將值控制在0~0.999之間
MIN(0.999, MAX(0, timeoffset - x))
MIN 讓值不大于0.999,MAX讓值不小于0.

-(void)pan:(UIPanGestureRecognizer * )pan
{
    CGFloat x =[pan translationInView:self.view].x;
    x /= 200.0;
    CFTimeInterval timeOffset = self.doorLayer.timeOffset;
    timeoffset = MIN(0.999, MAX(0, timeoffset - x));
    self.doorLayer.timeOffset = timeOffset;
    [pan setTranslation:CGPointZero inView:self.view];
}

static float randomFloatBetween(float from, float to)
{
    return from + ((float)rand()/(float)RAND_MAX) * (to - from);
}