精通iOS的Core Data：数据存储与持久化引擎

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"Pro Core Data for iOS, 2nd Edition" 本书是针对iOS开发者的深入指南，专注于使用核心数据(Core Data)这一苹果提供的数据存储和检索技术。随着iOS平台应用开发的日益火爆，开发者们越来越意识到在设备上持久化数据的重要性。这本书面向已掌握基本iOS开发技能的读者，旨在帮助他们提升应用的品质，将应用从不错提升到优秀。在"Getting Started"章节中，作者引导读者进入Core Data的世界，介绍如何设置项目并开始使用Core Data框架。这一部分通常包括创建新项目、导入Core Data框架和配置数据模型的初步步骤。 "Understanding Core Data"章节深入讲解Core Data的本质，解释它是如何作为对象图管理系统工作的，以及它与SQLite数据库的关系。它会涵盖实体、属性、关系等核心概念，帮助开发者理解Core Data的数据模型设计。 "Storing Data: SQLite and Other Options"章节探讨了Core Data如何使用SQLite作为默认的后台存储，同时也会讨论其他可能的数据存储选项，如In-Memory存储或云同步策略。 "Creating a Data Model"章节详细介绍了如何设计和创建数据模型，包括定义实体、属性和关系，以及如何利用Xcode的图形界面工具来完成这些工作。 "Working with Data Objects"章节则关注于如何操作和管理NSManagedObject子类，包括创建、检索、更新和删除数据对象，以及处理对象生命周期中的各种事件。 "Refining Result Sets"章节探讨如何通过查询优化来获取和过滤数据，使用NSPredicate和NSSortDescriptor来定制查询结果。 "Tuning Performance and Memory Usage"章节关注性能调优，讲解如何减少内存消耗，提高数据访问速度，以及如何利用缓存策略来提升用户体验。 "Versioning and Migrating Data"章节讲解如何在应用升级时处理数据模型的变化，包括轻量级迁移和重置映射等数据迁移策略。 "Managing TableViews Using a Fetched Results Controller"章节介绍了如何结合使用Fetched Results Controller与UITableView，实现动态加载和刷新表格数据，确保流畅的用户界面体验。 "Using Core Data in Advanced Applications"章节涵盖了更高级的主题，如后台上下文、多线程处理以及如何在Core Data与其他框架（如CloudKit或网络API）之间集成。本书索引和参考资源齐全，适合有经验的iOS开发者深入学习和查阅。通过阅读和实践书中的示例，读者可以掌握Core Data的各个方面，从而在自己的应用中实现高效且可靠的数据管理。

Chapter

Getting Started

If you misread this book’s title, thought it discussed and deciphered core dumps, and

hope it will help you debug a nasty application crash, you got the wrong book. Get a

debugger, memory tools, and an appointment with the optometrist. Otherwise, you

bought, borrowed, burglarized, or acquired this book somehow because you want to

better understand and implement Core Data in your iOS applications. You got the right

book.

You might read these words from a paper book, stout and sturdy and smelling faintly of

binding glue. You might digitally flip through these pages on a nook, iPad, Kindle, Sony

Reader, Kobo eReader, or some other electronic book reader. You might stare at a

computer screen, whether on laptop, netbook, or monitor, reading a few words at a time

while telling yourself to ignore your Twitter feed rolling CNN-like along the screen’s

edge. As you read, you know that not only can you stop at any time but that you can

resume at any time. Any time you want to read this book, you can pick it up. If you

marked the spot where you were last reading, you can even start from where you last

stopped. We take this for granted with books.

Users take it for granted with applications.

Users expect to find their data each time they launch their applications. Apple’s Core

Data framework helps you ensure that they will. This chapter introduces you to Core

Data, explaining what it is, how it came to be, and how to build simple Core Data–based

applications for iOS. This book walks through the simplicity and complexities of Core

Data. Use the information in the book to create applications that store and retrieve data

reliably and efficiently so that users can depend on their data. Code carefully, though—

you don’t want to write buggy code and have to deal with nasty application crashes.

What Is Core Data?

When people use computers, they expect to preserve any progress they make toward

completing their tasks. Saving progress, essential to office software, code editors, and

games involving small plumbers, is what programmers call persistence. Most software

requires persistence, or the ability to store and retrieve data, so that users don’t have to

CHAPTER 1: Getting Started

reenter all their data each time they use their applications. Some software can survive

without any data storage or retrieval; calculators, carpenter’s levels, and apps that make

annoying or obscene sounds spring to mind. Most useful applications, however,

preserve some state, whether configuration-oriented data, progress toward achieving

some goal, or mounds of related data that users create and care about. Understanding

how to persist data to iDevices is critical to most useful iOS development.

