SPAs not able to send notifications via Polkadot Apps

[the-right-joyce]

When an user interacts with a product via the SPA (e.g. the conference app), notifications to that user can only be received while the SPA is open. As soon as the user is not interacting with the product anymore (e.g. the SPA or app is closed), no notifications from the product are received any longer.

@P1sar suggested we can use the statement store and create a channel for these product notifications, where the user can subscribe to and listen if there's a notification for them coming in.
@ryanleecode is currently working on the statement store subscription for chats: https://hackmd.io/@1JCaGppGSUqHtJilikYaKw/r16YTVg5Ze

cc @SBalaguer @TorstenStueber

[SBalaguer]

Here's my take on this, although @TorstenStueber I need your input because it requires changes on the HostAPI.

Problem

The host today only exposes host_push_notification, which is fire-and-forget, immediate. Any product that wants a time-shifted notification ("remind me 10 min before this session", "ping me when voting opens") has no path that survives the sandbox: products can't touch UNUserNotificationCenter, can't keep a background timer (WebView suspends), can't talk APNs/FCM, and can't reach a relay directly.

Proposed primitive

A first-class scheduled notification API in the Host API. The product schedules by stable id with a future fireAt timestamp; the host fires the notification on-device when the time arrives, replicates the schedule to a backend so it also fires on the user's other devices.

┌──────────┐ host_notification_schedule ┌──────────┐  POST /v1/scheduled-…  ┌──────────────────┐
│ Product  │ ─────────────────────────▶ │   Host   │ ──────────────────────▶│ Identity backend │
│  (SPA)   │                            │ (via API)│                        │  (or relay)      │
└──────────┘                            └────┬─────┘                        └──────────┬───────┘
                                             │                                         │
                                  schedules local trigger                  fires APNs/FCM/web-push
                                  (UNCalendar / AlarmManager /              to other hosts
                                  service-worker showTrigger)

Proposed Methods

Sender == receiver throughout; id is decided by the product (e.g. session:0xabc:reminder).

• host_notification_schedule({ id, fireAt, title?, body, deeplink? }): Add / edit, idempotent on id. Re-issuing replaces both the local trigger and the backend row.
• host_notification_cancel({ ids }): Delete. Drops local trigger and backend row in lockstep.
• host_notification_list(): List. Returns all scheduled notifications for this product, with replicated and localScheduled flags so the product can reason about partial state.

Reconciliation

The product owns reconciliation: on open / on relevant state change, call list(), diff against its model, issue cancel/schedule to update the notifications accordingly. The host is not a calendar, it doesn't know when external state changes (a session is cancelled on-chain, a deadline moves), it only acts as scheduler and dispatcher.

Open question

1. How should the host talk to the backend service? Today it's implemented via direct HTTP, but there's a redesign to use statement store in the future.
2. PolkadotApp can manage locally stored notifications, but is this the case for Desktop and Browser too?
3. Permissions: user needs to allow notifications from the SPA.
4. Same user notification or cross-users notifications? Currently this is designed so that a user schedules a reminder to it's own self (e.g.: remind me when this talk that I favorited starts). Do we want to make it more generic in that it can include another account? In that case, what are the rules for the destination account?

[TorstenStueber]

Here is the RFC for the TrUAPI to add this capability: paritytech/truapi#66