Objective-C:NSTimer和倒计时

- (void)startTimerWithDuration:(NSTimeInterval)duration {
    NSDate *fireDate = [NSDate dateWithTimeIntervalSinceNow:duration];
    self.timer = [NSTimer timerWithFireDate:fireDate interval:1.0 target:self selector:@selector(timerFired:) userInfo:nil repeats:YES];
    [[NSRunLoop currentRunLoop] addTimer:self.timer forMode:NSRunLoopCommonModes];
}

- (void)timerFired:(NSTimer *)timer {
    NSTimeInterval remainingTime = [timer.fireDate timeIntervalSinceNow];
    if (remainingTime <= 0) {
        [self.timer invalidate];
        self.timer = nil;
        // 倒计时结束后的操作
    } else {
        // 更新倒计时显示
    }
}