读bootstrap的栅格系统

bootstrap中栅格布局源码。主要是为了多熟悉less，然后就是更加痛彻的明白bootstrap的原理。

(一)布局容器

lessgrid.less

.container {
  .container-fixed();

  @media (min-width: @screen-sm-min) {
     @container-sm;
  }
  @media (min- @screen-md-min) {
    width: @container-md;
  }
  @media (min- @screen-lg-min) {
    width: @container-lg;
  }
}


// Fluid container
//
// Utilizes the mixin meant for fixed width containers, but without any defined
// width for fluid, full width layouts.

.container-fluid {
  .container-fixed();
}

mixingrid.less

.container-fixed(@gutter: @grid-gutter-width) {
  margin-right: auto;
  margin-left: auto;
  padding-left:  floor((@gutter / 2));
  padding-right: ceil((@gutter / 2));
  &:extend(.clearfix all);
}

栅格布局分：

　　固定宽布局（.container）：容器有固定width值。

　　流式布局（.container-fluid）：容器不设置width值。

(二)行

lessgrid.less

.row {
  .make-row();
}

mixingrid.less

.make-row(@gutter: @grid-gutter-width) {
  margin-left:  ceil((@gutter / -2));
  margin-right: floor((@gutter / -2));
  &:extend(.clearfix all);
}

为列(col)设置一个row并设置负margin撑满container。

(三)列

lessgrid.less

.make-grid-columns();

mixingrid-framework.less

.make-grid-columns() {
  // Common styles for all sizes of grid columns, widths 1-12
  .col(@index) { // initial
    @item: ~".col-xs-@{index}, .col-sm-@{index}, .col-md-@{index}, .col-lg-@{index}";
    .col((@index + 1), @item);
  }
  .col(@index, @list) when (@index =< @grid-columns) { // general; "=<" isn't a typo
    @item: ~".col-xs-@{index}, .col-sm-@{index}, .col-md-@{index}, .col-lg-@{index}";
    .col((@index + 1), ~"@{list}, @{item}");
  }
  .col(@index, @list) when (@index > @grid-columns) { // terminal
    @{list} {
      position: relative;
      // Prevent columns from collapsing when empty
      min-height: 1px;
      // Inner gutter via padding
      padding-left:  ceil((@grid-gutter-width / 2));
      padding-right: floor((@grid-gutter-width / 2));
    }
  }
  .col(1); // kickstart it
}

.make-grid-columns生成的所有栅格列的类名后，添加每个类名的公共样式。

lessgrid.less

.make-grid(xs);

mixingrid-framework.less

.float-grid-columns(@class) {
  .col(@index) { // initial
    @item: ~".col-@{class}-@{index}";
    .col((@index + 1), @item);
  }
  .col(@index, @list) when (@index =< @grid-columns) { // general
    @item: ~".col-@{class}-@{index}";
    .col((@index + 1), ~"@{list}, @{item}");
  }
  .col(@index, @list) when (@index > @grid-columns) { // terminal
    @{list} {
      float: left;
    }
  }
  .col(1); // kickstart it
}

.calc-grid-column(@index, @class, @type) when (@type = width) and (@index > 0) {
  .col-@{class}-@{index} {
    width: percentage((@index / @grid-columns));
  }
}
.calc-grid-column(@index, @class, @type) when (@type = push) and (@index > 0) {
  .col-@{class}-push-@{index} {
    left: percentage((@index / @grid-columns));
  }
}
.calc-grid-column(@index, @class, @type) when (@type = push) and (@index = 0) {
  .col-@{class}-push-0 {
    left: auto;
  }
}
.calc-grid-column(@index, @class, @type) when (@type = pull) and (@index > 0) {
  .col-@{class}-pull-@{index} {
    right: percentage((@index / @grid-columns));
  }
}
.calc-grid-column(@index, @class, @type) when (@type = pull) and (@index = 0) {
  .col-@{class}-pull-0 {
    right: auto;
  }
}
.calc-grid-column(@index, @class, @type) when (@type = offset) {
  .col-@{class}-offset-@{index} {
    margin-left: percentage((@index / @grid-columns));
  }
}

// Basic looping in LESS
.loop-grid-columns(@index, @class, @type) when (@index >= 0) {
  .calc-grid-column(@index, @class, @type);
  // next iteration
  .loop-grid-columns((@index - 1), @class, @type);
}

