• (void)viewWillAppear:(BOOL)animated{
  [super viewWillAppear:animated];
  //创建通知
  [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(appDidEnterBackgroundNotification) name:UIApplicationDidEnterBackgroundNotification object:nil];
  [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(appWillEnterForegroundNotification) name:UIApplicationWillEnterForegroundNotification object:nil];
  }

• (void)viewDidDisappear:(BOOL)animated {
  [super viewDidDisappear:animated];
  //移除通知
  [[NSNotificationCenter defaultCenter]removeObserver:self];
  }

• (void)appDidEnterBackgroundNotification{
  NSDate *date = [NSDate date];
  NSUserDefaults *user = [NSUserDefaults standardUserDefaults];
  [user setObject:date forKey:@"date"];
  [user synchronize];
  NSLog(@"存储时间 =============== ======= %@",date);
  }

• (void)appWillEnterForegroundNotification {
  NSDate *dateLast = [[NSUserDefaults standardUserDefaults] objectForKey:@"date"];
  NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
  [dateFormatter setDateFormat:@"yyyy-MM-dd HH:mm:ss"];
  NSString *strDate = [dateFormatter stringFromDate:dateLast];
  [self intervalSinceNow:strDate];
  }

// 计算某一时间到当前时间

• (NSString *)intervalSinceNow: (NSString *) theDate
  {
  NSArray *timeArray=[theDate componentsSeparatedByString:@"."];
  theDate=[timeArray objectAtIndex:0];
  NSDateFormatter *date=[[NSDateFormatter alloc] init];
  [date setDateFormat:@"yyyy-MM-dd HH:mm:ss"];
  NSDate d=[date dateFromString:theDate];
  NSTimeInterval late=[d timeIntervalSince1970]1;
  NSDate dat = [NSDate date];
  NSTimeInterval now=[dat timeIntervalSince1970]1;
  NSString *timeString=@"";
  NSTimeInterval cha= fabs(late-now); // 求绝对值
  timeString = [NSString stringWithFormat:@"%f", cha];
  timeString = [timeString substringToIndex:timeString.length-7];
  if ([timeString intValue] > 60) {
  [self shouNoticeViewWithNoticeString:@"您登录超时了哦"];
  }
  return timeString;
  }

• (void)ensureLogAgain {
  LoginViewController *login = [[LoginViewController alloc] init];
  [self.navigationController pushViewController:login animated:YES];
  }