calendula/docs/design/calendar-visibility-model.md

Design: One calendar-visibility model (reminders silently never fire)

Status: implemented on fix/calendar-visibility-model (on-device review owed) Date: 2026-07-25, revised 2026-07-25 after code review

Built as specified, then revised in two places the review found (see §4, §6): the reconciliation no longer switches calendars on, and a calendar switched off without WRITE_CALENDAR is kept app-side instead of being lost. Other notes: CalendarSource gained a syncsEvents flag (read for #76's "not synced" label); the reconcile runs from CalendulaApp.onCreate and from PermissionViewModel.onGranted, so a launch that starts without the calendar permissions still catches up the moment they are granted; and the Settings hint string was reworded rather than dropped — it described the retired app-only behaviour, so leaving it would have been the one piece of false text on screen. Tracking: Codeberg #75. Split out of this work: #76 (read-only / not-synced communication), #77 (accounts merged by name).

The bug

Reminders never arrive for a calendar that is hidden at system level, and nothing in the app hints at it. The event shows up, its reminders are listed on the detail screen, and no notification ever fires.

The cause is a design defect, not a device quirk: the app has two visibility models. Calendula's own disabledCalendarIds decides what it shows, while the system's Calendars.VISIBLE decides whether the provider schedules reminder alarms at all. The app reads VISIBLE into CalendarSource.isVisibleInSystem and then never uses it — calendars() queries with a null selection. So the two models can disagree indefinitely, and when they do, reminder delivery dies quietly.

Not a regression (nothing in data/reminders/ changed in 2.14/2.15) and not Android-10-specific — the gate is identical on every supported version.

Verified provider behaviour

Checked against AOSP source and empirically on device; details in the calendars-visible-flag-facts note.

• AOSP CalendarAlarmManager.scheduleNextAlarmLocked selects reminders with WHERE Calendars.VISIBLE=1 AND … AND Reminders.METHOD=1, and its cleanup selector deletes CalendarAlerts rows whose event has visible=0. Identical in the android10, android13 and android15 branches.
• VISIBLE is one of only three Calendars columns Android documents as "writable by both an app and a sync adapter" (with CALENDAR_DISPLAY_NAME and SYNC_EVENTS). Writing it is sanctioned, not a workaround.
• VISIBLE is device-local. Google's sync adapter neither pushes a local change upstream nor overwrites one on a later sync — verified by flipping a Google account's primary calendar, forcing the upload sync (the adapter cleared DIRTY/MUTATORS) and observing no change on the web, and by forcing a full sync carrying sync_extra_get_settings=true without the local value being reverted. The adapter only seeds the initial value at calendar creation. Caveat: a negative observation, so "no propagation seen where it would have been visible" rather than proof of impossibility.
• VISIBLE and SYNC_EVENTS are independent. visible=1, sync_events=0 is reachable and persistent: a calendar that looks enabled everywhere and is permanently empty, because its events are not on the device at all.

Decision

Collapse to one model, the way Etar does it — the Settings → Calendars toggle is Calendars.VISIBLE.

The two existing levels map onto it directly:

Level	Before	After
Filter sheet	hiddenCalendarIds (app-local)	unchanged
Settings → Calendars	disabledCalendarIds (app-local)	Calendars.VISIBLE

The filter sheet stays a purely in-app declutter toggle and deliberately does not suppress reminders — hiding a calendar from view is not the same as asking not to be reminded. Settings-off does stop reminders, because the provider then schedules nothing.

Rejected alternatives:

• Own the reminder delivery (the Fossify model) — schedule our own alarms so VISIBLE stops mattering. Bigger, and it leaves two visibility models in place. Kept as a possible later step; see Non-goals.
• Warn about the mismatch — a banner papering over two sources of truth rather than removing one.
• Have the toggle write SYNC_EVENTS too — turning it off makes the provider drop the calendar's local events, so an innocuous switch becomes destructive. Never written.

No hint text and no confirmation dialog: the toggle now means "show this calendar", which is what a user assumes, and the change does not leave the device.

Work

1. Write path

• CalendarDataSource.setCalendarVisible(id: Long, visible: Boolean) — update ContentUris.withAppendedId(Calendars.CONTENT_URI, id), not the sync-adapter URI (must work on synced calendars) and not a WHERE _id IN (…) batch. See the gotcha below. Mirror in FakeCalendarDataSource; repository passthrough on io.
• No manual UI patching: the ContentObserver re-queries, and the provider re-runs checkNextAlarm() itself when VISIBLE changes. The observer-driven invariant holds for free.

Load-bearing gotcha. CalendarProvider2.updateInTransaction returns early with a raw mDb.update() unless the selection is _id=? or starts with _id= (:4281). That early return skips the dirty marking, the MUTATORS stamp and the checkNextAlarm() reschedule at :4314-4322 — i.e. it would silently skip the very reminder rescheduling this fix exists to trigger. Always address one calendar by appended id. A batch of per-row ContentProviderOperations is fine (those generate _id=n, which the provider handles explicitly); a single IN-clause op is not.

