以下是您可以用来做您想做的事情的类:

Here's a class you can use to do what you want:

@interface StopWatch()
@property ( nonatomic, strong ) NSTimer * displayTimer ;
@property ( nonatomic ) CFAbsoluteTime startTime ;
@end

@implementation StopWatch

-(void)dealloc
{
    [ self.displayTimer invalidate ] ;
}

-(void)startTimer
{
    self.startTime = CFAbsoluteTimeGetCurrent() ;
    self.displayTimer = [ NSTimer scheduledTimerWithTimeInterval:0.05 target:self selector:@selector( timerFired: ) userInfo:nil repeats:YES ] ;
}

-(void)stopTimer
{
    [ self.displayTimer invalidate ] ;
    self.displayTimer = nil ;

    CFAbsoluteTime elapsedTime = CFAbsoluteTimeGetCurrent() - self.startTime ;
    [ self updateDisplay:elapsedTime ] ;
}

-(void)timerFired:(NSTimer*)timer
{
    CFAbsoluteTime elapsedTime = CFAbsoluteTimeGetCurrent() - self.startTime ;
    [ self updateDisplay:elapsedTime ] ;
}

-(void)updateDisplay:(CFAbsoluteTime)elapsedTime
{
    // update your label here
}

@end

关键点是:

1. 通过将秒表启动时的系统时间保存到变量中来进行计时.
2. 秒表停止时，通过从当前时间中减去秒表开始时间来计算经过的时间
3. 使用计时器更新您的显示.无论您的计时器是否准确，这都不重要.如果您要保证至少每隔0.1秒更新一次显示，则可以尝试将计时器间隔设置为最小更新时间(0.05s)的1/2.