[Kde-pim] Recurrence Exception handling

[Christian Mollekopf]

Hey,

Since Dan Vratil and me have hit the same Problem more or less simultaneously 
for the Google-Resource respectively the Kolab-Resource, I'd like to share our 
plans with this specific feature of iCal.

The problematic feature, which isn't fully supported by KCalCore so far, is an 
exception to an event.
This feature allows to override a specific occurrence of a recurring event 
with a new event, allowing to change any/all of it's properties on that 
specific date.

In iCal this exception is represented by an additional event, with an 
identical UID and an additional RecurrenceID, which contains the DTSTART of 
the occurrence to override.

As this breaks the basic assumption that UID's are unique, kcalcore doesn't 
like it and can't deal with it properly. KOrganizer simply creates a new event 
and excludes the date from the original event, effectively loosing the 
connection of the exception to the main recurring event.

My suggestion to fix this shortcoming would be to an exception-property to the 
KCalCore::Incidence class, which holds a list of KCalCore::Incidence's. This 
doesn't reflect how iCalendar models it (in iCal the exception is next to the 
event and not contained), but I think a containment makes much more sense for 
the in-memory representation. This way deleting an event automatically also 
removes the corresponding exceptions. Further this way we don't have to care 
about the UID's, we can simply generate special UID's for the exceptions which 
are unique as well.

As this requires new API in kdepimlibs and we have to use that API from 
kdepim-runtime, I suppose this fix has to wait for 4.12 (4.11 for the 
kdepimlibs fix).

As temporary workaround we came up with the following solution, which can also 
be backported to 4.10 (e.g. for 4.10.1).
* We create a new event for each exception, and assign it a new UID with the 
pattern: UID_RECURRENCEID
* We exclude that specific date (RecurrenceID) on the original event.
* The original UID is stored in an attribute (Dan created a patch where it's 
stored in an akonadi attribute, but we could also use a KCalCore custom value)
* We additionally store the original exception-dates of the main event, so we 
can remove the additional date-exclusion again when writing the values back to 
the server.

This way we can implement the feature transparently to korganizer, which 
should work mostly fine, as exception and event can be modified from 
korganizer, although korganizer has no comprehension of the link between 
exception and original event. There are some glitches, such as the resource 
automatically deleting the exception after the main event has been deleted, 
but that's still better than just breaking or dismissing the feature 
alltogether.

Dan already prepared a patch for the google resource, I will then do the same 
for the kolab-resource, and possibly the ical serialization.

If you have any input or concerns, let us know.

Cheers,
Christian

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