先了解下scale 
scale 即屏幕缩放系数，根据当前设备屏幕的大小计算的比例
下面来看下不同几款设备对应的scale

Snip20170510_4.png

以上数据是根据下面代码获取scale 
CGFloat scale = [UIScreen mainScreen].scale

实例代码

设置高为1像素的视图
#define SINGLE_LINE_WIDTH           (1 / [UIScreen mainScreen].scale)
#define SINGLE_LINE_ADJUST_OFFSET   ((1 / [UIScreen mainScreen].scale) / 2)
lineWidth = SINGLE_LINE_WIDTH;
    CGFloat pixelAdjustOffset = SINGLE_LINE_WIDTH;
    //仅当要绘制的线宽为奇数像素时，绘制位置需要调整
if (((int)(lineWidth * [UIScreen mainScreen].scale) + 1) % 2 == 0) {
        pixelAdjustOffset = SINGLE_LINE_ADJUST_OFFSET;
    }

    UIView *line = [[UIView alloc] initWithFrame:CGRectZero];
//    xPos yPos
    line.frame = CGRectMake(0-pixelAdjustOffset, 100-pixelAdjustOffset, sys_w, lineWidth);
    line.backgroundColor = [UIColor redColor];
    [self.view addSubview:line];