feat(compiler): Use a single compiler for multiple plugin instances

index.js

@@ -84,12 +84,19 @@ class HtmlWebpackPlugin {
       this.options.filename = path.relative(compiler.options.output.path, filename);
     }
 
+    // Clear the cache once a new HtmlWebpackPlugin is added
+    childCompiler.clearCache(compiler);
+
+    compiler.hooks.compile.tap('HtmlWebpackPlugin', () => {
+      childCompiler.addTemplateToCompiler(compiler, this.options.template);
+    });
+
     // setup hooks for third party plugins
     compiler.hooks.compilation.tap('HtmlWebpackPluginHooks', getHtmlWebpackPluginHooks);
 
     compiler.hooks.make.tapAsync('HtmlWebpackPlugin', (compilation, callback) => {
       // Compile the template (queued)
-      compilationPromise = childCompiler.compileTemplate(self.options.template, compiler.context, self.options.filename, compilation)
+      compilationPromise = childCompiler.compileTemplate(self.options.template, self.options.filename, compilation)
         .catch(err => {
           compilation.errors.push(prettyError(err, compiler.context).toString());
           return {
@@ -114,6 +121,9 @@ class HtmlWebpackPlugin {
      * @param {() => void} callback
     */
       (compilation, callback) => {
+        // Clear the childCompilerCache
+        childCompiler.clearCache(compiler);
+
         // Get all entry point names for this html file
         const entryNames = Array.from(compilation.entrypoints.keys());
         const filteredEntryNames = self.filterChunks(entryNames, self.options.chunks, self.options.excludeChunks);

lib/compiler.js

@@ -1,114 +1,254 @@
-/*
+// @ts-check
+/** @typedef {import("webpack/lib/Compilation.js")} WebpackCompilation */
+/** @typedef {import("webpack/lib/Compiler.js")} WebpackCompiler */
+/** @typedef {import("webpack/lib/Chunk.js")} WebpackChunk */
+'use strict';
+/**
+ * @file
  * This file uses webpack to compile a template with a child compiler.
  *
  * [TEMPLATE] -> [JAVASCRIPT]
  *
  */
 'use strict';
-const path = require('path');
 const NodeTemplatePlugin = require('webpack/lib/node/NodeTemplatePlugin');
 const NodeTargetPlugin = require('webpack/lib/node/NodeTargetPlugin');
 const LoaderTargetPlugin = require('webpack/lib/LoaderTargetPlugin');
 const LibraryTemplatePlugin = require('webpack/lib/LibraryTemplatePlugin');
 const SingleEntryPlugin = require('webpack/lib/SingleEntryPlugin');
 
 /**
- * Compiles the template into a nodejs factory, adds its to the compilation.assets
- * and returns a promise of the result asset object.
- *
- * @param template relative path to the template file
- * @param context path context
- * @param outputFilename the file name
- * @param compilation The webpack compilation object
- *
- * Returns an object:
- * {
- *  hash: {String} - Base64 hash of the file
- *  content: {String} - Javascript executable code of the template
- * }
- *
+ * The HtmlWebpackChildCompiler is a helper to allow resusing one childCompiler
+ * for multile HtmlWebpackPlugin instances to improve the compilation performance.
  */
-module.exports.compileTemplate = function compileTemplate (template, context, outputFilename, compilation) {
-  // The entry file is just an empty helper as the dynamic template
-  // require is added in "loader.js"
-  const outputOptions = {
-    filename: outputFilename,
-    publicPath: compilation.outputOptions.publicPath
-  };
-  // Store the result of the parent compilation before we start the child compilation
-  const assetsBeforeCompilation = Object.assign({}, compilation.assets[outputOptions.filename]);
-  // Create an additional child compiler which takes the template
-  // and turns it into an Node.JS html factory.
-  // This allows us to use loaders during the compilation
-  const compilerName = getCompilerName(context, outputFilename);
-  const childCompiler = compilation.createChildCompiler(compilerName, outputOptions);
-  childCompiler.context = context;
-  new NodeTemplatePlugin(outputOptions).apply(childCompiler);
-  new NodeTargetPlugin().apply(childCompiler);
-  new LibraryTemplatePlugin('HTML_WEBPACK_PLUGIN_RESULT', 'var').apply(childCompiler);
-
-  // Using undefined as name for the SingleEntryPlugin causes a unexpected output as described in
-  // https://github.com/jantimon/html-webpack-plugin/issues/895
-  // Using a string as a name for the SingleEntryPlugin causes problems with HMR as described in
-  // https://github.com/jantimon/html-webpack-plugin/issues/900
-  // Until the HMR issue is fixed we keep the ugly output:
-  new SingleEntryPlugin(this.context, template, undefined).apply(childCompiler);
-
-  new LoaderTargetPlugin('node').apply(childCompiler);
-
-  // Fix for "Uncaught TypeError: __webpack_require__(...) is not a function"
-  // Hot module replacement requires that every child compiler has its own
-  // cache. @see https://github.com/ampedandwired/html-webpack-plugin/pull/179
-  childCompiler.hooks.compilation.tap('HtmlWebpackPlugin', compilation => {
-    if (compilation.cache) {
-      if (!compilation.cache[compilerName]) {
-        compilation.cache[compilerName] = {};
-      }
-      compilation.cache = compilation.cache[compilerName];
+class HtmlWebpackChildCompiler {
+
+  constructor () {
+    /**
+     * @type {string[]} templateIds
+     * The template array will allow us to keep track which input generated which output
+     */
+    this.templates = [];
+    /**
+     * @type {Promise<{[templatePath: string]: { content: string, hash: string, entry: WebpackChunk }}>}
+     */
+    this.compilationPromise;
+
+    /**
+     * @type {Date}
+     */
+    this.compilationStarted;
+  }
+
+  /**
+   * Add a templatePath to the child compiler
+   * The given template will be compiled by `compileTemplates`
+   * @param {string} template - The webpack path to the template e.g. `'!!html-loader!index.html'`
+   */
+  addTemplate (template) {
+    const templateId = this.templates.indexOf(template);
+    // Don't add the template to the compiler if a similar template was already added
+    if (templateId !== -1) {
+      return templateId;
     }
-  });
+    // A child compiler can compile only once
+    // throw an error if a new template is added after the compilation started
+    if (this.compilationPromise) {
+      throw new Error('New templates can only be added before `compileTemplates` was called.');
+    }
+    // Add the template to the childCompiler
+    const newTemplateId = this.templates.length;
+    this.templates.push(template);
+    return newTemplateId;
+  }
 