.make-grid(@class) {
  .float-grid-columns(@class);
  .loop-grid-columns(@grid-columns, @class, width);
  .loop-grid-columns(@grid-columns, @class, pull);
  .loop-grid-columns(@grid-columns, @class, push);
  .loop-grid-columns(@grid-columns, @class, offset);
}

栅格列偏移（offset）计算Mixin，列排序（pull or push）计算Mixin，列宽（width）计算Mixin，浮动。

bootstrap3以xs屏优先，直接初始化.make-grid(xs)。

@media (min- @screen-sm-min) {
  .make-grid(sm);
}


// Medium grid
//
// Columns, offsets, pushes, and pulls for the desktop device range.

@media (min- @screen-md-min) {
  .make-grid(md);
}


// Large grid
//
// Columns, offsets, pushes, and pulls for the large desktop device range.

@media (min- @screen-lg-min) {
  .make-grid(lg);
}

其他屏幕写入@media中。

不明白float-grid-columns直接添加了变量（@class）以小屏（xs）优先写在了@media外边，而make-grid-columns并没有这样。

（4）Mixin 列

// Generate the extra small columns
.make-xs-column(@columns; @gutter: @grid-gutter-width) {
  position: relative;
  float: left;
  width: percentage((@columns / @grid-columns));
  min-height: 1px;
  padding-left:  (@gutter / 2);
  padding-right: (@gutter / 2);
}
.make-xs-column-offset(@columns) {
  margin-left: percentage((@columns / @grid-columns));
}
.make-xs-column-push(@columns) {
  left: percentage((@columns / @grid-columns));
}
.make-xs-column-pull(@columns) {
  right: percentage((@columns / @grid-columns));
}

// Generate the small columns
.make-sm-column(@columns; @gutter: @grid-gutter-width) {
  position: relative;
  min-height: 1px;
  padding-left:  (@gutter / 2);
  padding-right: (@gutter / 2);

  @media (min-width: @screen-sm-min) {
    float: left;
    width: percentage((@columns / @grid-columns));
  }
}
.make-sm-column-offset(@columns) {
  @media (min-width: @screen-sm-min) {
    margin-left: percentage((@columns / @grid-columns));
  }
}
.make-sm-column-push(@columns) {
  @media (min-width: @screen-sm-min) {
    left: percentage((@columns / @grid-columns));
  }
}
.make-sm-column-pull(@columns) {
  @media (min-width: @screen-sm-min) {
    right: percentage((@columns / @grid-columns));
  }
}

// Generate the medium columns
.make-md-column(@columns; @gutter: @grid-gutter-width) {
  position: relative;
  min-height: 1px;
  padding-left:  (@gutter / 2);
  padding-right: (@gutter / 2);

  @media (min-width: @screen-md-min) {
    float: left;
    width: percentage((@columns / @grid-columns));
  }
}
.make-md-column-offset(@columns) {
  @media (min-width: @screen-md-min) {
    margin-left: percentage((@columns / @grid-columns));
  }
}
.make-md-column-push(@columns) {
  @media (min-width: @screen-md-min) {
    left: percentage((@columns / @grid-columns));
  }
}
.make-md-column-pull(@columns) {
  @media (min-width: @screen-md-min) {
    right: percentage((@columns / @grid-columns));
  }
}

// Generate the large columns
.make-lg-column(@columns; @gutter: @grid-gutter-width) {
  position: relative;
  min-height: 1px;
  padding-left:  (@gutter / 2);
  padding-right: (@gutter / 2);

  @media (min-width: @screen-lg-min) {
    float: left;
    width: percentage((@columns / @grid-columns));
  }
}
.make-lg-column-offset(@columns) {
  @media (min-width: @screen-lg-min) {
    margin-left: percentage((@columns / @grid-columns));
  }
}
.make-lg-column-push(@columns) {
  @media (min-width: @screen-lg-min) {
    left: percentage((@columns / @grid-columns));
  }
}
.make-lg-column-pull(@columns) {
  @media (min-width: @screen-lg-min) {
    right: percentage((@columns / @grid-columns));
  }
}

不使用预定义栅格类名的方法，而是直接使用Mixin划分.row。需要使用less编写。