移动端1px的处理

一起养成写作习惯！这是我参与「掘金日新计划 · 4 月更文挑战」的第1天，点击查看活动详情。

问题描述： css的1px在PC端就是1px；在移动端就往往就大于 1px

产生原因：

设备像素比：dpr=window.devicePixelRatio，也就是设备的物理像素与逻辑像素的比值。
在retina屏的手机上, dpr为2或3，css里写的1px宽度映射到物理像素上就有2px或3px宽度。
例如：iPhone6的dpr为2，物理像素是750（x轴）,它的逻辑像素为375。也就是说，1个逻辑像素，在x轴和y轴方向，需要2个物理像素来显示，即：dpr=2时，表示1个CSS像素由4个物理像素点组成

解决方案：

方案一：伪元素+transform缩放

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>1px 问题</title>
    <style>
        body {margin:0;padding:19px}
        div { padding:1em;margin:1em;}
        .box1{border-top: 1px solid #000;}
        .box2{position: relative;border-bottom:1px solid #000;}
        .box2::after{
            content:'';
            position: absolute;
            left:0;top:0;width: 100%;
            height: 1px; 
            background-color: #000;
            transform: scaleY(0.5);
        }
    </style>
</head>
<body>
    <div class="box1">
        1px的上边框
    </div>

    <div class="box2">
        0.5px的上边框
    </div>
</body>
</html>

注意：伪元素::after或::before独立于当前元素，可以单独对其缩放而不影响元素本身的缩放

方案2：创建公共样式

// src/assets/styles/hairline.scss

@mixin scale-hairline-common($color, $top, $right, $bottom, $left) {
  content: '';
  position: absolute;
  display: block;
  z-index: 1;
  top: $top;
  right: $right;
  bottom: $bottom;
  left: $left;
  background-color: $color;
}


@mixin hairline($direction, $color: #000, $radius: 0) {
  @if $direction == top {
    border-top: 1px solid $color;

    // min-resolution 用来检测设备的最小像素密度
    @media (min-resolution: 2dppx) {
      border-top: none;

      &::before {
        @include scale-hairline-common($color, 0, auto, auto, 0);
        width: 100%;
        height: 1px;
        transform-origin: 50% 50%;
        transform: scaleY(0.5);

        @media (min-resolution: 3dppx) {
          transform: scaleY(0.33);
        }
      }
    }
  } @else if $direction == right {
    border-right: 1px solid $color;

    @media (min-resolution: 2dppx) {
      border-right: none;

      &::after {
        @include scale-hairline-common($color, 0, 0, auto, auto);
        width: 1px;
        height: 100%;
        background: $color;
        transform-origin: 100% 50%;
        transform: scaleX(0.5);

        @media (min-resolution: 3dppx) {
          transform: scaleX(0.33);
        }
      }
    }
  } @else if $direction == bottom {
    border-bottom: 1px solid $color;

    @media (min-resolution: 2dppx) {
      border-bottom: none;

      &::after {
        @include scale-hairline-common($color, auto, auto, 0, 0);
        width: 100%;
        height: 1px;
        transform-origin: 50% 100%;
        transform: scaleY(0.5);

        @media (min-resolution: 3dppx) {
          transform: scaleY(0.33);
        }
      }
    }
  } @else if $direction == left {
    border-left: 1px solid $color;

    @media (min-resolution: 2dppx) {
      border-left: none;

      &::before {
        @include scale-hairline-common($color, 0, auto, auto, 0);
        width: 1px;
        height: 100%;
        transform-origin: 100% 50%;
        transform: scaleX(0.5);

        @media (min-resolution: 3dppx) {
          transform: scaleX(0.33);
        }
      }
    }
  } @else if $direction == all {
    border: 1px solid $color;
    border-radius: $radius;

    @media (min-resolution: 2dppx) {
      position: relative;
      border: none;

      &::before {
        content: '';
        position: absolute;
        left: 0;
        top: 0;
        width: 200%;
        height: 200%;
        border: 1px solid $color;
        border-radius: $radius * 2;
        transform-origin: 0 0;
        transform: scale(0.5);
        box-sizing: border-box;
        pointer-events: none;
      }
    }
  }
}

// 移除边框
@mixin hairline-remove($position: all) {
  @if $position == left {
    border-left: 0;
    &::before {
      display: none !important;
    }
  } @else if $position == right {
    border-right: 0;
    &::after {
      display: none !important;
    }
  } @else if $position == top {
    border-top: 0;
    &::before {
      display: none !important;
    }
  } @else if $position == bottom {
    border-bottom: 0;
    &::after {
      display: none !important;
    }
  } @else if $position == all {
    border: 0;
    &::before {
      display: none !important;
    }
    &::after {
      display: none !important;
    }
  }
}

使用步骤：

1.在某个Scss文件中导入所封装的Scss文件

2.给某个选择器添加1px的边框

语法：

选择器 {
  @include hairline(参数，xxxx)
}

示例：

@import '~@scss/hairline.scss';

.box1 {
  margin: 10px 0;
  position:relative;
  @include hairline(bottom, #000); // 添加边框
  // 移除边框
  // @include hairline-remove(bottom); 
}