feat(server): add callback support for invalidate (#1900)

lib/Server.js

@@ -980,9 +980,9 @@ class Server {
     this.contentBaseWatchers.push(watcher);
   }
 
-  invalidate() {
+  invalidate(callback) {
     if (this.middleware) {
-      this.middleware.invalidate();
+      this.middleware.invalidate(callback);
     }
   }
 }

test/InvalidateCallback.test.js

@@ -0,0 +1,41 @@
+'use strict';
+
+const webpack = require('webpack');
+const { noop } = require('webpack-dev-middleware/lib/util');
+const Server = require('../lib/Server');
+const config = require('./fixtures/simple-config/webpack.config');
+
+describe('Invalidate Callback', () => {
+  describe('Testing callback functions on calling invalidate without callback', () => {
+    it('should be `noop` (the default callback function)', (done) => {
+      const compiler = webpack(config);
+      const server = new Server(compiler);
+
+      server.invalidate();
+      expect(server.middleware.context.callbacks[0]).toBe(noop);
+
+      compiler.hooks.done.tap('webpack-dev-server', () => {
+        server.close(done);
+      });
+
+      compiler.run(() => {});
+    });
+  });
+
+  describe('Testing callback functions on calling invalidate with callback', () => {
+    it('should be `callback` function', (done) => {
+      const compiler = webpack(config);
+      const callback = jest.fn();
+      const server = new Server(compiler);
+      server.invalidate(callback);
+
+      expect(server.middleware.context.callbacks[0]).toBe(callback);
+
+      compiler.hooks.done.tap('webpack-dev-server', () => {
+        server.close(done);
+      });
+
+      compiler.run(() => {});
+    });
+  });
+});