically which would make the ROI very low.
And we don't want to start to encourage using a custom UI anyway.

Instead you should use other means of storing data to be preserved in the UI - like in case of using CDI/Spring DI - use a @RouteScope bean that is tied with @RouteScopeOwner to the root layout that has the @PreserveOnRefresh.

Acceptance Criteria

• For now the idea is to just update the migrating from v7/v8 documentation (latest, v14 branches) to highlight that storing data needs to happen elsewhere than in the UI.
• Add a section to CDI scopes documentation on how to retain beans with @NormalRouteScope + @PreserveOnRefresh https://vaadin.com/docs/latest/flow/integrations/cdi/contexts
  (The Spring Scopes documentation already highlights this)

[Frettman]

It's still unfortunate if not downright detrimental that @PreserveOnRefresh and @UIScope are effectively mutually exclusive now. If any library or add-on wants to be usable with @PreserveOnRefresh it cannot use @UIScope (or just store stuff in the UI object). But the combination of @RouteScope and @RouteScopeOwner (or any other shared location) is application-specific and therefore not usable in libraries or add-ons. E.g. I have an add-on that provides an event bus for session and UI scopes. It simply will not work with @PreserveOnRefresh.
As @UIScope seems much more valuable to me, I have pretty much given up on @PreserveOnRefresh. At least for now.