Add ready for use RRule instance with caching

[polRk]

Add RRule ready for use instance with result caching

Checklist

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

[polRk]

I do not know how to cache after method without recurrence rule until value. Can u help me?

[polRk]

I'm inspired by this repository https://github.com/jakubroztocil/rrule

[JonasWanke]

Thanks for this contribution! Caching seems like a really useful feature as the internal calculations in this package can get quite complex.

I have a few questions/comments/suggestions about the architecture of this caching:

• Having a class called RRule (the common abbreviation for Recurrence Rule) next to the class RecurrenceRule seems a bit confusing. Could we either integrate this functionality directly in RecurrenceRule or maybe rename RRule to CachingRecurrenceRule?

• rrule.js's before and after only return a single value (the first event before/after the given instant) as opposed to all values before/after that instant, but I think that the latter is useful as well. (And if we need the former in the future, we can still add methods calculating and returning only the single next event.) However, I'm not sure whether we really need three separate methods (before, after, and between). Maybe we can combine those into a single method with optional before and after parameters?

  List<DateTime> getAllInstances({
    required DateTime start, // Only necessary if this is integrated into `RecurrenceRule` directly.
    DateTime? after,
    bool includeAfter = false,
    DateTime? before,
    bool includeBefore = false,
  }) {/* ... */}

• Currently, a hash of all relevant parameters (rrule, start, and method arguments) is used as the cache key. While hash functions try to minimize the risk of collisions, those are not impossible. Hence, I recommend creating a utility class like _CacheKey which stores those arguments directly and, as the name suggests, is used as the cache key. And as the cache is currently stored per recurrence rule, the rrule doesn't even have to be part of this key.

• RRule accepts an initialCache, similar to rrule.js. Do you know of any use-cases for supplying that initial cache state, especially since the cache itself is not made public (and hence can't be persisted otherwise by users of this package)?

[polRk]

RRule accepts an initialCache, similar to rrule.js. Do you know of any use-cases for supplying that initial cache state, especially since the cache itself is not made public (and hence can't be persisted otherwise by users of this package)?

This can reduce the cold start. If I have 1000 recurring tasks, it will be better to use cached values rather than calculated ones.

[JonasWanke]

This looks great! I've added some minor comments, and then this can get merged :)

(You can ignore the failing lint actions.)

[JonasWanke]

This is now published as part of v0.2.3. Thanks for contributing!