Initial commit for CKSyncEngine sample app

Author: Tim Mahoney

.gitignore

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+# macOS
+.DS_Store
+
+# Xcode
+**/*.xcodeproj/project.xcworkspace/*
+.build/
+build/
+*.pbxuser
+!default.pbxuser
+*.mode1v3
+!default.mode1v3
+*.mode2v3
+!default.mode2v3
+*.perspectivev3
+!default.perspectivev3
+xcuserdata
+*.xccheckout
+*.moved-aside
+DerivedData
+*.hmap
+*.ipa
+*.xcuserstate
+*.xcuserdatad
+*.dSYM*

README.md

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+# CloudKit Samples: CKSyncEngine
+
+### Goals
+
+This project demonstrates using `CKSyncEngine` to sync data in an app.
+
+### Prerequisites
+
+* A Mac with [Xcode 15](https://developer.apple.com/xcode/) (or later) installed is required to build and test this project.
+* An iOS device running iOS 17 or later, or Mac running macOS 14 or later, is required to run this app.
+* An active [Apple Developer Program membership](https://developer.apple.com/support/compare-memberships/) is needed to create a CloudKit container and sign the app to run on a device.
+
+**Note**: `CKSyncEngine` relies on remote notifications in order to sync properly. Simulators cannot register for remote push notifications, so running this sample on a real device or Mac is required for this app to properly sync.
+
+### Setup Instructions
+
+1. Ensure you are logged into your developer account in Xcode with an active membership.
+1. In the “Signing & Capabilities” tab of the SyncEngine target, ensure your team is selected in the Signing section, and there is a valid container selected under the “iCloud” section.
+1. Ensure that all devices are logged into the same iCloud account.
+
+#### Using Your Own iCloud Container
+
+* Create a new iCloud container through Xcode’s “Signing & Capabilities” tab of the SyncEngine app target.
+* Update the `CKContainer` in [SyncedDatabase.swift](SyncEngine/SyncedDatabase.swift) with your new iCloud container identifier.
+
+### How it Works
+
+* The main `CKSyncEngine` integration is contained in [SyncedDatabase.swift](SyncEngine/SyncedDatabase.swift).
+* On first launch, the app initializes a `SyncedDatabase`, which syncs a local store with the server.
+* The app’s main UI displays a list of Contacts. When the user adds a new Contact through the UI, this contact is saved to a local store and to the server.
+* Saving the Contact record triggers a push notification, which tells other devices to fetch the record from the server.
+* When other devices fetch the record, they save them to the local store, and the UI shows the new data.
+
+### Example Flow
+
+1. Run the app on a device or Mac. Latest changes are fetched from the server.
+1. Repeat the above on another device and add a new contact through the UI.
+1. The first device fetches the changes and shows the contact in the UI.
+
+### Tests
+
+This project includes a few basic tests for `CKSyncEngine` integration in [SyncTests.swift](Tests/SyncTests.swift). This shows one possible way to test your CloudKit sync code by simulating multiple devices syncing back and forth. The test suite only exercises a few basic scenarios, and there are many more complex scenarios to test in your own application.
+
+### Things To Learn
+
+* Syncing data with `CKSyncEngine`.
+* Adding, deleting, and merging remote changes into a local store, and reflecting those changes live in a UI.
+* Testing your integration with `CKSyncEngine`.
+
+### Further Reading
+
+* [CKSyncEngine Documentation](https://developer.apple.com/documentation/cloudkit/cksyncengine)