local-notifications: On iOS, scheduled notifications do not respect timezone changes

[gregzo]

Bug Report

Plugin(s)

@capacitor/local-notifications 7.0.1

Platform(s)

iOS, macOS
Android untested

Current Behavior

In LocalNotificationsPlugin.swift, the method handleScheduledNotification instantiates the notification trigger for at punctual notifications using UNTimeIntervalNotificationTrigger. For this to work, the requested at date is first converted to a date interval from now. The result is that the notification will fire at a fixed interval in the future, regardless of the timezone which the user is in.

Expected Behavior

If there are no repeats, on notifications should result in a UNCalendarNotificationTrigger trigger with repeat: false. That way, the scheduled notification should respect timezone changes so that a notification scheduled for 8am whilst in London doesn't fire at 2am in New York.

Edited: removed erroneous mention of UNTimeIntervalNotificationTrigger

[gregzo]

Re-reading this, I realize my explanation is a bit confusing. Here's a more detailed overview of the current implementation and what is wrong with it.

The plugin exposes 3 ways to schedule notifications, none of which map to timezone-aware scheduling iOS side. The mapping to native notifications is done in LocalNotificationsPlugin.handleScheduleNotification here: https://github.com/ionic-team/capacitor-plugins/blob/main/local-notifications/ios/Sources/LocalNotificationsPlugin/LocalNotificationsPlugin.swift#L277

1. schedule.at

Use: Schedule a notification at a specific date and time
Maps to: UNTimeIntervalNotificationTrigger which points to a fixed time in the future, which won't respect time zone changes

2. schedule.on

Use: Schedule a notification on particular intervals (Cron-like)
Maps to: UNCalendarNotificationTrigger with repeat: true, which is timezone aware BUT because the repeat argument is hardcoded to true, cannot be used to schedule one time notifications.

3. schedule.every

Use: Schedule a notification which repeats at a fixed time interval
Maps to: UNTimeIntervalNotificationTrigger

Suggestions

A. Modify schedule.at implementation code path

In order to allow for punctual, time-zone sensitive notifications, we could map schedule.at to UNCalendarNotificationTrigger. We would lose the support for repeats when scheduling with at, but if I read the code correctly, the current implementation is wrong anyway - it schedules a repeated notification at an interval between now and the schedule.at date, which does not make much sense. Of course, we could always fallback to UNTimeIntervalNotificationTrigger if repeats=true to guarantee backwards compatibility.

B. Modify schedule.on implementation code path

Another way to go about it would be to honor the repeat parameter in the schedule.on code path, but this would break behavior for existing implementations which expect schedule.on to always repeat, cron-like.

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Happy to discuss further and to work on a PR. From what I understand of the current implementation, option A is perfectly viable.