说明

table-header是表头组件，较为复杂，直接看源码解读。

源码解读

import ElCheckbox from 'element-ui/packages/checkbox';
import ElTag from 'element-ui/packages/tag';
import Vue from 'vue';
import FilterPanel from './filter-panel.vue';

/**
 * 获取所有的列，因为会有嵌套所以使用了递归
 * @param columns 原始的所有列
 * @return 所有列
 */
const getAllColumns = (columns) => {
  const result = [];
  columns.forEach((column) => {
    if (column.children) {
      result.push(column);
      result.push.apply(result, getAllColumns(column.children));
    } else {
      result.push(column);
    }
  });
  return result;
};

/**
 * 将所有的列信息转换成表头的行信息
 * @param originColumns 列信息
 * @return 表头信息
 */
const convertToRows = (originColumns) => {
  // 最大层级
  let maxLevel = 1;
  // 遍历列来判断表头每个单元格需要占多少格
  const traverse = (column, parent) => {
    if (parent) {
      column.level = parent.level + 1;
      if (maxLevel < column.level) {
        maxLevel = column.level;
      }
    }
    if (column.children) {
      let colSpan = 0;
      column.children.forEach((subColumn) => {
        traverse(subColumn, column);
        colSpan += subColumn.colSpan;
      });
      column.colSpan = colSpan;
    } else {
      column.colSpan = 1;
    }
  };

  // 获取每一列的层级
  originColumns.forEach((column) => {
    column.level = 1;
    traverse(column);
  });

  const rows = [];
  for (let i = 0; i < maxLevel; i++) {
    rows.push([]);
  }

  const allColumns = getAllColumns(originColumns);

  // 相同的层级作为同一行
  allColumns.forEach((column) => {
    if (!column.children) {
      column.rowSpan = maxLevel - column.level + 1;
    } else {
      column.rowSpan = 1;
    }
    rows[column.level - 1].push(column);
  });

  return rows;
};

export default {
  name: 'ElTableHeader',

  // 渲染函数
  render(h) {
    // 原始列信息
    const originColumns = this.store.states.originColumns;
    // 表头信息
    const columnRows = convertToRows(originColumns, this.columns);

    return (
      // table 上的 style 是为了清除默认的间距
      <table
        class="el-table__header"
        cellspacing="0"
        cellpadding="0"
        border="0">
        {/* colgroup 是用来存储列信息的 */}
        <colgroup>
          {/* 列信息 */}
          {
            this._l(this.columns, column =>
              <col
                name={ column.id }
                width={ column.realWidth || column.width }
              />)
          }
          {/* 如果左侧有固定还需要记录滚动条的宽度 */}
          {
            !this.fixed && this.layout.gutterWidth
              ? <col name="gutter" width={ this.layout.scrollY ? this.layout.gutterWidth : '' }></col>
              : ''
          }
        </colgroup>
        <thead>
          {
            // 按行渲染
            this._l(columnRows, (columns, rowIndex) =>
              <tr>
              {
                // 渲染每个单元格
                this._l(columns, (column, cellIndex) =>
                  <th
                    // 列高
                    colspan={ column.colSpan }
                    // 行宽
                    rowspan={ column.rowSpan }
                    // 鼠标移动事件
                    on-mousemove={ ($event) => this.handleMouseMove($event, column) }
                    // 鼠标移出事件
                    on-mouseout={ this.handleMouseOut }
                    // 鼠标按下事件
                    on-mousedown={ ($event) => this.handleMouseDown($event, column) }
                    // 鼠标单击事件
                    on-click={ ($event) => this.handleHeaderClick($event, column) }
                    class={
                      [
                        column.id,
                        column.order,
                        column.headerAlign,
                        column.className || '',
                        // 判断是否隐藏，为了处理 fixed
                        rowIndex === 0 && this.isCellHidden(cellIndex, columns) ? 'is-hidden' : '',
                        // 判断是不是最后一级
                        !column.children ? 'is-leaf' : ''
                      ]
                    }>
                    <div
                      class={
                        [
                          'cell',
                          // 如果这列有选择筛选条件就高亮
                          column.filteredValue && column.filteredValue.length > 0 ? 'highlight' : ''
                        ]
                      }>
                    {
                      // 渲染单元格内部的内容
                      column.renderHeader
                        ? column.renderHeader.call(
                          this._renderProxy,
                          h,
                          {
                            column,
                            $index: cellIndex,
                            store: this.store,
                            _self: this.$parent.$vnode.context
                          })
                        : column.label
                    }
                    {
                      // 渲染排序的标志
                      column.sortable
                        ? <span class="caret-wrapper" on-click={ ($event) => this.handleSortClick($event, column) }>
                            <i class="sort-caret ascending"></i>
                            <i class="sort-caret descending"></i>
                          </span>
                        : ''
                    }
                    {
                      // 渲染筛选器的箭头
                      column.filterable
                         ? <span
                            class="el-table__column-filter-trigger"
                            on-click={ ($event) => this.handleFilterClick($event, column) }>
                            <i
                              class={
                                [
                                  'el-icon-arrow-down',
                                  column.filterOpened ? 'el-icon-arrow-up' : ''
                                ]
                              }>
                            </i>
                          </span>
                        : ''
                    }
                    </div>
                  </th>
                )
              }
              {
                // 弥补滚动条的宽度
                !this.fixed && this.layout.gutterWidth
                  ? <th class="gutter" style={{ width: this.layout.scrollY ? this.layout.gutterWidth + 'px' : '0' }}></th>
                  : ''
              }
              </tr>
            )
          }
        </thead>
      </table>
    );
  },

