nsuserdefaults未写入磁盘

我将一些值设置为nsuserdefaults,如果不退出应用程序就检索到这些值,一切正常。 当我重新启动应用程序时,95%的时间内更改都会消失。在5%的时间内,一切都保存完好。

我第一次面对这个问题。在我的其他应用程序中,我通常不会将nsstring保存为nsuserdefaults。保存nsstring的事实是否会导致问题?

以下是我的代码:

    -(void) mayShowFeedbackMessage
    {
     NSDateFormatter *dateFormatter = [ [NSDateFormatter alloc] init];
     [dateFormatter setTimeStyle: NSDateFormatterLongStyle];
     [dateFormatter setDateStyle: NSDateFormatterMediumStyle];

     if ([ [NSUserDefaults standardUserDefaults] objectForKey: @"ratingsMessage_shown_on_date"])
     {
          NSString *lastRatingDateString = [ [NSUserDefaults standardUserDefaults]
                                           objectForKey: @"ratingsMessage_shown_on_date"];
          NSDate *lastRatingDate = [dateFormatter dateFromString: lastRatingDateString];

          DebugLog(@"lastRatingDateString: %@", lastRatingDateString);
          DebugLog(@"time since last message: %.0f", [ [NSDate date] timeIntervalSinceDate: lastRatingDate]);

          if ( [ [NSDate date] timeIntervalSinceDate: lastRatingDate] >= 120.0f )
          {
              [self showFeedbackMessage];

              NSDateFormatter *dateFormatter = [ [NSDateFormatter alloc] init];
              [dateFormatter setTimeStyle: NSDateFormatterLongStyle];
              [dateFormatter setDateStyle: NSDateFormatterMediumStyle];

              NSString *dateString = [dateFormatter stringFromDate: [NSDate date] ];
             [dateFormatter release];

             [ [NSUserDefaults standardUserDefaults] setObject: dateString
                                                        forKey: @"ratingsMessage_shown_on_date" ];

             lastRatingDateString = [ [NSUserDefaults standardUserDefaults]
                                     objectForKey: @"ratingsMessage_shown_on_date"];
             DebugLog(@"new date: %@", lastRatingDateString);

         }
     }
     else
     {  
         NSDateFormatter *dateFormatter = [ [NSDateFormatter alloc] init];
         [dateFormatter setTimeStyle: NSDateFormatterLongStyle];
         [dateFormatter setDateStyle: NSDateFormatterMediumStyle];

         NSString *dateString = [dateFormatter stringFromDate: [NSDate date] ];
         [dateFormatter release];

         [ [NSUserDefaults standardUserDefaults] setObject: dateString
                                                        forKey: @"ratingsMessage_shown_on_date" ];

         NSString *lastRatingDateString = [ [NSUserDefaults standardUserDefaults]
                                                objectForKey: @"ratingsMessage_shown_on_date"];
         DebugLog(@"new date: %@", lastRatingDateString);

     }

         [dateFormatter release];
}

谢谢。