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];
}




posted on 2012-03-31 15:46  老Zhan  阅读(1564)  评论(0编辑  收藏  举报