毛毛虫滑块

/**
 根据总体偏移比例和相关固定值计算滑块起点坐标x, 滑块长度.

 @param sliderWidth 滑块静止时的长度
 @param points 滑块静止的站点数,也就是标题数量
 @param overallLength 按滑块从第一静止站点的左端到最后静止站点右端的总长度
 @param rate 外部输入滑动的比例
 @return poin.x 为滑块起点坐标x, point.y 为滑块的长度
 */
- (CGPoint)sliderLocationAndLengthWithSliderWidth:(CGFloat)sliderWidth points:(NSUInteger)points overallLength:(CGFloat)overallLength offsetRate:(CGFloat)rate {
    
    if (rate < 0) {
        return CGPointMake(0.0, sliderWidth);
    }
    
    if (rate > 1) {
        return CGPointMake(overallLength - sliderWidth, sliderWidth);
    }
    
    // space 滑块之间的间隔
    CGFloat space = (overallLength - points * sliderWidth) / (points - 1);
    // 细分滑动段数量, 每个锚点之间分起始段和抵达段两段
    NSInteger section = (points - 1) * 2;
    // 计算总体的滑动比例rate在细分后的哪一段
    NSInteger index = 0;
    for (int i = 0; i < section; i++) {
        CGFloat max = (CGFloat)(i + 1) / section;
        index = i;
        if (rate < max) {
            break;
        }
    }
    
    // 判断是起始段还是抵达段
    BOOL isStart = (index % 2 == 0);

    // 计算总体的滑动比例rate, 在细分段中占该段的比例
    CGFloat sectionRate = 1.0 / section;
    CGFloat minRate = index * sectionRate;
    CGFloat progress = (rate - minRate) / sectionRate;
    // 计算起始坐标和对应起始坐标的偏移长度
    CGFloat x = 0.0;
    CGFloat lenght = 0.0;
    if (isStart) {
        x = (index / 2) * (sliderWidth + space);
        lenght =  sliderWidth + (sliderWidth + space) * progress;
    }else{
        x = (index / 2) * (sliderWidth + space) + (sliderWidth + space) * progress;
        lenght = sliderWidth + (sliderWidth + space) * (1 - progress);
    }
    
    return CGPointMake(x, lenght);
}