使用Xcode 4.2和iOS SDK 5的Core Data实战

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“Pro Core Data for IOS”是一本专注于在iOS平台上使用Core Data框架的专业书籍，特别针对Xcode 4.2和iOS SDK 5进行了全面更新。本书旨在帮助开发者掌握如何利用Core Data来存储和管理应用程序的数据。 Core Data是苹果提供的一种数据管理框架，它简化了iOS应用中数据的持久化。这本书共分为10章，详细介绍了从入门到高级的Core Data使用技巧： 1. 第一章：开始使用 - 介绍如何启动一个Core Data项目，设置Xcode工程，并创建第一个数据模型。 2. 第二章：理解Core Data - 深入探讨Core Data的概念，包括实体（Entities）、属性（Attributes）、关系（Relationships）以及上下文（Context）。 3. 第三章：存储数据：SQLite和其他选项 - 讨论了Core Data如何使用SQLite作为默认的后台存储，以及如何选择和配置其他数据存储方案。 4. 第四章：创建数据模型 - 阐述如何设计和构建数据模型，包括定义实体、属性和关系，以及处理版本控制。 5. 第五章：操作数据对象 - 教导读者如何实例化、查询、修改和删除Core Data中的数据对象。 6. 第六章：细化结果集 - 介绍如何使用NSPredicate来筛选和排序数据，以及如何执行复杂的查询。 7. 第七章：性能与内存使用优化 - 提供关于提升Core Data性能和有效管理内存的策略。 8. 第八章：版本控制与数据迁移 - 讲解如何处理数据模型的版本更新，以及如何平滑地迁移用户数据。 9. 第九章：使用FetchedResultsController管理表格视图 - 展示如何利用FetchedResultsController来动态地更新UITableView，实现高效的表格数据展示。 10. 第十章：在高级应用中使用Core Data - 探讨在复杂应用场景下如何有效利用Core Data，包括多线程和分布式数据管理。本书适合已经掌握iOS开发基础知识的开发者阅读，通过学习，他们将能够充分利用Core Data的强大功能，创建出具备高效数据管理的iOS应用程序。书中的索引和目录结构使得读者可以快速定位到感兴趣的主题，而作者的经验分享则有助于开发者避免常见陷阱，提升开发效率。

CHAPTER 1: Getting Started

The implementation of the MasterViewController, found in MasterViewController.m,

shows how to interact with the Core Data framework to store and retrieve data. The

MasterViewController implementation provides an explicit getter for the

fetchedResultsController property that preconfigures it to fetch data from the data store.

The first step in creating the fetch controller consists of creating a request that will retrieve

Event entities, as shown in this code from the fetchedResultsController accessor:

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];

NSEntityDescription *entity = [NSEntityDescription entityForName:@"Event"

inManagedObjectContext:self.managedObjectContext];

[fetchRequest setEntity:entity];

The result of the request can be ordered using the sort descriptor from the Cocoa

Foundation framework. The sort descriptor defines the field to use for sorting and

whether the sort is ascending or descending. In this case, you sort by descending

chronological order, like so:

NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:

@"timeStamp" ascending:NO];

NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];

[fetchRequest setSortDescriptors:sortDescriptors];

Once you define the request, you can use it to construct the fetch controller. Because

the MasterViewController implements NSFetchedResultsControllerDelegate, it can be

set as the NSFetchedResultsController’s delegate so that it is automatically notified as

the result set changes and so that it updates its view appropriately. You could get the

same results by invoking the executeFetchRequest of the managed object context, but

you would not benefit from the other advantages that come from using the

NSFetchedResultsController such as the seamless integration with the UITableView, as

you’ll see later in this section and in Chapter 9. Here is the code that constructs the

fetch controller:

NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController

alloc] initWithFetchRequest:fetchRequest managedObjectContext:

self.managedObjectContext sectionNameKeyPath:nil cacheName:@"Master"];

aFetchedResultsController.delegate = self;

self.fetchedResultsController = aFetchedResultsController;

NOTE: You may have noticed that the initWithFetchRequest shown earlier uses a

parameter called cacheName. You could pass nil for the cacheName parameter to prevent

caching, but naming a cache indicates to Core Data to check for a cache with a name matching

the passed name and see whether it already contains the same fetch request definition. If it does

find a match, it will reuse the cached results. If it finds a cache entry by that name but the

request doesn’t match, then it is deleted. If it doesn’t find it at all, then the request is executed,

and the cache entry is created for the next time. This is obviously an optimization that aims to

prevent executing the same request over and over. Core Data manages its caches intelligently so

that if the results are updated by another call, the cache is removed if affected.

CHAPTER 1: Getting Started

Finally, you tell the controller to execute its query to start retrieving results. To do this,

use the performFetch method.

NSError *error = nil;

if (![self.fetchedResultsController performFetch:&error]) {

NSLog(@"Unresolved error %@, %@", error, [error userInfo]);

abort();

}

You can see the entire getter method for fetchedResultsController in Listing 1–1.

Listing 1–1. The Entire Getter Method for fetchedResultsController

- (NSFetchedResultsController *)fetchedResultsController

{

if (__fetchedResultsController != nil)

{

return __fetchedResultsController;

}

Set up the fetched results controller.

// Create the fetch request for the entity.

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];

// Edit the entity name as appropriate.

