iphone开发中的数据存储：Core Data

- (void)applicationWillResignActive:(NSNotification *)notification {
    //获取appDelegate
    BIDAppDelegate *appDelegate = [[UIApplication sharedApplication] delegate];
    //获取上下文context
    NSManagedObjectContext *context = [appDelegate managedObjectContext];
    
    NSError *error;
    
    for (int i = 1; i <= 4; i++) {
        NSString *fieldName = [NSString stringWithFormat:@"line%d", i];
        UITextField *theField = [self valueForKey:fieldName];
        //创建提取请求
        NSFetchRequest *request = [[NSFetchRequest alloc] init];
        //创建托管对象（Managed Objects）
        NSEntityDescription *entityDescription = [NSEntityDescription
                                                  entityForName:@"Line"
                                                  inManagedObjectContext:context];
        //为请求设置Entity，指定希望的对象实体
        [request setEntity:entityDescription];

        
        //谓词（predicate）（断言？）类似于SQL中的where语言
        //这句的意思是通知提取请求仅搜索lineNum 属性设置为i 的对象
        NSPredicate *pred = [NSPredicate
                             predicateWithFormat:@"(lineNum = %d)", i];
        //设置断言
        [request setPredicate:pred];
        
        NSManagedObject *theLine = nil;
        
        //执行提取请求,执行之后，context将跟踪你对该数组（objects）中返回的托管对象（theLine）的任何修改，最终发送sava：时保存
        NSArray *objects = [context executeFetchRequest:request
                                                  error:&error];
        
        if (objects == nil) {
            NSLog(@"There was an error!");
            // Do whatever error handling is appropriate
        }
        if ([objects count] > 0)//原本存在，因为有设置断言，所以这里的count=1
            theLine = [objects objectAtIndex:0];
        else//初次存储
            theLine = [NSEntityDescription
                       insertNewObjectForEntityForName:@"Line"
                       inManagedObjectContext:context];
        
        [theLine setValue:[NSNumber numberWithInt:i] forKey:@"lineNum"];
        [theLine setValue:theField.text forKey:@"lineText"];
        
    }
    //保存数据
    [context save:&error];
}

- (void)viewDidLoad

{

[super viewDidLoad];

// Do any additional setup after loading the view from its nib.

    BIDAppDelegate *appDelegate =

[[UIApplication sharedApplication] delegate];

NSManagedObjectContext *context = [appDelegate managedObjectContext];

NSEntityDescription *entityDescription = [NSEntityDescription

entityForName:@"Line"

inManagedObjectContext:context];

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

[request setEntity:entityDescription];



NSError *error;

NSArray *objects = [context executeFetchRequest:request error:&error];

if (objects == nil) {

NSLog(@"There was an error!");

// Do whatever error handling is appropriate

    }



for (NSManagedObject *oneObject in objects) {

NSNumber *lineNum = [oneObject valueForKey:@"lineNum"];

NSString *lineText = [oneObject valueForKey:@"lineText"];



NSString *fieldName = [NSString

stringWithFormat:@"line%d", [lineNum integerValue]];

UITextField *theField = [self valueForKey:fieldName];

theField.text = lineText;

}



UIApplication *app = [UIApplication sharedApplication];

[[NSNotificationCenter defaultCenter] addObserver:self

selector:@selector(applicationWillResignActive:)

name:UIApplicationWillResignActiveNotification

object:app];

}