  props: {
    fixed: String,
    store: {
      required: true
    },
    layout: {
      required: true
    },
    border: Boolean,
    defaultSort: {
      type: Object,
      default() {
        return {
          prop: '',
          order: ''
        };
      }
    }
  },

  components: {
    ElCheckbox,
    ElTag
  },

  computed: {
    // 判断是不是全选了
    isAllSelected() {
      return this.store.states.isAllSelected;
    },

    // 判断总的列数
    columnsCount() {
      return this.store.states.columns.length;
    },

    // 左侧固定列数
    leftFixedCount() {
      return this.store.states.fixedColumns.length;
    },

    // 右侧固定列数
    rightFixedCount() {
      return this.store.states.rightFixedColumns.length;
    },

    // 所有的列
    columns() {
      return this.store.states.columns;
    }
  },

  created() {
    this.filterPanels = {};
  },

  mounted() {
    if (this.defaultSort.prop) {
      const states = this.store.states;
      // 排序的属性
      states.sortProp = this.defaultSort.prop;
      // 升序或降序
      states.sortOrder = this.defaultSort.order || 'ascending';
      this.$nextTick(_ => {
        for (let i = 0, length = this.columns.length; i < length; i++) {
          let column = this.columns[i];
          // 如果是要排序的属性
          if (column.property === states.sortProp) {
            column.order = states.sortOrder;
            states.sortingColumn = column;
            break;
          }
        }

        if (states.sortingColumn) {
          this.store.commit('changeSortCondition');
        }
      });
    }
  },

  beforeDestroy() {
    const panels = this.filterPanels;
    for (let prop in panels) {
      // 销毁全部的筛选面板
      if (panels.hasOwnProperty(prop) && panels[prop]) {
        panels[prop].$destroy(true);
      }
    }
  },

  methods: {
    // 判断单元格是否应当隐藏
    isCellHidden(index, columns) {
      // 左侧固定的 wrapper 中，那么除了固定的这些列都应该隐藏掉
      if (this.fixed === true || this.fixed === 'left') {
        return index >= this.leftFixedCount;
      } else if (this.fixed === 'right') {  // 右侧固定的 wrapper 中，规定列之前的也都应当隐藏
        let before = 0;
        for (let i = 0; i < index; i++) {
          before += columns[i].colSpan;
        }
        return before < this.columnsCount - this.rightFixedCount;
      } else {  // 剩下的就是隐藏固定的列
        return (index < this.leftFixedCount) || (index >= this.columnsCount - this.rightFixedCount);
      }
    },

    // 切换全选
    toggleAllSelection() {
      this.store.commit('toggleAllSelection');
    },

    // 处理点击筛选器，应当显示对应的筛选面板
    handleFilterClick(event, column) {
      event.stopPropagation();
      const target = event.target;
      const cell = target.parentNode;
      const table = this.$parent;

      // 查找对应的筛选面板
      let filterPanel = this.filterPanels[column.id];

      // 如果存在，并且打开了，就关闭它
      if (filterPanel && column.filterOpened) {
        filterPanel.showPopper = false;
        return;
      }

      // 如果不存在，就创建它
      if (!filterPanel) {
        filterPanel = new Vue(FilterPanel);
        this.filterPanels[column.id] = filterPanel;

        filterPanel.table = table;
        filterPanel.cell = cell;
        filterPanel.column = column;
        !this.$isServer && filterPanel.$mount(document.createElement('div'));
      }

      // 创建后打开
      setTimeout(() => {
        filterPanel.showPopper = true;
      }, 16);
    },

    // 处理表头点击事件
    handleHeaderClick(event, column) {
      if (!column.filters && column.sortable) {  // 排序
        this.handleSortClick(event, column);
      } else if (column.filters && !column.sortable) {  // 筛选
        this.handleFilterClick(event, column);
      }

      this.$parent.$emit('header-click', column, event);
    },

    // 鼠标按下
    handleMouseDown(event, column) {
      if (this.$isServer) return;
      // 如果这一列还有孩子直接返回，应该操作孩子的宽度
      if (column.children && column.children.length > 0) return;
      /* istanbul ignore if */
      // 如果有拖拽的列，并且有边框
      if (this.draggingColumn && this.border) {
        // 表示正在拖动
        this.dragging = true;

