iphone开发中的数据存储：Core Data

同样是之前的存储四个textField的例子

在项目中创建后缀为” .xcdatamodel “的存储文件，在其attribute中创建属性，lineNum和lineText，类型为integer116和String

然后直接上代码：

- (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];
}

viewDidLoad中

- (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];
}