Context (web app)
To store state available to our entire Vaadin app, we can get and set "Attribute" on the VaadinContext object that represents our entire Vaadin-based web app at runtime. These attributes act as a key-value collection, where the key is of type String and the value is of type Object.
We access the context by calling UI.getCurrent().getSession().getService().getContext().
Session (per user)
To store state available to any one user’s session, we can similarly get and set "attributes" on the VaadinSession object.
We access the session by calling UI.getCurrent().getSession().
UI (web browser window/tab)
These two levels of scope, context & session, are wrappers around their equivalents defined in the Java Servlet specification. But Vaadin effectively has a third, finer level of scope. Vaadin supports multi-window apps, where each web browser window (or tab) has its own content handled by a UI object. If a user has three windows open within our Vaadin app, that user has three UI object instances on the server housed within a single VaadinSession object.
So it seems like a common need would be storing state per UI (web browser window/tab). So I would expect to see the same kind of getAttribute & setAttribute methods on UI as seen on VaadinSession & VaadinContext. But, no, I do not see such methods on UI.
➥ Is there an appropriate place to store state per UI object?
In the olden days, in previous generations of Vaadin, we always wrote our own subclass of UI. So we could always store state by defining member variables on our own UI-subclass. Now, in the days of Vaadin Flow (v10+, currently 14), we are discouraged (forbidden?) from writing a subclass of UI.
Before filing a feature-request for such attributes, I want to ask if I missed out on a usual place where folks store their per-UI state in current Vaadin-based apps.