        // 显示 resize
        this.$parent.resizeProxyVisible = true;

        const tableEl = this.$parent.$el;
        const tableLeft = tableEl.getBoundingClientRect().left;
        // 拖动列
        const columnEl = this.$el.querySelector(`th.${column.id}`);
        const columnRect = columnEl.getBoundingClientRect();
        const minLeft = columnRect.left - tableLeft + 30;

        columnEl.classList.add('noclick');

        this.dragState = {
          startMouseLeft: event.clientX,  // 鼠标开始位置
          startLeft: columnRect.right - tableLeft,  // 开始位置距离表格最左边的距离
          startColumnLeft: columnRect.left - tableLeft,  // 开始时列左边离表格最左边的距离
          tableLeft
        };

        // 显示拖拽位置的线
        const resizeProxy = this.$parent.$refs.resizeProxy;
        resizeProxy.style.left = this.dragState.startLeft + 'px';

        document.onselectstart = function() { return false; };
        document.ondragstart = function() { return false; };

        // 鼠标移动的时候，计算移动距离并且移动辅助线
        const handleMouseMove = (event) => {
          const deltaLeft = event.clientX - this.dragState.startMouseLeft;
          const proxyLeft = this.dragState.startLeft + deltaLeft;

          resizeProxy.style.left = Math.max(minLeft, proxyLeft) + 'px';
        };

        // 鼠标抬起
        const handleMouseUp = () => {
          if (this.dragging) {
            const finalLeft = parseInt(resizeProxy.style.left, 10);  // 最终停止的位置
            const columnWidth = finalLeft - this.dragState.startColumnLeft;  // 应该变成的列宽
            column.width = column.realWidth = columnWidth;  // 应用列宽改变

            this.store.scheduleLayout();  // 重新更新布局

            document.body.style.cursor = '';
            this.dragging = false;
            this.draggingColumn = null;
            this.dragState = {};

            this.$parent.resizeProxyVisible = false;
          }

          // 移除相应的监听器
          document.removeEventListener('mousemove', handleMouseMove);
          document.removeEventListener('mouseup', handleMouseUp);
          document.onselectstart = null;
          document.ondragstart = null;

          setTimeout(function() {
            columnEl.classList.remove('noclick');
          }, 0);
        };

        document.addEventListener('mousemove', handleMouseMove);
        document.addEventListener('mouseup', handleMouseUp);
      }
    },

    // 鼠标移动事件
    handleMouseMove(event, column) {
      if (column.children && column.children.length > 0) return;
      let target = event.target;
      while (target && target.tagName !== 'TH') {  // 寻找 th 标签
        target = target.parentNode;
      }

      // 如果没有列，或者不能改变大小
      if (!column || !column.resizable) return;

      // 如果正在拖动并且有边框
      if (!this.dragging && this.border) {
        let rect = target.getBoundingClientRect();

        const bodyStyle = document.body.style;
        if (rect.width > 12 && rect.right - event.pageX < 8) {
          bodyStyle.cursor = 'col-resize';
          this.draggingColumn = column;
        } else if (!this.dragging) {
          bodyStyle.cursor = '';
          this.draggingColumn = null;
        }
      }
    },

    // 鼠标移除后
    handleMouseOut() {
      if (this.$isServer) return;
      document.body.style.cursor = '';
    },

    // 切换排序顺序
    toggleOrder(order) {
      return !order ? 'ascending' : order === 'ascending' ? 'descending' : null;
    },

    // 点击排序
    handleSortClick(event, column) {
      event.stopPropagation();
      // 切换排序顺序
      let order = this.toggleOrder(column.order);

      // 寻找 TH
      let target = event.target;
      while (target && target.tagName !== 'TH') {
        target = target.parentNode;
      }

      // 如果这时候有 `noclick` 类，就移除
      if (target && target.tagName === 'TH') {
        if (target.classList.contains('noclick')) {
          target.classList.remove('noclick');
          return;
        }
      }

      // 如果不能排序就直接返回
      if (!column.sortable) return;

      const states = this.store.states;
      let sortProp = states.sortProp;
      let sortOrder;
      const sortingColumn = states.sortingColumn;

      // 如果排序列不是当前列，就切换成当前列
      if (sortingColumn !== column) {
        if (sortingColumn) {
          sortingColumn.order = null;
        }
        states.sortingColumn = column;
        sortProp = column.property;
      }

      // 如果没有顺序
      if (!order) {
        sortOrder = column.order = null;
        states.sortingColumn = null;
        sortProp = null;
      } else {
        sortOrder = column.order = order;
      }

      states.sortProp = sortProp;
      states.sortOrder = sortOrder;

      this.store.commit('changeSortCondition');
    }
  },

  data() {
    return {
      draggingColumn: null,
      dragging: false,
      dragState: {}
    };
  }
};