iCloud is a free service that lets users access their personal content on all their devices—wirelessly and automatically via Apple ID. iCloud does this by combining network-based storage with dedicated APIs, supported by full integration with the operating system. Apple provides server infrastructure, backup, and user accounts, so you can focus on building great iCloud-enabled apps.
iCloud 是一项让用户通过他们的AppleID在他们的无线设备自动存取数据的服务。iCloud使用操作系统完全集成的网络存储和专用API实现这个服务。Apple提供了服务器基础构建、备份和用户账户系统，你只需集中于开发基于iCloud的应用。
There are three iCloud storage services: key-value storage, document storage, and CloudKit. The core idea behind key-value and document storage is to eliminate explicit synchronization between devices. A user never needs to think about syncing, and your app never interacts directly with iCloud servers. When you adopt these APIs as described in this document, changes appear automatically on all the devices attached to an iCloud account. Your users get safe, consistent, and transparent access to their personal content everywhere.
CloudKit allows you to store app and user data as records in a public database, that is shared between users of your app, or a private database accessible only by the current user. However, it’s your responsibility to determine when to fetch and save records. Because the data is shared, your app also needs to keep local records synchronized. For native apps, CloudKit provides the CloudKit framework, and for web apps, the CloudKit JS library and web services to access these databases.
