Building a Shopping List Application With CloudKit: Introduction

Back in 2012, Apple introduced iCloud alongside iOS 5. At the same time, the company announced that developers would have access to iCloud through a number of APIs. At first, developers had three options:

These APIs aren't perfect, though. A major shortcoming is their lack of transparency. Core Data integration, in particular, has led to frustration and confusion among even the most experienced developers. When something goes wrong, developers have no idea what or who the culprit is. It could be a problem in their code or in Apple's.

CloudKit

At WWDC 2014, Apple introduced CloudKit, a brand new framework that directly interacts with Apple's iCloud servers. The framework is comparable to a number of PaaS (Platform as a Service) solutions, such as Firebase. Like Firebase, Apple provides a flexible API and a dashboard that offers developers a peek into the data stored on Apple's iCloud servers.

What I like most about CloudKit is Apple's own commitment to the framework. According to the company, iCloud Drive and iCloud Photo Library are built on top of CloudKit. This shows that the CloudKit framework and its infrastructure are robust and reliable.

As a developer, this sign of trust and commitment is important. In the past, Apple occasionally released APIs that were plagued by bugs or lacking key features simply because the company wasn't eating its own dog food. This isn't true for CloudKit. And that is promising.

Should You Use CloudKit?

Key-value storage and document storage have their uses, and Apple emphasizes that CloudKit doesn't replace or deprecate existing iCloud APIs. The same is true for Core Data. CloudKit doesn't offer local storage, for example. This means that an application running on a device without a network connection is pretty much useless if it solely relies on CloudKit.

Apple also emphasizes that error handling is critical when working with CloudKit. If a save operation fails, for example, and the user isn't notified, then she may not even know that her data wasn't saved—and lost.

CloudKit is a great solution for storing structured as well as non-structured data in the cloud. If you need a solution to access data on multiple devices, then CloudKit is certainly an option to consider.

At the 2015 WWDC, Apple did what few developers had expected or hoped for. It announced a web service for CloudKit. This means that CloudKit can be used on virtually any platform, including Android and Windows Phone.

Apple's pricing is pretty competitive too. Getting started with CloudKit is free, and it remains free for most applications. Again, CloudKit is certainly worth considering if you plan to store data in the cloud.

CloudKit Concepts

Developers struggling with Core Data are often unfamiliar with the building blocks of the framework. If you don't take the time to learn about and understand the Core Data stack, then you will inevitably run into problems. The same is true for CloudKit.

Before we start working on a sample application that uses CloudKit, I want to spend a few minutes introducing you to a number of key concepts of the CloudKit framework and infrastructure. Let's start with containers, databases, and sandboxing.

Privacy and Containment

Apple makes it very clear that privacy is an important aspect of CloudKit. The first thing to know is that each application has its own container in iCloud. This concept is very similar to how iOS applications each have their own sandbox. However, it is possible to share a container with other applications as long as those applications are associated with the same developer account. As you can imagine, this opens up a number of interesting possibilities for developers.

A CloudKit container contains several databases. Each container has one public database that can be used to store data that is accessible to every user of your application. In addition to the public database, a container also contains a private database for each user of your application. The user's private database is used to store data that is specific to that particular user. Data segregation and encapsulation is a key component of the CloudKit and iCloud infrastructure.

Even though an application's container can hold many databases, from a developer's perspective a container holds only two databases: the public database and the private database of the user that is currently signed in to their iCloud account. As of 2017, Apple introduced a third database, shared database, providing apps with the ability to share a subset of records that is shared on another user's private database, inviting them to contribute to those exposed records. 

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Doron Katz