关于CoreData的理解和使用.

#import <UIKit/UIKit.h>
#import <CoreData/CoreData.h>
 
@interface AppDelegate : UIResponder <UIApplicationDelegate>
 
@property (strong, nonatomic) UIWindow *window;
 
//被管理者对象上下文
@property (readonly, strong, nonatomic) NSManagedObjectContext *managedObjectContext;
 
//数据模型器管理类
@property (readonly, strong, nonatomic) NSManagedObjectModel *managedObjectModel;
 
//数据连接器工具类 (连接助理)
@property (readonly, strong, nonatomic) NSPersistentStoreCoordinator *persistentStoreCoordinator;
 
//通过对上下文的保存,将上下文管理的对象保存到实体数据库.
- (void)saveContext;
 
//获取数据库所在的路径
- (NSURL *)applicationDocumentsDirectory;
 
 
@end

#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.lanou.LessonCoreData" in the application's documents directory.
    NSLog(@"%@",[[[NSFileManager defaultManager] URLsForDirectory:NSDocumentDirectory inDomains:NSUserDomainMask] lastObject]);
     
    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;
    }
     
    //momd 去查询xcdatamodeld
    NSURL *modelURL = [[NSBundle mainBundle] URLForResource:@"LessonCoreData" withExtension:@"momd"];
    //创建数据器模型工具类.
    _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
     
    //使用数据模型器工具类实例化数据连接器工具类.
    _persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:[self managedObjectModel]];
    //数据库的绝对路径.
    NSURL *storeURL = [[self applicationDocumentsDirectory] URLByAppendingPathComponent:@"LessonCoreData.sqlite"];
    NSError *error = nil;
    NSString *failureReason = @"There was an error creating or loading the application's saved data.";
     
    //创建数据库的过程.
    //1.数据持久化类型
    //2.设置信息
    //3.数据库的绝对路径.
    //4.配置SQL数据的选项
    //5.错误信息的赋值.
    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:9999 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]);
        //程序中断
        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;
    }
     
    //通过GET方法创建数据连接器工具类
    NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator];
    //判断数据器连接工具类是否为空
    if (!coordinator) {
        return nil;
    }
    //创建数据管理器工具类
    _managedObjectContext = [[NSManagedObjectContext alloc] init];
    //设置数据管理器工具类的数据连接器.
    [_managedObjectContext setPersistentStoreCoordinator:coordinator];
    return _managedObjectContext;
}
 
#pragma mark - Core Data Saving support
//保存你当前的操作到数据库中.
- (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();
        }
    }
}

#import "ViewController.h"
#import "AppDelegate.h"
#import "ClassEntity.h"
 
@interface ViewController ()
 
@end
 
@implementation ViewController
 
- (void)viewDidLoad {
    [super viewDidLoad];
    //获取当前的AppDelegate的实例.
    AppDelegate *appdelegate = [UIApplication sharedApplication].delegate;
     
    ////为数据库中增加数据
     
    //创建当前实体类的实体描述
    NSEntityDescription *entityDescription = [NSEntityDescription entityForName:@"ClassEntity" inManagedObjectContext:appdelegate.managedObjectContext];
     
    //通过实体描述文件来创建当前类的实例.
    ClassEntity *classEntity = [[ClassEntity alloc] initWithEntity:entityDescription insertIntoManagedObjectContext:appdelegate.managedObjectContext];
     
    //为实例的属性赋值.
    classEntity.name = @"张三";
    classEntity.sex = @"男";
    classEntity.age = @"21";
     
    //保存操作到真实的数据库内.
    [appdelegate saveContext];
     
    ////查询数据库内存放的实体内容.
     
    //创建查询语句
    NSFetchRequest *request = [[NSFetchRequest alloc] initWithEntityName:@"ClassEntity"];
     
    //通过AppDelegate来查询你想要的数据库.
    //注意返回值为数组.
    NSArray *arr = [appdelegate.managedObjectContext executeFetchRequest:request error:nil];
     
    //打印数组
    NSLog(@"%@",arr);
     
    ////删除数据库中存放的实体.
     
    //通过查询获取到你需要删除的表单中的所有内容.
     
    //通过返回值获取到的数组来查找你当前想要的实体位置.
    ClassEntity *classEntity1 = [arr objectAtIndex:0];
     
    //通过AppDelegate
    [appdelegate.managedObjectContext deleteObject:classEntity1];
     
    //通过AppDelegate来保存到你的真实数据库中.
    [appdelegate saveContext];
     
     
     
    // Do any additional setup after loading the view, typically from a nib.
}