fix(perf): add virtual css timestamps to webpack compilation (#381)

fixes webpack issue whereby timestamps for virtual css files are always
undefined when webpack is in watch mode

src/VirtualModulePlugin.js

@@ -60,6 +60,25 @@ class VirtualModulePlugin {
       compiler.resolvers.normal.fileSystem = fs;
       compiler.resolvers.context.fileSystem = fs;
       compiler.resolvers.loader.fileSystem = fs;
+    } else {
+      /**
+       * When webpack is in watch mode, the map of file timestamps is computed
+       * from the watcher instance, which uses the real filesystem and as a
+       * result the virtual css files are not found in this map.
+       * To correct this, we manually add these files to the map here.
+       * @see https://github.com/4Catalyzer/astroturf/pull/381
+       */
+      compiler.hooks.watchRun.tapAsync(
+        'astroturf',
+        ({ fileTimestamps }, callback) => {
+          this.fs.getPaths().forEach((value, key) => {
+            const mtime = +value.mtime;
+            fileTimestamps.set(key, mtime);
+          });
+
+          callback();
+        },
+      );
     }
 
     this.augmented = true;

src/memory-fs.js

@@ -79,6 +79,8 @@ class MemoryFs {
     });
   };
 
+  getPaths = () => this.paths;
+
   exists = (p, cb) => cb(this.existsSync(p));
 
   existsSync = p => this.paths.has(path.normalize(p));

test/helpers.js

@@ -45,7 +45,9 @@ export function runLoader(src, options, filename = 'MyStyleFile.js') {
       resourcePath: filename,
       request: `babel-loader!css-literal-loader!${filename}`,
       _compiler: {},
-      _compilation: {},
+      _compilation: {
+        fileTimestamps: new Map(),
+      },
       _module: {},
       resolve(request, cb) {
         cb(null, relative(dirname(filename), request));