NSEntityDescription *entity = [NSEntityDescription entityForName:@"Event"

inManagedObjectContext:self.managedObjectContext];

[fetchRequest setEntity:entity];

// Set the batch size to a suitable number.

[fetchRequest setFetchBatchSize:20];

// Edit the sort key as appropriate.

NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"timeStamp"

ascending:NO];

NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];

[fetchRequest setSortDescriptors:sortDescriptors];

// Edit the section name key path and cache name if appropriate.

// nil for section name key path means "no sections".

NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController

alloc] initWithFetchRequest:fetchRequest managedObjectContext:self.managedObjectContext

sectionNameKeyPath:nil cacheName:@"Master"];

aFetchedResultsController.delegate = self;

self.fetchedResultsController = aFetchedResultsController;

NSError *error = nil;

if (![self.fetchedResultsController performFetch:&error])

{

Replace this implementation with code to handle the error appropriately.

abort() causes the application to generate a crash log and terminate. You should

not use this function in a shipping application, although it may be useful during

development. If it is not possible to recover from the error, display an alert panel

that instructs the user to quit the application by pressing the Home button.

CHAPTER 1: Getting Started

NSLog(@"Unresolved error %@, %@", error, [error userInfo]);

abort();

}

return __fetchedResultsController;

}

NSFetchedResultsController behaves as a collection of managed objects, similar to an

NSArray, which makes it easy to use. In fact, it exposes a read-only property called

fetchedObjects that is of type NSArray to make things even easier to access the objects

it fetches. The MasterViewController class, which also extends UITableViewController,

demonstrates just how suited the NSFetchedResultsController is to manage the table’s

content.

Inserting New Objects

A quick glance at the insertNewObject: method shows how new events (the managed

objects) are created and added to the persistent store. Managed objects are defined by

the entity description from the data model and can live only within a context. The first

step is to get a hold of the current context as well as the entity definition. In this case,

instead of explicitly naming the entity, you reuse the entity definitions that are attached

to the fetched results controller:

NSManagedObjectContext *context = [self.fetchedResultsController managedObjectContext];

NSEntityDescription *entity = [[self.fetchedResultsController fetchRequest] entity];

Now that you’ve gathered all the elements needed to bring the new managed object to

existence, you create the Event object and set its timeStamp value.

NSManagedObject *newManagedObject = [NSEntityDescription

insertNewObjectForEntityForName:[entity name] inManagedObjectContext:context];

[newManagedObject setValue:[NSDate date] forKey:@"timeStamp"];

The last step of the process is to tell Core Data to save changes to its context. The

obvious change is the object you just created, but keep in mind that calling the save.

method will also affect any other unsaved changes to the context.

NSError *error = nil;

if (![context save:&error]) {

NSLog(@"Unresolved error %@, %@", error, [error userInfo]);

abort();

}

The complete method for inserting the new Event object is shown in Listing 1–2.

Listing 1–2. The Complete Method for Inserting the New Event Object

- (void)insertNewObject {

// Create a new instance of the entity managed by the fetched results controller.

NSManagedObjectContext *context = [self.fetchedResultsController

managedObjectContext];

NSEntityDescription *entity = [[self.fetchedResultsController fetchRequest] entity];

NSManagedObject *newManagedObject = [NSEntityDescription

insertNewObjectForEntityForName:[entity name] inManagedObjectContext:context];

CHAPTER 1: Getting Started

// If appropriate, configure the new managed object.

// Normally you should use accessor methods, but using KVC here avoids the need to add

a custom class to the template.

[newManagedObject setValue:[NSDate date] forKey:@"timeStamp"];

// Save the context.

NSError *error = nil;

if (![context save:&error]) {

NSLog(@"Unresolved error %@, %@", error, [error userInfo]);

abort();

}

Initializing the Managed Context

Obviously, none of this can happen without initializing the managed context first. This is

the role of the application delegate. In a Core Data–enabled application, the delegate

must expose three properties.

@property (readonly, strong, nonatomic) NSManagedObjectContext *managedObjectContext;

@property (readonly, strong, nonatomic) NSManagedObjectModel *managedObjectModel;

@property (readonly, strong, nonatomic) NSPersistentStoreCoordinator

*persistentStoreCoordinator;

Note that they are all marked as read-only, which prevents any other component in the

application from setting them directly. A closer look at BasicApplicationAppDelegate.m

shows that all three properties have explicit getter methods.

First, the managed object model is derived from the data model

(BasicApplication.xcdatamodel) and loaded.

- (NSManagedObjectModel *)managedObjectModel

{

if (__managedObjectModel != nil)

{

return __managedObjectModel;

}

NSURL *modelURL = [[NSBundle mainBundle] URLForResource:@"BasicApplication"

withExtension:@"momd"];

__managedObjectModel = [[NSManagedObjectModel alloc]

initWithContentsOfURL:modelURL];

return __managedObjectModel;

}

Then a persistent store is created to support the model. In this case, as well as in most

Core Data scenarios, the persistent store is backed by a SQLite database. The managed

object model is a logical representation of the data store, while the persistent store is

the materialization of that data store.

- (NSPersistentStoreCoordinator *)persistentStoreCoordinator {

if (__persistentStoreCoordinator != nil) {

return __persistentStoreCoordinator;

}

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