fix koa hmr (#693)

Author: ambar

examples/with-koa/src/index.js

@@ -4,15 +4,14 @@ import http from 'http';
 // Use `app#callback()` method here instead of directly
 // passing `app` as an argument to `createServer` (or use `app#listen()` instead)
 // @see https://github.com/koajs/koa/blob/master/docs/api/index.md#appcallback
-const server = http.createServer(app.callback());
-
-let currentApp = app;
+let currentHandler = app.callback();
+const server = http.createServer(currentHandler);
 
 server.listen(process.env.PORT || 3000, (error) => {
   if (error) {
     console.log(error)
   }
-  
+
   console.log('🚀 started')
 });
 
@@ -21,9 +20,9 @@ if (module.hot) {
 
   module.hot.accept('./server', () => {
     console.log('🔁  HMR Reloading `./server`...');
-    server.removeListener('request', currentApp);
-    const newApp = require('./server').default;
-    server.on('request', newApp);
-    currentApp = newApp;
+    server.removeListener('request', currentHandler);
+    const newHandler = require('./server').default.callback();
+    server.on('request', newHandler);
+    currentHandler = newHandler;
   });
 }