How does iCloud work when logged in on someone else's device

fs_tigre wrote:

It now starting to make sense. The reason for the main question was to confirm that this wouldn't work since I'm actually the developer of an app in the App Store which does NOT support Sharing only iCloud/CloudKit storage. I was under the impression that all data would sync just by downloading the app with the same Apple ID on multiple devices, but it doesn't seem to be the case, I tried it, and the data only syncs on devices where using the same iCloud account but doesn't sync where other users are logged in with their own iCloud account. Thanks

iCloud data is stored and accessible per Apple ID until and unless the app itself enables sharing.

Mixing Apple IDs means mixing what’s stored locally in app-local databases and/or cached locally in “Other” storage and that then gets uploaded and merged with iCloud, and mixing what is hosted in iCloud and that then gets downloaded and merged locally.

Sync across multiple users is trickier to create, and is not the default. The developer (you) need to enable CloudKit sharing. Alternatively, there are some discussions of this for third-party sync solutions used by and posted by various developers posted around the net, too. Those for Panic sync, for instance.

When sharing an Apple ID, and if not exceedingly careful, your data gets uploaded merged with theirs, their data merged with yours, and a right fine mess will be had by all. Things can get much worse from there, too.

Usual sequence here for an app would be a discussion with the app developer about their user of Family Sharing or some other shared storage.

Or you can, for instance, use the built-in Reminders app to share:

… Share lists and collaborate in Reminders on iPhone - Apple Support

Notes too can be shared:

… Share notes and collaborate on iPhone - Apple Support

And sharing with Pages, Numbers, and Keynote, too:

… Collaborate on documents in Pages, Numbers, and Keynote on iPhone, iPad, and Mac - Apple Support

Sharing an Apple ID, however, does not end well.

Maybe. If the app offers sharing then you should be able to do so. Some apps offer collaboration too so you can both make edits. Some users simply use reminders or notes to share.

If you use the same Apple ID on multiple device, all data will sync between the devices; not a good idea.

fs_tigre Said:

"How does iCloud work when logged in on someone else's deviceL Hi, can someone please help me understand how iCloud works? I'm specifically interested in understanding how iCloud works when logged in on someone else's device. For instance, let's say I have an app that lets you sync data between multiple devices[...]"

Accessing Data On Another's Device:

Is this just a onetime occurrence? Or is this to occur for a long time to come? Do not grant Account Access on a device that is not yours, for a one time occurrence.

A. Using iCloud Drive:

If a one time occurrence, then just use iCloud Drive. Go here: https://www.icloud.com/iclouddrive

B. Using Family Sharing:

If this is to office for a time time to come, then use Family Sharing. Go Here: Set Up Family Sharing - Apple Support

fs_tigre Said:

"Just to be clear, this is an app that I really want to share data with let's say my wife, and let's say is a shopping app where I want her to be able to modify the entire data from her device, this way we both see the same thing, is this possible?"

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Thank you for Clarifying:

Yes. This is possible. The only option whey would perform the formatting on their own device. You cannot have remote sharing of your screen. Then, have them hand it over to you. If it is large in size, then use Family Sharing to access it. Remove them from Family Sharing, once completed. Go Here: Set Up Family Sharing - Apple Support

Just for the Note:

You'd be shocked by how many participants on these forums report that someone they granted access to their account actually took advantage of it. So, keep the credentials to yourself, and do not share them. Uploading to a server is wise, but, if only two people, Family Sharing is what is best to perform here.

You are welcome.