详细讲解了 Core Data 的框架以及设计的类，下面我们来讲解一个完全手动编写代码 使用这些类的示例，这个例子来自 苹果官方示例。在这个例子里面，我们打算做这样一件事情：记录程序运行记录（时间与 process id），并保存到xml文件中。我们使用 Core Data 来做这个事情。
示例代码下载：
一，建立一个新的 Mac command-line tool application 工程，命名为 CoreDataTutorial。为支持垃圾主动回收机制，点击项目名称，在右边的 Build Setting 中查找 garbage 关键字，将找到的 Objective-C Garbage Collection 设置为 Required [-fobj-gc-only]。并将 main.m 中 的 main() 方法修改为如下：
[cpp]
int main (int argc, const char * argv[])
{
NSLog(@" === Core Data Tutorial ===");
// Enable GC
//
objc_startCollectorThread();
return 0;
}
在 main() 之前添加如下方法：
[cpp]
NSManagedObjectModel *managedObjectModel()
{
static NSManagedObjectModel *moModel = nil;
if (moModel != nil) {
return moModel;
}
moModel = [[NSManagedObjectModel alloc] init];
// Create the entity
//
NSEntityDescription *runEntity = [[NSEntityDescription alloc] init];
[runEntity setName:@"Run"];
[runEntity setManagedObjectClassName:@"Run"];
[moModel setEntities:[NSArray arrayWithObject:runEntity]];
// Add the Attributes
//
NSAttributeDescription *dateAttribute = [[NSAttributeDescription alloc] init];
[dateAttribute setName:@"date"];
[dateAttribute setAttributeType:NSDateAttributeType];
[dateAttribute setOptional:NO];
NSAttributeDescription *idAttribute = [[NSAttributeDescription alloc] init];
[idAttribute setName:@"processID"];
[idAttribute setAttributeType:NSInteger32AttributeType];
[idAttribute setOptional:NO];
[idAttribute setDefaultValue:[NSNumber numberWithInteger:-1]];
// Create the validation predicate for the process ID.
// The following code is equivalent to validationPredicate = [NSPredicate predicateWithFormat:@"SELF > 0"]
//
NSExpression *lhs = [NSExpression expressionForEvaluatedObject];
NSExpression *rhs = [NSExpression expressionForConstantValue:[NSNumber numberWithInteger:0]];
NSPredicate *validationPredicate = [NSComparisonPredicate
predicateWithLeftExpression:lhs
rightExpression:rhs
modifier:NSDirectPredicateModifier
type:NSGreaterThanPredicateOperatorType
options:0];
NSString *validationWarning = @"Process ID < 1";
[idAttribute setValidationPredicates:[NSArray arrayWithObject:validationPredicate]
withValidationWarnings:[NSArray arrayWithObject:validationWarning]];
// set the properties for the entity.
//
NSArray *properties = [NSArray arrayWithObjects: dateAttribute, idAttribute, nil];
[runEntity setProperties:properties];
// Add a Localization Dictionary
//
NSMutableDictionary *localizationDictionary = [NSMutableDictionary dictionary];
[localizationDictionary setObject:@"Date" forKey:@"Property/date/Entity/Run"];
[localizationDictionary setObject:@"Process ID" forKey:@"Property/processID/Entity/Run"];
[localizationDictionary setObject:@"Process ID must not be less than 1" forKey:@"ErrorString/Process ID < 1"];
[moModel setLocalizationDictionary:localizationDictionary];
return moModel;
}
在上面的代码中：
1）我们创建了一个全局模型 moModel；
2）并在其中创建一个名为 Run 的 Entity，这个 Entity 对应的 ManagedObject 类名为 Run（很快我们将创建这样一个类）；3）给 Run Entity 添加了两个必须的 Property：date 和 processID，分别表示运行时间以及进程 ID；并设置默认的进程 ID 为 -1；4）给 processID 特性设置检验条件：必须大于 0；5）给模型设置本地化描述词典；本地化描述提供对 Entity，Property，Error信息等的便于理解的描述，其可用的键值对如下表：
Key | Value | |
"Entity/NonLocalizedEntityName" | "LocalizedEntityName" | |
"Property/NonLocalizedPropertyName/Entity/EntityName" | "LocalizedPropertyName" | |
"Property/NonLocalizedPropertyName" | "LocalizedPropertyName" | |