Apple’s Core Data provides a versatile persistence framework. Core Data isn’t the only

data storage option, nor is it necessarily the best option in all scenarios, but it fits well

with the rest of the Cocoa Touch development framework and maps well to objects.

Core Data hides most of the complexities of data storage and allows you to focus on

what makes your application fun, unique, or usable.

Although Core Data can store data in a relational database (such as SQLite), it is not a

database engine. It doesn’t even have to use a relational database to store its data.

Though Core Data provides an entity-relationship diagramming tool, it is not a data

modeler. It isn’t a data access layer like Hibernate, though it provides much of the same

object-relational mapping functionality. Instead, Core Data wraps the best of all these

tools into a data management framework that allows you to work with entities,

attributes, and relationships in a way that resembles the object graphs you’re used to

working with in normal object-oriented programming.

Early iPhone programmers didn’t have the power of the Core Data framework to store

and retrieve data. The next section shows you the history behind persistence in iOS.

History of Persistence in iOS

Core Data evolved from a NeXT technology called Enterprise Objects Framework (EOF)

by way of WebObjects, another NeXT technology that still powers parts of Apple’s web

site. It debuted in 2005 as part of Mac OS X 10.4 (“Tiger”), but didn’t appear on iPhones

until version 3.0 of the SDK, released in June 2009. Before Core Data, iPhone

developers had the following options in terms of persistence:

 Use property lists, which contain nested lists of key/value pairs of

various data types.

 Serialize objects to files using the SDK’s NSCoding protocol.

 Take advantage of the iPhone’s support for the relational database

SQLite.

 Persist data to the Internet cloud.

Developers used all these mechanisms for data storage as they built the first wave of

applications that flooded Apple’s App Store. Each one of these storage options remains

viable, and developers continue to employ them as they build newer applications using

newer SDK versions.

None of these options, however, compares favorably to the power, ease of use, and

Cocoa-fitness of Core Data. Despite the invention of frameworks like FMDatabase or

CHAPTER 1: Getting Started

ActiveRecord to make dealing with persistence on iOS easier in the pre–Core Data days,

developers gratefully leapt to Core Data when it became available.

Although Core Data might not solve all persistence problems best and you might solve

some of your persistence scenarios using other means like the options listed earlier,

you’ll turn to Core Data more often than not. As you work through this book and learn

the problems that Core Data solves and how elegantly it solves them, you’ll likely use

Core Data any time you can. As new persistence opportunities arise, you won’t ask

yourself, “Should I use Core Data for this?” but rather, “Is there any reason not to use

Core Data?”

The next section shows you how to build a basic Core Data application using Xcode’s

project templates. Even if you’ve already generated an Xcode Core Data project and

know all the buttons and check boxes to click, don’t skip the next section. It explains

the Core Data–related sections of code that the templates generate and forms a base of

understanding on which the rest of the book builds.

Creating a Basic Core Data Application

The many facets, classes, and nuances of Core Data merit artful analysis and deep

discussions to teach you all you need to know to gain mastery of Core Data’s

complexities. Building a practical foundation to support the theory, however, is just as

essential to mastery. This section builds a simple Core Data–based application using

one of Xcode’s built-in templates and then dissects the most important parts of its Core

Data–related code to show what they do and how they interact. At the end of this

section, you will understand how this application interacts with Core Data to store and

retrieve data.

Understanding the Core Data Components

Before building this section’s basic Core Data application, you should have a high-level

understanding of the components of Core Data. Figure 1–1 illustrates the key elements

of the application you will build in this section. Review this figure for a bird’s-eye view of

what this application accomplishes, where all its pieces fit, and why you need them.

As a user of Core Data, you should never interact directly with the underlying persistent

store. One of the fundamental principles of Core Data is that the persistent store should

be abstracted from the user. A key advantage of that is the ability to seamlessly change

the backing store in the future without having to modify the rest of your code. You

should try to picture Core Data as a framework that manages the persistence of objects

rather than thinking about databases. Not surprisingly, the objects managed by the

framework must extend NSManagedObject and are typically referred to as, well, managed

objects. Don’t think, though, that the lack of imagination in the naming conventions for

the components of Core Data reveals an unimaginative or mundane framework. In fact,

Core Data does an excellent job at keeping all the object graph interdependencies,

optimizations, and caching in a predictable state so that you don’t have to worry about

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