第十五篇、程序返回前台的时间差（常用于显示广告）

如果app在后台待机太久，再次进来前台的时候也应该展示广告，所以在applicationDidEnterBackground的时候应该把时间存起来：

 //程序切入后台,这里要注意GMT时间
    NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
    NSTimeZone* sourceTimeZone = [NSTimeZone timeZoneWithAbbreviation:@"GMT"];
    [formatter setTimeZone:sourceTimeZone];
    [formatter setDateFormat:@"yyyy-MM-dd HH:mm:ss"];
    _lastTimeEnterBackGroundStr = [formatter stringFromDate:[NSDate date]];//当前时间

在applicationWillEnterForeground的时候对比时间差，判断是否显示:

NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
        NSTimeZone* sourceTimeZone = [NSTimeZone timeZoneWithAbbreviation:@"GMT"];
        [formatter setTimeZone:sourceTimeZone];
        [formatter setDateFormat:@"yyyy-MM-dd HH:mm:ss"];
        NSDate * lastDate = [formatter dateFromString:_lastTimeEnterBackGroundStr];
        NSDate * now = [formatter dateFromString:[formatter stringFromDate:[NSDate date]]];
        NSTimeInterval IntervalTime = [now timeIntervalSince1970]*1 - [lastDate timeIntervalSince1970]*1;
        if (IntervalTime>(2*60*60)) {
            [_mainController loadAdvertisedView];
        }