"ErrorString/NonLocalizedErrorString" | "LocalizedErrorString" |
三，创建并设置运行时类和对象
由于要用到存储功能，所以我们必须定义持久化数据的存储路径。我们在 main() 之前添加如下方法设置存储路径：
[cpp]
NSURL *applicationLogDirectory()
{
NSString *LOG_DIRECTORY = @"CoreDataTutorial";
static NSURL *ald = nil;
if (ald == nil)
{
NSFileManager *fileManager = [[NSFileManager alloc] init];
NSError *error = nil;
NSURL *libraryURL = [fileManager URLForDirectory:NSLibraryDirectory inDomain:NSUserDomainMask
appropriateForURL:nil create:YES error:&error];
if (libraryURL == nil) {
NSLog(@"Could not access Library directory\n%@", [error localizedDescription]);
}
else
{
ald = [libraryURL URLByAppendingPathComponent:@"Logs"];
ald = [ald URLByAppendingPathComponent:LOG_DIRECTORY];
NSLog(@" >> log path %@", [ald path]);
NSDictionary *properties = [ald resourceValuesForKeys:[NSArray arrayWithObject:NSURLIsDirectoryKey] error:&error];
if (properties == nil)
{
if (![fileManager createDirectoryAtPath:[ald path] withIntermediateDirectories:YES attributes:nil error:&error])
{
NSLog(@"Could not create directory %@\n%@",
[ald path], [error localizedDescription]);
ald = nil;
}
}
}
}
return ald;
}
下面，我们来创建运行时对象：ManagedObjectContext 和 PersistentStoreCoordinator。
[cpp]
NSManagedObjectContext *managedObjectContext()
{
static NSManagedObjectContext *moContext = nil;
if (moContext != nil) {
return moContext;
}
moContext = [[NSManagedObjectContext alloc] init];
// Create a persistent store coordinator, then set the coordinator for the context.
//
NSManagedObjectModel *moModel = managedObjectModel();
NSPersistentStoreCoordinator *coordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:moModel];
[moContext setPersistentStoreCoordinator: coordinator];
// Create a new persistent store of the appropriate type.
//
NSString *STORE_TYPE = NSXMLStoreType;
NSString *STORE_FILENAME = @"CoreDataTutorial.xml";
NSError *error = nil;
NSURL *url = [applicationLogDirectory() URLByAppendingPathComponent:STORE_FILENAME];
NSPersistentStore *newStore = [coordinator addPersistentStoreWithType:STORE_TYPE
configuration:nil
URL:url
options:nil
error:&error];
if (newStore == nil) {
NSLog(@"Store Configuration Failure\n%@", ([error localizedDescription] != nil) ? [error localizedDescription] : @"Unknown Error");
}
return moContext;
}
好，至此万事具备，只欠 ManagedObject 了！下面我们就来定义这个数据对象类。向工程添加 Core Data->NSManagedObject subclass 的类，名为 Run (模型中 Entity 定义的类名) 。
Run.h
[cpp]
#import <CoreData/NSManagedObject.h>
@interface Run : NSManagedObject
{
NSInteger processID;
}
@property (retain) NSDate *date;
@property (retain) NSDate *primitiveDate;
@property NSInteger processID;
@end
[cpp]
//
// Run.m
// CoreDataTutorial
//
// Created by kesalin on 8/29/11.
// Copyright 2011 kesalin@gmail.com. All rights reserved.
//
#import "Run.h"
@implementation Run
@dynamic date;
@dynamic primitiveDate;
- (void) awakeFromInsert
{
[super awakeFromInsert];
self.primitiveDate = [NSDate date];
}
#pragma mark -
#pragma mark Getter and setter
- (NSInteger)processID
{
[self willAccessValueForKey:@"processID"];
NSInteger pid = processID;
[self didAccessValueForKey:@"processID"];
return pid;
}
- (void)setProcessID:(NSInteger)newProcessID
{
[self willChangeValueForKey:@"processID"];
processID = newProcessID;
[self didChangeValueForKey:@"processID"];
}
// Implement a setNilValueForKey: method. If the key is “processID” then set processID to 0.
