Enable external calendar read-write.

[smallandsimple]

Is your feature request related to a problem? Please describe.
It is frustrating that external calendars can only be added for viewing. For my use case, that functionality limitation impedes my family's entire ability to use the Nextcloud Calendar app

Describe the solution you'd like
I would like to see an option implemented to add external calendars via a link that will enable calendar synchronization with an external CalDAV server. This would extend the functionality of the NextCloud app as a true one-stop-shop calendar where a user could both create and have full access to any calendars they need, including existing calendars from other services.

Describe alternatives you've considered
Honestly, I can't think of any alternatives that would provide the needed functionality. I have already attempted using various third party applications and displaying them as 'external sites', however, so far, none provide the type of functionality that such a feature would enable if added to the Nextcloud Calendar. Aside from that, using the external sites requires an additional login, where added functionality could maintain saved credentials.

Additional context
Here is an idea of what it might look like in the interface:

[dagguh]

This is an important step when gradually de-Googling your personal life, family, friends and work.

[silopolis]

That'd surely be a major enhancement !

[miguelarios]

Yes! I want Nextcloud to be my calendar client just as it can be my mail client.

[ant0nwax]

I vote +1 if I may vote here, otherwise I would not be able to use nextcloud easily for mail AND appointments AND accept them

a calendar nowadays runs on the same system like the mail system imho

i will anyway make tries to use the internal nextcloud calendar instead of the one which is running and paired with the mail system inbox

[rasos]

SabreDAV also has a php library for a WebDAV/CalDAV/DardDAV client. This may be a starting point for nextcloud acting not only as a CalDAV server, but also as a client, even if the code on github seems quite outdated.

Their more actual serializers and deserializers also can be used both for the client and server use-cases. For example:
https://github.com/sabre-io/dav/blob/master/lib/DAV/Xml/Response/MultiStatus.php

[rasos]

We might try to re-use parts of the Calcom's CalDav server code https://github.com/calcom/cal.com

Cal.com is a service for appointments handling and can play the client role for several calendar services.

Cal.com code is GPLv3 and uses:

• Next.js with TypeScript
• React
• Tailwind
• Prisma

Code to handle CalDAV calendar is in calcom/packages/app-store/caldavcalendar and in calcom/packages/lib/CalendarService.ts

[tcitworld]

That's not especially helpful, but thanks anyway.

The Sabre DAV client is already used at Nextcloud for pretty much the same things with WebDAV (basis of the federation feature) and the address book sync. It will have to get extended before anyway to support the REPORT WebDAV method.

As this issue is a required basis for Calendar sharing federation nextcloud/server#1440, I'm starting to look at it more closely, but there's really a lot to do. :)

[rasos]

The tsdav library might be another candidate to fetch calendar events via CalDav into nextcloud. Has been tested with various cloud providers.

[battosai30]

+1 for this feature.