核心数据：未保存

Question

我无法保存到一个变量letsMeet.startTimeLabel。然而，在选择NSLog显示正确的值后，我保存到另一个变量（letsMeet.endTimeLabel）后，letsMeet.startTimeLabel变为（NULL）。下面是代码：

- (void)actionSheet:(UIActionSheet *)actionSheet clickedButtonAtIndex:(NSInteger)buttonIndex 
{ 
letsMeet = (LetsMeet *) [NSEntityDescription insertNewObjectForEntityForName:@"LetsMeet" inManagedObjectContext:managedObjectContext]; 
switch (actionSheet.tag) 
{ 
    case 1: 
    { 
     if (buttonIndex == 0) 
     { 
      UIDatePicker *startDatePicker = (UIDatePicker *)[actionSheet viewWithTag:kDatePickerTag1]; 

      NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init]; 
      [dateFormatter setDateFormat:@"dd"]; 
      NSDate *selectedDate = [startDatePicker date]; 

      NSDateFormatter *dayFormatter = [[NSDateFormatter alloc] init]; 
      [dayFormatter setDateFormat:@"EEEE"]; 
      NSDate *selectedDay= [startDatePicker date]; 

      NSDateFormatter *monthFormatter = [[NSDateFormatter alloc] init]; 
      [monthFormatter setDateFormat:@"MMMM"]; 
      NSDate *selectedMonth = [startDatePicker date]; 

      NSString *date = [[NSString alloc] initWithFormat:@"%@", [dateFormatter stringFromDate:selectedDate]]; 
      DateLabel.text = date; 
      [letsMeet setDateLabel:date]; 

      NSString *month = [[NSString alloc] initWithFormat:@"%@", [dayFormatter stringFromDate:selectedMonth]]; 
      MonthLabel.text = month; 
      [letsMeet setMonthLabel:month]; 

      NSString *day = [[NSString alloc] initWithFormat:@"%@", [monthFormatter stringFromDate:selectedDay]]; 
      DayLabel.text = day; 
      [letsMeet setDateLabel:day]; 

      NSDateFormatter *timeFormatter = [[NSDateFormatter alloc] init]; 
      [timeFormatter setDateFormat: @"h:mm a"]; 
      NSDate *selectedStartTime = [startDatePicker date]; 

      NSString *startTime = [[NSString alloc] initWithFormat:@"%@", [timeFormatter stringFromDate:selectedStartTime]]; 
      StartTimeLabel.text = startTime; 
      [letsMeet setStartTimeLabel:startTime]; 

      NSError *error = nil; 
      if (![managedObjectContext save:&error]){ 

       NSLog(@"Error Saving"); 
      } 
     } 
     NSLog (@"This is the StartTime after selecting %@", letsMeet.startTimeLabel); 
    } 
    break; 

    case 2: 
    { 
     if (buttonIndex == 0) 
     { 
      UIDatePicker *endTimePicker = (UIDatePicker *)[actionSheet viewWithTag:kDatePickerTag2]; 
      NSDateFormatter *endTimeFormatter = [[NSDateFormatter alloc] init]; 
      [endTimeFormatter setDateFormat: @"h:mm a"]; 
      NSDate *endSelectedTime = [endTimePicker date]; 
      NSString *endTime = [[NSString alloc] initWithFormat:@"%@", [endTimeFormatter stringFromDate:endSelectedTime]]; 
      EndTimeLabel.text = endTime; 
      [letsMeet setEndTimeLabel:endTime]; 
      NSLog (@"This is the EndTime %@", letsMeet.endTimeLabel); 
      NSLog (@"This is the StartTime after selecting BOTH %@", letsMeet.startTimeLabel); 
     } 
     else if (buttonIndex == 1) 
     { 
      EndTimeLabel.text = @"Whenever"; 
      [letsMeet setEndTimeLabel:EndTimeLabel.text]; 
     } 
     NSError *error = nil; 
     if (![managedObjectContext save:&error]) { 
    } 

}break; 

    // Handle the error. 
} 

} 


-(void) prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender 
{ 
    UIViewController *destinationViewController = segue.destinationViewController; 
    NSLog (@"Prepare For Segue StartTime %@", letsMeet.startTimeLabel); 
    NSLog (@"Prepare For Segue EndTime%@", letsMeet.endTimeLabel); 
} 

这里是日志：

2013-02-20 21:38:24.253 AppointmentTime[3129:c07] This is the StartTime after selecting 9:30 AM 
2013-02-20 21:38:32.325 AppointmentTime[3129:c07] This is the EndTime 12:15 PM 
2013-02-20 21:38:32.325 AppointmentTime[3129:c07] This is the StartTime after Selecting BOTH (null) 
2013-02-20 21:38:34.069 AppointmentTime[3129:c07] Prepare For Segue StartTime (null) 
2013-02-20 21:38:34.069 AppointmentTime[3129:c07] Prepare For Segue EndTime12:15 PM 

问：为什么会letsMeet.startTimeLabel显示纠正第一次选择结束时间之后，它改变为NULL。请注意EndTime继续显示正确​​的值，直到prepareForSegue。奇怪的！

Answer 1

根据您的日志和代码，您正在输入切换块两次。这意味着您要输入两次actionSheet:clickedButtonAtIndex:方法。所以每次你输入方法

letsMeet = (LetsMeet *) [NSEntityDescription insertNewObjectForEntityForName:@"LetsMeet" inManagedObjectContext:managedObjectContext]; 

语句被执行两次，依次创建两个对象。您可以通过从商店获取进行查看。

所以你检查两个不同的对象，因此null属性。

如果您只使用一个管理对象，则可能在执行insertNewObjectForEntityForName:inManagedObjectContext:之前为对象添加nil检查。这将确保您使用的是同一个对象。
如果您同时使用多个对象，请使用对象ID或某个唯一键来标识对象并对其进行操作。

编辑：
您可以检查与下面的代码零：

if(letsMeet==Nil){ 
    letsMeet = (LetsMeet *) [NSEntityDescription insertNewObjectForEntityForName:@"LetsMeet" inManagedObjectContext:managedObjectContext]; 
} 

这只会工作，如果对象您所呼叫的actionSheet:clickedButtonAtIndex:方法始终是在内存中。但是，由于您坚持不懈，您可能需要从商店获取对象，然后检查否。的物体。

NSError *error; 
NSFetchRequest *request = [[NSFetchRequest alloc] init]; 
[request setEntity:[NSEntityDescription entityForName:@"LetsMeet" inManagedObjectContext:managedObjectContext]]; 
NSArray *objectArray = [managedObjectContext executeFetchRequest:request error:&error] 
if(objectArray.count==0){ 
    letsMeet = (LetsMeet *) [NSEntityDescription insertNewObjectForEntityForName:@"LetsMeet" inManagedObjectContext:managedObjectContext]; 
}else{ 
    letsMeet = (LetsMeet *)[objectArray objectAtIndex:0]; 
} 

注：如果你需要坚持只有一对夫妇的变量，核心数据可能是矫枉过正。改为使用NSUserDefaults，并保持简单。