"isWritableFileAtPath" returns "true" for files in a shared readonly iCloud Drive folder

Hello,

My app supports to open file via UIDocumentPickerViewController, and the app needs to detect if the opened file is writable.

We are using the "isWritableFileAtPath" method to detect if the file is writable, but iCloud Drive behaviors strangly:

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Case one:

Sharing a file directly with readonly permission from iCloud account A to iCloud account B, "isWritableFileAtPath" return "false" on the device of the B user - that's what we expected.

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Case two:

Sharing a FOLD with readonly permission from iCloud account A to iCloud account B, "isWritableFileAtPath" for a file in the shared folder returns "true" on the device of the B user - that's NOT what we expected.

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Would you please help us on it?

- Is it a bug of the iCloud Drive?

- Or if there is another way to detect if the app has permission to write the file opened using UIDocumentPickerViewController ?

Thanks,

Philip