Support for iCloud KV Storage

[KeithBird]

Support for iCloud KV Storage

Motivation

NSUbiquitousKeyValueStore enables developers to seamlessly store and sync app preferences, configuration, and state data across all devices linked to a user's iCloud account using lightweight key-value storage. It would be beneficial to add support for this feature to enable swift-sharing, addressing the limitations of @AppStorage in SwiftUI.

Implementation

The interface for NSUbiquitousKeyValueStore is similar to UserDefaults:

@Shared(.iCloudKV("isOn")) var isOn = true

Any changes made to this value will be synchronized to iCloud key-value storage. Further, any change made to the "isOn" key in iCloud will be immediately played back to the @Shared value, as demonstrated in this test:

@Test
func externalWrite() {
  NSUbiquitousKeyValueStore.default.removeObject(forKey: "isOn")

  @Shared(.iCloudKV("isOn")) var isOn = true
  #expect(isOn == true)
  NSUbiquitousKeyValueStore.default.set(false, forKey: "isOn")
  NSUbiquitousKeyValueStore.default.synchronize()
  #expect(isOn == false)
}

The implementation of the iCloudKV strategy will be similar to AppStorageKey.swift, but it will replace UserDefaults with NSUbiquitousKeyValueStore.

Limitations

• NSUbiquitousKeyValueStore has a size limit of 1MB total for all key-value pairs
• Key length is limited to 64 bytes
• The developer needs to enable the iCloud capability.

We will address the above limitations by warning the developer with IssueReporting.

[KeithBird]

If this seems worth implementing for the library, are there any issues with the proposal? If it looks good to you, I would like to begin the implementation. @stephencelis @mbrandonw

[stephencelis]

@KeithBird I think what you've sketched out sounds good, but there's no need for it to be a core strategy. It sounds like the perfect opportunity for a downstream plugin, and we'd be happy to link to your package in the Sharing README 😄

[KeithBird]

I've completed the plugin! Writing test cases for iCloud is challenging, but I tested it on my physical devices (Mac, iPhone, and Apple Watch), and it works well in my released apps.
Could you please review the iCloudKVKey file when you have a chance? I'm new to open source and would appreciate your feedback. @stephencelis