webpack/lib/cache/MemoryCachePlugin.js

Packs ECMAScript/CommonJs/AMD modules for the browser. Allows you to split your codebase into multiple bundles, which can be loaded on demand. Supports loaders to preprocess files, i.e. json, jsx, es7, css, less, ... and your custom stuff.

/*
	MIT License http://www.opensource.org/licenses/mit-license.php
	Author Tobias Koppers @sokra
*/

"use strict";

const Cache = require("../Cache");

/** @import { Data, Etag } from "../Cache" */
/** @import Compiler from "../Compiler" */

class MemoryCachePlugin {
	/**
	 * Applies the plugin by registering its hooks on the compiler.
	 * @param {Compiler} compiler the compiler instance
	 * @returns {void}
	 */
	apply(compiler) {
		/** @type {Map<string, { etag: Etag | null, data: Data } | null>} */
		const cache = new Map();
		compiler.cache.hooks.store.tap(
			{ name: "MemoryCachePlugin", stage: Cache.STAGE_MEMORY },
			(identifier, etag, data) => {
				cache.set(identifier, { etag, data });
			}
		);
		compiler.cache.hooks.get.tap(
			{ name: "MemoryCachePlugin", stage: Cache.STAGE_MEMORY },
			(identifier, etag, gotHandlers) => {
				const cacheEntry = cache.get(identifier);
				// A recorded miss: the whole chain was asked for this identifier already.
				if (cacheEntry === null) return null;
				// Etags are compared by identity — a lazy one is interned per source
				// object, so equal content reached through a second object is a
				// different etag. Hashing to tell those apart would cost every hit what
				// laziness saves, so a mismatch falls through to the next stage instead:
				// the file cache compares etags by value and can still answer. Returning
				// `null` here would bail the hook and lose that (`Cache.get` maps it to
				// `undefined` regardless, so it buys the caller nothing).
				if (cacheEntry !== undefined && cacheEntry.etag === etag) {
					return cacheEntry.data;
				}
				gotHandlers.push((result, callback) => {
					if (result !== undefined) {
						cache.set(identifier, { etag, data: result });
					} else if (cacheEntry === undefined) {
						// Record the miss only for an identifier nothing was known about:
						// an entry reached with a different etag still answers its own.
						cache.set(identifier, null);
					}
					return callback();
				});
			}
		);
		compiler.cache.hooks.shutdown.tap(
			{ name: "MemoryCachePlugin", stage: Cache.STAGE_MEMORY },
			() => {
				cache.clear();
			}
		);
	}
}

module.exports = MemoryCachePlugin;