webpack中学习到的拓扑排序

在html-webpack-plugin中看到可以通过拓扑排序的方式来对不同的chunks进行处理，所以把代码记录下来：

'use strict';
var toposort = require('toposort');
var _ = require('lodash');
/* Sorts dependencies between chunks by their "parents" attribute. This function sorts chunks based on their dependencies with each other. The parent relation between chunks as generated by Webpack for each chunk is used to define a directed (and hopefully acyclic) graph, which is then topologically sorted in order to retrieve the correct order in which chunks need to be embedded into HTML. A directed edge in this graph is describing a "is parent of" relationship from a chunk to another (distinct) chunk. Thus topological sorting orders chunks from bottom-layer chunks to highest level chunks that use the lower-level chunks. @param {Array} chunks an array of chunks as generated by the html-webpack-plugin. It is assumed that each entry contains at least the properties "id" (containing the chunk id) and "parents" (array containing the ids of the parent chunks). @return {Array} A topologically sorted version of the input chunks 因为最上面的通过commonchunkplugin产生的chunk具有webpack的runtime，所以排列在前面 */
module.exports.dependency = function (chunks) {
  if (!chunks) {
    return chunks;
  }
  // We build a map (chunk-id -> chunk) for faster access during graph building.
  // 通过chunk-id -> chunk这种Map结构更加容易绘制图
  var nodeMap = {};
  chunks.forEach(function (chunk) {
    nodeMap[chunk.id] = chunk;
  });
  // Next, we add an edge for each parent relationship into the graph
  var edges = [];
  chunks.forEach(function (chunk) {
    if (chunk.parents) {
      // Add an edge for each parent (parent -> child)
      chunk.parents.forEach(function (parentId) {
        // webpack2 chunk.parents are chunks instead of string id(s)
        var parentChunk = _.isObject(parentId) ? parentId : nodeMap[parentId];
        // If the parent chunk does not exist (e.g. because of an excluded chunk)
        // we ignore that parent
        if (parentChunk) {
          edges.push([parentChunk, chunk]);
        }
      });
    }
  });
  // We now perform a topological sorting on the input chunks and built edges
  return toposort.array(chunks, edges);
};

通过这种方式，我们通过commonchunkplugin抽取出来的含有webpack的runtime的chunk就会排列在前面~

参考资料：

topSort