//
- (void)setNilValueForKey:(NSString *)key {
if ([key isEqualToString:@"processID"]) {
self.processID = 0;
}
else {
[super setNilValueForKey:key];
}
}
@end
三，创建或读取数据对象，设置 其值，保存
好，至此真正的万事具备，我们可以创建或从持久化文件中读取数据对象，设置其值，并将其保存到持久化文件中。本例中持久化文件为 xml 文件。修改 main() 中代码如下：[cpp]
int main (int argc, const char * argv[])
{
NSLog(@" === Core Data Tutorial ===");
// Enable GC
//
objc_startCollectorThread();
NSError *error = nil;
NSManagedObjectModel *moModel = managedObjectModel();
NSLog(@"The managed object model is defined as follows:\n%@", moModel);
if (applicationLogDirectory() == nil) {
exit(1);
}
NSManagedObjectContext *moContext = managedObjectContext();
// Create an Instance of the Run Entity
//
NSEntityDescription *runEntity = [[moModel entitiesByName] objectForKey:@"Run"];
Run *run = [[Run alloc] initWithEntity:runEntity insertIntoManagedObjectContext:moContext];
NSProcessInfo *processInfo = [NSProcessInfo processInfo];
run.processID = [processInfo processIdentifier];
if (![moContext save: &error]) {
NSLog(@"Error while saving\n%@", ([error localizedDescription] != nil) ? [error localizedDescription] : @"Unknown Error");
exit(1);
}
// Fetching Run Objects
//
NSFetchRequest *request = [[NSFetchRequest alloc] init];
[request setEntity:runEntity];
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"date" ascending:YES];
[request setSortDescriptors:[NSArray arrayWithObject:sortDescriptor]];
error = nil;
NSArray *array = [moContext executeFetchRequest:request error:&error];
if ((error != nil) || (array == nil))
{
NSLog(@"Error while fetching\n%@", ([error localizedDescription] != nil) ? [error localizedDescription] : @"Unknown Error");
exit(1);
}
// Display the Results
//
NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
[formatter setDateStyle:NSDateFormatterMediumStyle];
[formatter setTimeStyle:NSDateFormatterMediumStyle];
NSLog(@"%@ run history:", [processInfo processName]);
for (run in array)
{
NSLog(@"On %@ as process ID %ld", [formatter stringForObjectValue:run.date], run.processID);
}
return 0;
}
大功告成！编译运行，我们可以得到如下显示：
[cpp]
2011-09-03 21:42:47.556 CoreDataTutorial[992:903] CoreDataTutorial run history:
2011-09-03 21:42:47.557 CoreDataTutorial[992:903] On 2011-9-3 下午09:41:56 as process ID 940
2011-09-03 21:42:47.557 CoreDataTutorial[992:903] On 2011-9-3 下午09:42:16 as process ID 955
2011-09-03 21:42:47.558 CoreDataTutorial[992:903] On 2011-9-3 下午09:42:20 as process ID 965
2011-09-03 21:42:47.558 CoreDataTutorial[992:903] On 2011-9-3 下午09:42:24 as process ID 978
2011-09-03 21:42:47.559 CoreDataTutorial[992:903] On 2011-9-3 下午09:42:47 as process ID 992
通过这个例子，我们可以更好理解 Core Data 的运作机制。在 Core Data 中我们最常用的就是 ManagedObjectContext，它几乎参与对数据对象的所有操作，包括对 undo/redo 的支持；而 Entity 对应的运行时类为 ManagedObject，我们可以理解为抽象数据结构 Entity 在内存中由 ManagedObject 来体现，而 Perproty 数据类型在内存中则由 ManagedObject 类的成员属性来体现。一般我们不需要与 PersistentStoreCoordinator 打交道，对数据文件的读写操作都由 ManagedObjectContext 为我们代劳了。