-  // Compile and return a promise
-  return new Promise((resolve, reject) => {
-    childCompiler.runAsChild((err, entries, childCompilation) => {
-      // Resolve / reject the promise
-      if (childCompilation && childCompilation.errors && childCompilation.errors.length) {
-        const errorDetails = childCompilation.errors.map(error => error.message + (error.error ? ':\n' + error.error : '')).join('\n');
-        reject(new Error('Child compilation failed:\n' + errorDetails));
-      } else if (err) {
-        reject(err);
-      } else {
-        // Replace [hash] placeholders in filename
-        const outputName = compilation.mainTemplate.hooks.assetPath.call(outputOptions.filename, {
-          hash: childCompilation.hash,
-          chunk: entries[0]
-        });
+  /**
+   * This function will start the template compilation
+   * once it is started no more templates can be added
+   *
+   * @param {WebpackCompilation} mainCompilation
+   * @returns {Promise<{[templatePath: string]: { content: string, hash: string, entry: WebpackChunk }}>}
+   */
+  compileTemplates (mainCompilation) {
+    // To prevent multiple compilations for the same template
+    // the compilation is cached in a promise.
+    // If it already exists return
+    if (this.compilationPromise) {
+      return this.compilationPromise;
+    }
+
+    // The entry file is just an empty helper as the dynamic template
+    // require is added in "loader.js"
+    const outputOptions = {
+      filename: '__child-[name]',
+      publicPath: mainCompilation.outputOptions.publicPath
+    };
+    const compilerName = 'HtmlWebpackCompiler';
+    // Create an additional child compiler which takes the template
+    // and turns it into an Node.JS html factory.
+    // This allows us to use loaders during the compilation
+    const childCompiler = mainCompilation.createChildCompiler(compilerName, outputOptions);
+    // The file path context which webpack uses to resolve all relative files to
+    childCompiler.context = mainCompilation.compiler.context;
+    // Compile the template to nodejs javascript
+    new NodeTemplatePlugin(outputOptions).apply(childCompiler);
+    new NodeTargetPlugin().apply(childCompiler);
+    new LibraryTemplatePlugin('HTML_WEBPACK_PLUGIN_RESULT', 'var').apply(childCompiler);
+    new LoaderTargetPlugin('node').apply(childCompiler);
 
