iOS-Core Data基础

Core Data基础

Core Data是一个API集合，被设计用来简化数据对象的持久存储。

在此先不普及概念，先通过一个简单的案例使用来感受一下Core Data的精妙之处。

在创建工程的时候勾选Use Core Data.

创建好项目，我们可以看到在左侧任务栏多了一个CoreDataDemo.xcdatamodeld。暂且先不管这个文件。

此时如果我们打开AppDelegate.h和AppDelegate.m文件，会发现比平时多了很多的内容。

下面是生成的声明文件和实现文件。

 #import <UIKit/UIKit.h>
 #import <CoreData/CoreData.h>

 @interface AppDelegate : UIResponder <UIApplicationDelegate>

 @property (strong, nonatomic) UIWindow *window;

 @property (readonly, strong, nonatomic) NSManagedObjectContext *managedObjectContext;
 @property (readonly, strong, nonatomic) NSManagedObjectModel *managedObjectModel;
 @property (readonly, strong, nonatomic) NSPersistentStoreCoordinator *persistentStoreCoordinator;

 - (void)saveContext;
 - (NSURL *)applicationDocumentsDirectory;
 @end

AppDelegate.h

 #import "AppDelegate.h"

 @interface AppDelegate ()

 @end

 @implementation AppDelegate

 - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
     // Override point for customization after application launch.
     return YES;
 }

 - (void)applicationWillResignActive:(UIApplication *)application {
     // Sent when the application is about to move from active to inactive state. This can occur for certain types of temporary interruptions (such as an incoming phone call or SMS message) or when the user quits the application and it begins the transition to the background state.
     // Use this method to pause ongoing tasks, disable timers, and throttle down OpenGL ES frame rates. Games should use this method to pause the game.
 }

 - (void)applicationDidEnterBackground:(UIApplication *)application {
     // Use this method to release shared resources, save user data, invalidate timers, and store enough application state information to restore your application to its current state in case it is terminated later.
     // If your application supports background execution, this method is called instead of applicationWillTerminate: when the user quits.
 }

 - (void)applicationWillEnterForeground:(UIApplication *)application {
     // Called as part of the transition from the background to the inactive state; here you can undo many of the changes made on entering the background.
 }

 - (void)applicationDidBecomeActive:(UIApplication *)application {
     // Restart any tasks that were paused (or not yet started) while the application was inactive. If the application was previously in the background, optionally refresh the user interface.
 }

 - (void)applicationWillTerminate:(UIApplication *)application {
     // Called when the application is about to terminate. Save data if appropriate. See also applicationDidEnterBackground:.
     // Saves changes in the application's managed object context before the application terminates.
     [self saveContext];
 }

 #pragma mark - Core Data stack

 @synthesize managedObjectContext = _managedObjectContext;
 @synthesize managedObjectModel = _managedObjectModel;
 @synthesize persistentStoreCoordinator = _persistentStoreCoordinator;

 - (NSURL *)applicationDocumentsDirectory {
     // The directory the application uses to store the Core Data store file. This code uses a directory named "com.wyg.CoreDataDemo" in the application's documents directory.
     return [[[NSFileManager defaultManager] URLsForDirectory:NSDocumentDirectory inDomains:NSUserDomainMask] lastObject];
 }

 - (NSManagedObjectModel *)managedObjectModel {
     // The managed object model for the application. It is a fatal error for the application not to be able to find and load its model.
     if (_managedObjectModel != nil) {
         return _managedObjectModel;
     }
     NSURL *modelURL = [[NSBundle mainBundle] URLForResource:@"CoreDataDemo" withExtension:@"momd"];
     _managedObjectModel = [[NSManagedObjectModel alloc] initWithContentsOfURL:modelURL];
     return _managedObjectModel;
 }

 - (NSPersistentStoreCoordinator *)persistentStoreCoordinator {
     // The persistent store coordinator for the application. This implementation creates and return a coordinator, having added the store for the application to it.
     if (_persistentStoreCoordinator != nil) {
         return _persistentStoreCoordinator;
     }

     // Create the coordinator and store

     _persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:[self managedObjectModel]];
     NSURL *storeURL = [[self applicationDocumentsDirectory] URLByAppendingPathComponent:@"CoreDataDemo.sqlite"];
     NSError *error = nil;
     NSString *failureReason = @"There was an error creating or loading the application's saved data.";
     if (![_persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:storeURL options:nil error:&error]) {
         // Report any error we got.
         NSMutableDictionary *dict = [NSMutableDictionary dictionary];
         dict[NSLocalizedDescriptionKey] = @"Failed to initialize the application's saved data";
         dict[NSLocalizedFailureReasonErrorKey] = failureReason;
         dict[NSUnderlyingErrorKey] = error;
         error = [NSError errorWithDomain:@"YOUR_ERROR_DOMAIN" code: userInfo:dict];
         // Replace this 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.
         NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
         abort();
     }

     return _persistentStoreCoordinator;
 }

 - (NSManagedObjectContext *)managedObjectContext {
     // Returns the managed object context for the application (which is already bound to the persistent store coordinator for the application.)
     if (_managedObjectContext != nil) {
         return _managedObjectContext;
     }

     NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator];
     if (!coordinator) {
         return nil;
     }
     _managedObjectContext = [[NSManagedObjectContext alloc] init];
     [_managedObjectContext setPersistentStoreCoordinator:coordinator];
     return _managedObjectContext;
 }

 #pragma mark - Core Data Saving support

 - (void)saveContext {
     NSManagedObjectContext *managedObjectContext = self.managedObjectContext;
     if (managedObjectContext != nil) {
         NSError *error = nil;
         if ([managedObjectContext hasChanges] && ![managedObjectContext save:&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.
             NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
             abort();
         }
     }
 }

