iPhone实例学习之iphone sdk2.0 miss和invalid operands to binary问题

 这些天在学习iPhone软件开发，在学习到31个iphone软件开发实例教程的时候，首先遇到的是iphone sdk 2.0 miss的问题，我用的是Xcode3.23，it's for Mac OS X10.6 and iPhone OS 4.0。编译下载下来的软件的时候出现错误提示：error: There is no SDK with the name or path 'iphoneos2.0'。

解决的办法是：

1.选择 xCode目录中的"Project".
2.选择 "Edit Project Settings" 子目录.
3.在 "Base SDK for All Configurations"选项中选择iPhone Simulator 4.0.
4.build and run.

但是这个时候编译第一个例程又弹出如下错误：

error: invalid operands to binary - (have 'int' and 'id')

将这段出错代码

NSDate* now = [NSDate date];

int hour = 23 - [[now dateWithCalendarFormat:nil timeZone:nil] hourOfDay];

int min = 59 - [[now dateWithCalendarFormat:nil timeZone:nil] minuteOfHour];

int sec = 59 - [[now dateWithCalendarFormat:nil timeZone:nil] secondOfMinute];

countdownLabel.text = [NSString stringWithFormat:@"%02d:%02d:%02d", hour, min,sec];

替换成如下代码即可顺利通过编译：

NSDate* now = [NSDate date];

NSCalendar *gregorian = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];

NSDateComponents *dateComponents = [gregorian components:(NSHourCalendarUnit | NSMinuteCalendarUnit | NSSecondCalendarUnit) fromDate:now];

NSInteger hour = [dateComponents hour];

NSInteger minute = [dateComponents minute];

NSInteger second = [dateComponents second];

[gregorian release];

countdownLabel.text = [NSString stringWithFormat:@"%02d:%02d:%02d", hour, minute, second];