-        // Restore the parent compilation to the state like it
-        // was before the child compilation
-        compilation.assets[outputName] = assetsBeforeCompilation[outputName];
-        if (assetsBeforeCompilation[outputName] === undefined) {
-          // If it wasn't there - delete it
-          delete compilation.assets[outputName];
+    // Fix for "Uncaught TypeError: __webpack_require__(...) is not a function"
+    // Hot module replacement requires that every child compiler has its own
+    // cache. @see https://github.com/ampedandwired/html-webpack-plugin/pull/179
+    childCompiler.hooks.compilation.tap('HtmlWebpackPlugin', compilation => {
+      if (compilation.cache) {
+        if (!compilation.cache[compilerName]) {
+          compilation.cache[compilerName] = {};
         }
-        resolve({
-          // Hash of the template entry point
-          hash: entries[0].hash,
-          // Output name
-          outputName: outputName,
-          // Compiled code
-          content: childCompilation.assets[outputName].source()
-        });
+        compilation.cache = compilation.cache[compilerName];
       }
     });
+
+    // Add all templates
+    this.templates.forEach((template, index) => {
+      new SingleEntryPlugin(childCompiler.context, template, `HtmlWebpackPlugin_${index}`).apply(childCompiler);
+    });
+
+    this.compilationStarted = new Date();
+    this.compilationPromise = new Promise((resolve, reject) => {
+      childCompiler.runAsChild((err, entries, childCompilation) => {
+        // Extract templates
+        const compiledTemplates = entries
+          ? extractHelperFilesFromCompilation(mainCompilation, childCompilation, outputOptions.filename, entries)
+          : [];
+        // Reject the promise if the childCompilation contains error
+        if (childCompilation && childCompilation.errors && childCompilation.errors.length) {
+          const errorDetails = childCompilation.errors.map(error => error.message + (error.error ? ':\n' + error.error : '')).join('\n');
+          reject(new Error('Child compilation failed:\n' + errorDetails));
+          return;
+        }
+        // Reject if the error object contains errors
+        if (err) {
+          reject(err);
+          return;
+        }
+        /**
+         * @type {{[templatePath: string]: { content: string, hash: string, entry: WebpackChunk }}}
+         */
+        const result = {};
+        compiledTemplates.forEach((templateSource, entryIndex) => {
+          // The compiledTemplates are generated from the entries added in
+          // the addTemplate function.
+          // Therefore the array index of this.templates should be the as entryIndex.
+          result[this.templates[entryIndex]] = {
+            content: templateSource,
+            hash: childCompilation.hash,
+            entry: entries[entryIndex]
+          };
+        });
+        resolve(result);
+      });
+    });
+
+    return this.compilationPromise;
+  }
+}
+
+/**
+ * The webpack child compilation will create files as a side effect.
+ * This function will extract them and clean them up so they won't be written to disk.
+ *
+ * Returns the source code of the compiled templates as string
+ *
+ * @returns Array<string>
+ */
+function extractHelperFilesFromCompilation (mainCompilation, childCompilation, filename, childEntryChunks) {
+  const helperAssetNames = childEntryChunks.map((entryChunk, index) => {
+    return mainCompilation.mainTemplate.hooks.assetPath.call(filename, {
+      hash: childCompilation.hash,
+      chunk: entryChunk,
+      name: `HtmlWebpackPlugin_${index}`
+    });
   });
-};
+
+  helperAssetNames.forEach((helperFileName) => {
+    delete mainCompilation.assets[helperFileName];
+  });
+
+  const helperContents = helperAssetNames.map((helperFileName) => {
+    return childCompilation.assets[helperFileName].source();
+  });
+
+  return helperContents;
+}
+
+/**
+ * @type {WeakMap<WebpackCompiler, HtmlWebpackChildCompiler>}}
+ */
+const childCompilerCache = new WeakMap();
+
+/**
+ * Get child compiler from cache or a new child compiler for the given mainCompilation
+ *
+ * @param {WebpackCompiler} mainCompiler
+ */
+function getChildCompiler (mainCompiler) {
+  if (!childCompilerCache[mainCompiler]) {
+    childCompilerCache[mainCompiler] = new HtmlWebpackChildCompiler();
+  }
+  return childCompilerCache[mainCompiler];
+}
 
 /**
- * Returns the child compiler name e.g. 'html-webpack-plugin for "index.html"'
+ * Remove the childCompiler from the cache
+ *
+ * @param {WebpackCompiler} mainCompiler
+ */
+function clearCache (mainCompiler) {
+  delete (childCompilerCache[mainCompiler]);
+}
+
+/**
+ * Register a template for the current main compiler
+ * @param {WebpackCompiler} mainCompiler
+ * @param {string} templatePath
  */
-function getCompilerName (context, filename) {
-  const absolutePath = path.resolve(context, filename);
-  const relativePath = path.relative(context, absolutePath);
-  return 'html-webpack-plugin for "' + (absolutePath.length < relativePath.length ? absolutePath : relativePath) + '"';
+function addTemplateToCompiler (mainCompiler, templatePath) {
+  getChildCompiler(mainCompiler).addTemplate(templatePath);
 }
+
+/**
+ * Starts the compilation for all templates.
+ * This has to be called once all templates where added.
+ *
+ * If this function is called multiple times it will use a cache inside
+ * the childCompiler
+ *
+ * @param {string} templatePath
+ * @param {string} outputFilename
+ * @param {WebpackCompilation} mainCompilation
+ */
+function compileTemplate (templatePath, outputFilename, mainCompilation) {
+  const childCompiler = getChildCompiler(mainCompilation.compiler);
+  return childCompiler.compileTemplates(mainCompilation).then((compiledTemplates) => {
+    const compiledTemplate = compiledTemplates[templatePath];
+    // Replace [hash] placeholders in filename
+    const outputName = mainCompilation.mainTemplate.hooks.assetPath.call(outputFilename, {
+      hash: compiledTemplate.hash,
+      chunk: compiledTemplate.entry
+    });
+    return {
+      // Hash of the template entry point
+      hash: compiledTemplate.hash,
+      // Output name
+      outputName: outputName,
+      // Compiled code
+      content: compiledTemplate.content
+    };
+  });
+}
+
+module.exports = {
+  addTemplateToCompiler,
+  compileTemplate,
+  clearCache
+};