CSSでnマスレイアウトを実装する方法

<!-- a-grid是一个flex容器, 方便他的内容做"水平/垂直居中" -->
<div class="a-grid">
  <!-- a-grid__item用来占位实现正方形 -->
  <div class="a-grid__item">
      <!-- item__content才是真正装内容的容器 -->
      <div class="item__content">
        内容...
      </div>
  </div>
</div>

.a-grid {
    display: flex;
    flex-wrap: wrap;
    width: 100%;
 
    .a-grid__item {
        text-align:center;
        position:relative;
        >.item__content {
            display:flex
            flex-flow: column;
            align-items: center;
            justify-content: center;
        }
    }
}
 
@mixin grid($row:3, $column:3, $hasBorder:false, $isSquare:true) {
    @extend .a-grid;
 
    .a-grid__item {
 
        flex-basis: 100%/$column;
 
        @if($isSquare) {
            padding-bottom: 100%/$column;
            height: 0;
        }
 
        >.item__content {
 
            @if($isSquare) {
                position:absolute;
                top:0;left:0;right:0;bottom:0;
            }
        }
    }
 
    @for $index from 1 to (($row - 1) * $column + 1) {
        .a-grid__item:nth-child(#{$index}) {
            @if($hasBorder) {
                border-bottom: 1px solid #eee;
            }
        }
    }
 
    @for $index from 1 to $column {
        .a-grid__item:nth-child(#{$column}n + #{$index}) {
            @if($hasBorder) {
                border-right: 1px solid #eee;
            }
        }
    }
}

// 生成一个 3行3列, 正方形格子的宫格
.a-grid-3-3 {
    @include grid(3, 3, true);
}
 
// 生成一个 2行5列, 无边框宫格, 每个格子由内容决定高度
.a-grid-2-5 {
    @include grid(2, 5, false, false);
}