Question

我不知道发生了什么。我试图通过比较日期来制作一个有效的计时器。当我启动计时器时，它有时会工作，然后随机停止，有时它只返回timeInterval的值。当时间间隔为负时，它似乎只能正常工作。这是我的方法：

-(IBAction)startTimer:(id)sender{
    if (timer == nil) {
        [startButton setTitle:@"Start" forState:UIControlStateNormal];
        timer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(timerAction:) userInfo:nil repeats:YES];
        date = [NSDate dateWithTimeIntervalSinceNow:testTask.timeInterval]; //instance variable
    } else {
        [startButton setTitle:@"Stop" forState:UIControlStateNormal];
        [timer invalidate];
        timer = nil;
    }

}
-(void)timerAction:(NSTimer *)t
{
    NSDate *currentDate = [NSDate date];
    NSTimeInterval updatedTimeInterval = [date timeIntervalSinceDate:currentDate];
    if (updatedTimeInterval > 0){
        if (self.timer)
        {
            [self timerExpired];
            [self.timer invalidate];
            self.timer = nil;
        }
    }
    else
    {
        testTask.timeInterval = updatedTimeInterval;
        NSLog(@"%.2f", testTask.timeInterval);
        NSError *error;
        if (![self.context save:&error]) {
            NSLog(@"couldn't save: %@", [error localizedDescription]);
        }
    }
    NSUInteger seconds = (NSUInteger)round(testTask.timeInterval);
    NSString *string = [NSString stringWithFormat:@"%02u:%02u:%02u",
                        seconds / 3600, (seconds / 60) % 60, seconds % 60];
    timerLabel.text = string;
    NSLog(@"%f", testTask.timeInterval);
}

Answer 1

您从较早的currentDate中减去较晚的日期date。我告诉过你这样做：[[NSDate date] timeIntervalSinceDate:date]

Answer 2

有一个原因timeIntervalSinceNow未被称为timeIntervalUntilNow ...

也称为：如果相关日期早于当前日期/时间，则过去的时间现在为负数。如果日期晚于当前日期，那将是积极的。（只是一点逻辑和/或英语语义：））

   Earlier date      Current date
        ^                  ^
        +------------------+
              since now: (earlier date) - (current date)... that's < 0

为什么我的时间间隔得到负值？

2 个答案: