CoreData的增删改查和数据迁移示例

作为管理器，最基本的功能就是增删改查了。

1、插入

AppDelegate *app = [[UIApplication sharedApplication] delegate];
NSManagedObjectContext *context = [app managedObjectContext];
NSManagedObject ＊newManagedObject = [NSEntityDescription insertNewObjectForEntityForName:@"entityname" inManagedObjectContext:context];
[newManagedObject setValue:value forKey:@"propertyname"];
NSError *error;
if (![context save:&error]) {

    // Handle the error…
}

2、查询

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSManagedObjectContext *managedObjectContext = appDelegate.managedObjectContext;
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Hero" inManagedObjectContext:managedObjectContext];
NSSortDescriptor *sortDescriptor1 = [[NSSortDescriptor alloc]initWithKey:@”name” ascending:YES];
NSSortDescriptor *sortDescriptor2 = [[NSSortDescriptor alloc]initWithKey:@”secretIdentity” ascending:YES];
NSArray *sortDescriptors = [[NSArray alloc]initWithObjects:sortDescriptor1, sortDescriptor2, nil];
[fetchRequest setSortDescriptors:sortDescriptors];
[fetchRequest setEntity:entity];
[fetchRequest setFetchBatchSize:20];
NSArray *objecs = [context executeFetchRequest: fetchRequest error:&error];
if(objets==nil || error != nil){　　//做处理
}

3、修改

-(void)applicationWillResignActive:(NSNotification *)notification{
    NSLog(@"applicationWillResignActive");
    //创建托管对象上下文
    NSManagedObjectContext *context = [appDelegate managedObjectContext];
    NSFetchRequest *request = [[NSFetchRequest alloc]init];
    NSEntityDescription *entityDescription = [NSEntityDescription entityForName:@"User" inManagedObjectContext:context];
    [request setEntity:entityDescription];

    NSManagedObject *user = nil;
    NSError *error;
    NSArray *objets = [context executeFetchRequest:request error:&error];
    if(objets==nil){
        NSLog(@"There has a error!");
        //做错误处理
    }
    if([objets count]>0){
        //非第一次，更新数据
        NSLog(@"更新操作");
        user = [objets objectAtIndex:0];
    }else{
        NSLog(@"插入操作");
        //第一次保存，插入新数据
        user = [NSEntityDescription insertNewObjectForEntityForName:@"User" inManagedObjectContext:context];
    }
    [user setValue:self.serverIp.text forKeyPath:@"serverIp"];
    [user setValue:self.userName.text forKeyPath:@"userName"];
    [context save:&error];
}

4、删除

NSManagedObjectContext *managedObjectContext = appDelegate.managedObjectContext;
[context deleteObject:[objecs objectIndex:index];
NSError *error;
if (![context save:&error]) {
    NSLog(@”Unresolved error %@, %@”, error, [error userInfo]);
    exit(-1); // Fail
}

5、数据迁移

当程序变化时，在很多简单的情况下，Coredata提供了轻量级的自动数据迁移，比如以下三个情况可以通过开启CoreData自动迁移实现：
1.简单的增加一个字段
2.把一个必填字段改为可选字段
3.把可选字段改为必填字段（但一定要定义默认值）