 @end

AppDelegate.m

可能初次接触感觉密密麻麻的，那我们暂且也不用管它，既然系统为我们自动生成，我们直接使用就行了。

现在我们点击进去CoreDataDemo.xcdatamodeld这个文件，我们可以看到下面的界面。

Add Entity(添加实体)  Add Attribute(添加属性)   Editor Style(编辑风格)

现在我们添加一个Entity,命名为People,添加属性name,age

现在我们再添加一个Entity,命名位Book,添加属性name,author

现在我们可以添加People和Book的关系。

在上图中我们可以看到Relationships这个词(显示的是People，Book这一同理)。

我们想要在People中添加一个Book，表名Book是People的一个属性，可以简单理解为人有书

在Book中添加一个People,表名People是Book的一个属性，可以简单理解为书有主人

此时选择Editor Style,我们可以看到下图：

我们使用界面图形工具创建了一个People,一个Book，我们如何让他们生成实际的类呢？

选择菜单栏中的 Editor->create NSManagedObject subclass(也可以在command+n->Core Data->NSManagedObject subclass)

执行完上述步骤系统会为我们生成People，Book类。

下面是Book和People类文件，其中People类中我们发现NSManagedObject修饰book，我们将其修改位Book就行，并写上@class Book;有时会出现这个小问题。

 #import <Foundation/Foundation.h>
 #import <CoreData/CoreData.h>

 @class People;

 @interface Book : NSManagedObject

 @property (nonatomic, retain) NSNumber * name;
 @property (nonatomic, retain) NSString * author;
 @property (nonatomic, retain) People *people;

 @end

Book.h

 #import "Book.h"
 #import "People.h"

 @implementation Book

 @dynamic name;
 @dynamic author;
 @dynamic people;

 @end

Book.m

 #import <Foundation/Foundation.h>
 #import <CoreData/CoreData.h>

 @interface People : NSManagedObject

 @property (nonatomic, retain) NSString * name;
 @property (nonatomic, retain) NSNumber * age;
 @property (nonatomic, retain) NSManagedObject *book;

 @end

People.h

 #import "People.h"

 @implementation People

 @dynamic name;
 @dynamic age;
 @dynamic book;

 @end

People.m

到现在为止，万事俱备，只欠代码了。

我们对Core Data的操作，无外乎增删改查，直接上代码

在AppDelegate启动函数中，添加以下代码是往Core Data中添加一条记录：

 //托管对象上下文，有点类似于数据库
     NSManagedObjectContext *context = self.managedObjectContext;
     //实例，有点类似于表，插入一条数据，并给对象属性赋值
     People *people = [NSEntityDescription insertNewObjectForEntityForName:@"People" inManagedObjectContext:context];
     people.name = @"wyg";

     Book *book =[NSEntityDescription insertNewObjectForEntityForName:@"Book" inManagedObjectContext:context];
     book.name = @"三国演义";

     people.book = book;

     [self saveContext];

增

 NSManagedObjectContext *context = self.managedObjectContext;
     NSEntityDescription *entity = [NSEntityDescription entityForName:@"People" inManagedObjectContext:context];
     NSFetchRequest *request = [NSFetchRequest new];
     request.entity = entity;
     NSArray *arr = [context executeFetchRequest:request error:nil];

     for (People *object in arr)
     {
         NSLog(@"%@,%@",object.name,object.book.name);
     }

查

查得结果：

关于修改和删除，跟上面原理差不多，首先是查找结果集，假设要删除某条数据，可以使用：[context deleteObject:object];

对某条数据修改，可以直接使用：[object setValue:@"小明" forKey:@"name"];//将姓名修改为小明。

 

 概念普及

Core Data的关键组件：数据存储（data store）,持久存储协调器（Persistent Store Coordinator）,托管对象模型（Managed Object Modeal）,托管对象上下文（Managed Object Context）,这些术语可能难以理解，没有关系，以后在实践中我们会逐渐理解。

1.数据存储：数据存储是保存数据的一个或一组文件。当消息发送到Core Data，实际写入到了磁盘文件。创建数据存储依赖创建数据存储是的参数，数据存储可以是一个二进制文件，一个SQLite数据库，或者内存中的数据文件。

2.持久存储协调器：持久存储协调器是NSPersistentStoreCoordinator的实例，扮演上下文和数据存储的中间角色，协调器从上下文中获得数据请求并将他们转发给合适的数据存储。

3.托管对象模型：NSManagedObjectModel的实例，模型是一组实体，定义了应用程序中的数据对象。

4.托管对象上下文：NSManagedObjectContext实例，用于保存所有托管数据对象。可以将上下文想想成保存所有应用程序数据的沙盒，可以在上下文中添加对象，删除对象，以及修改等。

 

遇到问题

1.Key 0x889f2f0(:Key:0x889e400<-://981A476D-55AC-4CB4-BBD8-E0285E522412/Key/p1489> ; data: <fault>)
出现的可能原因：当时我创建的时候，勾选了core data，它会在delegate.m文件中生成一些操作，但是如果你在其它控制器中想要操作的话，就要获取core data的相关对象，我的当时的做法是：

     app = [AppDelegate new];
     NSManagedObjectContext *context = app.managedObjectContext;
     NSEntityDescription *entity = [NSEntityDescription entityForName:@"NoteModel" inManagedObjectContext:context];
     NSFetchRequest *request = [NSFetchRequest new];
     request.entity = entity;
     [request setReturnsObjectsAsFaults:NO];
     notes = [context executeFetchRequest:request error:nil];
     [collection reloadData];

code

如果我将app设置位局部变量，就会出现上述错误，我的解决办法是将其设置为全局变量。具体原因可以参考：http://imtx.me/archives/1888.html