2. Read path

• Settings → Calendars row state derives from isVisibleInSystem.
• Swap the display predicate from id in disabledIds to !isVisibleInSystem in CalendarRepositoryImpl.instances/searchEvents, EventEditViewModel (form picker), ImportViewModel (import picker).
• hiddenCalendarIds keeps working exactly as it does today, unioned as before.

3. Deletions

With VISIBLE=0 the provider creates no alert rows, so there is nothing to suppress, stash or recover. Remove:

• postableAlerts / isForDisabledCalendar (EventReminderReceiver.kt:29-41)
• SuppressedReminderStore and CalendarsViewModel.recoverReminders

The gate in ReminderNotifier.post stays (review): two paths reach it without a provider alert row behind them — a snooze re-shown from our own exact alarm, scheduled before the calendar was switched off, and a read-only install whose switch lives in the pending set. One choke point covers both receivers.

Net code reduction. Update the now-false KDoc on CalendarPrefs claiming the toggle "never touches the system's VISIBLE/SYNC_EVENTS flags, so other calendar apps are unaffected".

4. Reconciliation (hide-only, standing)

CalendarVisibilityReconciler runs on every launch and again when the permission screen grants the permissions. It drains CalendarPrefs.pendingDisabledCalendarIds — the retired disabledCalendarIds key, re-read under a new name — into the provider:

• Pending and visible=1 → write visible=0, then drop the id.
• Pending but already visible=0, or gone from the device → drop the id.
• Everything else untouched. Nothing is ever switched on.

Each id leaves the set as its own write lands, so a run that dies part-way resumes where it stopped and can never re-apply a hide the user has since undone by hand. The set empties itself; no separate "migration done" flag exists.

Hide-only was the review's correction. The first cut also switched calendars on (enabled in-app + visible=0 + sync_events=1 → visible=1) to keep the upgrade invisible. Two problems, both real:

• VISIBLE is device-wide. "Not disabled in Calendula" is the default for every calendar, including ones the user deliberately hid in Google Calendar, Etar or DAVx5 — those would pop back there and start firing reminders nobody asked for, from a switch the user never touched.
• The sync_events=1 guard doesn't hold. It is sound for sync-adapter accounts, but an ACCOUNT_TYPE_LOCAL calendar another app created can sit at sync_events=0 while holding real device-local events, so the guard would have hidden events it claimed could not exist.

The cost is that events from a system-hidden calendar stop appearing in Calendula. That is confined to this app, visible and reversible in Settings → Calendars, and announced: on the first run that finds such a calendar, the reconciler arms a one-time dialog (CalendarVisibilityNoticeDialog) explaining that visibility now follows the device and where to change it. The answer — including "nothing to say" — is stored, so it can never resurface later.

4b. No WRITE_CALENDAR

Only READ_CALENDAR gates the app (RootScreen), and PermissionScreen says as much: declining write keeps Calendula usable read-only. Those users can't have the flag written for them, so the switch keeps working app-side — CalendarRepositoryImpl.setCalendarsVisible writes the pending set instead — and that set filters instances/searchEvents and gates ReminderNotifier.post exactly as VISIBLE would. If WRITE_CALENDAR ever arrives, the reconciler flushes it and the app-side copy disappears. This is the one place a second visibility model still exists, and it exists only where the first one is unwritable.

5. sync_events = 0 rows

No toggle at all — flipping it cannot produce an event. The row still shows, labelled "not synced", sorted to the bottom of its account group regardless of alphabetical order, and dimmed using the screen's existing dimmed/dimIf convention. Consult the material-3 skill before designing the label. Tracked in #76, not here.

Non-goals

The missed-broadcast class of failure: a reminder lost because the provider's EVENT_REMINDER broadcast never arrives (OEM app-sleeping, an OEM-modified provider) or because a competing calendar app marked the alert FIRED first. There is no rescan, and dueAlerts only accepts STATE_SCHEDULED, so one missed broadcast loses that reminder permanently.

If we take that on later, note that Etar's AlarmScheduler self-disables once a real EVENT_REMINDER is ever seen (PROVIDER_REMINDER_PREF_KEY) — that latch must not be copied. It assumes the only failure is "broadcast never arrives"; ours includes "broadcast arrives fine for other calendars, but no alert row was ever created". Any scheduler of ours must run unconditionally and dedupe by notification identity.

Testing

All JVM-testable:

• the reconciliation as a pure function over (calendars, pendingIds): hide, settled (already off / gone), never a switch-on, and the notice predicate
• the display-predicate swap, and the pending set filtering alongside it
• the read-only path: no provider write, the choice parked in the pending set, events filtered from it
• FakeCalendarDataSource assertions that setCalendarVisible addresses a single calendar by id
• one Calendars query per provider tick, however many flows are collecting

On-device review before release (UI-touching). Local repro for the #75 shape: force a test calendar to visible=0 via the provider, confirm its reminders stop and that the Settings toggle brings them back.

Release

Behaviour change plus a migration and a device-wide-flag change, so this reads as a minor (2.17.0) rather than a patch. New strings land in values/strings.xml only; translations come back through Weblate.