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                class Plugin {
  apply(compiler) {
    compiler.plugin('emit', function (compilation, callback) {
      // compilation.chunks 存放所有代码块,是一个数组
      compilation.chunks.forEach(function (chunk) {
        // chunk 代表一个代码块
        // 代码块由多个模块组成,通过 chunk.forEachModule 能读取组成代码块的每个模块
        chunk.forEachModule(function (module) {
          // module 代表一个模块
          // module.fileDependencies 存放当前模块的所有依赖的文件路径,是一个数组
          module.fileDependencies.forEach(function (filepath) {

        // Webpack 会根据 Chunk 去生成输出的文件资源,每个 Chunk 都对应一个及其以上的输出文件
        // 例如在 Chunk 中包含了 CSS 模块并且使用了 ExtractTextPlugin 时,
        // 该 Chunk 就会生成 .js 和 .css 两个文件
        chunk.files.forEach(function (filename) {
          // compilation.assets 存放当前所有即将输出的资源
          // 调用一个输出资源的 source() 方法能获取到输出资源的内容
          let source = compilation.assets[filename].source();

      // 这是一个异步事件,要记得调用 callback 通知 Webpack 本次事件监听处理结束。
      // 如果忘记了调用 callback,Webpack 将一直卡在这里而不会往后执行。