Question

我无法保存到一个变量letsMeet.startTimeLabel。选择NSLog后立即显示正确的值，但是，在保存到另一个变量（letsMeet.endTimeLabel）后，letsMeet.startTimeLabel更改为（NULL）。以下是代码：

- (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：为什么第一次让letMeet.startTimeLabel显示正确，选择EndTime后，它会变为NULL。请注意EndTime继续显示正确的值，直到prepareForSegue。怪异！

Answer 1

根据您的日志和代码，您将进入两次切换块。这意味着您要两次输入actionSheet:clickedButtonAtIndex:方法。所以每次进入方法

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

语句执行两次，依次创建两个对象。您可以通过从商店进行提取来查看此内容。

所以你要检查两个不同对象的属性，因此检查null。

如果您只使用一个托管对象，则可以在执行nil之前为该对象添加insertNewObjectForEntityForName:inManagedObjectContext:检查。这将确保您使用相同的对象。
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您的对象并对其进行操作。

修改
您可以使用以下代码检查nil：

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

只有在调用actionSheet:clickedButtonAtIndex:方法的对象始终在内存中时，这才有效。但是因为你是持久的，你可能想要从商店中获取对象，然后检查否。对象。

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]; }

注意：如果您只需要保留几个变量，核心数据可能会过度。请改用NSUserDefaults并保持简单。

核心数据：不保存

1 个答案: