What is Hot Module Replacement (HMR) ?

As the Webpack documentation says:

Hot Module Replacement (HMR) exchanges, adds, or removes modules while an application is running, without a full reload. This can significantly speed up development in a few ways: Retain application state which is lost during a full reload. Save valuable development time by only updating what’s changed. * Modifications made to CSS/JS in the source code results in an instant browser update which is almost comparable to changing styles directly in the browser’s dev tools.

For a Cozy application, having this feature allows you to have the specific changed part reloaded in your browser without reloading again the cozy-bar, the cozy-client and other layout components.

Notice: The HMR will be explained here only for applications build using Webpack.

Is it available for my app?

If you created your application (React or Vue) using Create Cozy App a.k.a CCA (from v1) and:

Then, you dont’ have anything to do. See the next part of this tutorial to use HMR 😎

If you did change these parts or you didn’t create your application initialy using Create Cozy App, you can see the part Make HMR available in my application.

Run my app with HMR

To have a full HMR experience, you’ll have to do two things here:

But thanks to the last version of cozy-scripts, all of that are handled for you with the command cozy-scripts start:

yarn cozy-scripts start --hot --browser
# or if you have the last package.json from CCA, you can do
yarn start

How can I run my webpack and my stack in different terminals?

First, you can pass an option to disable the stack handling by the start command:

yarn cozy-scripts start --hot --browser --no-stack
# or if you have the last package.json from CCA, you can do
yarn start --no-stack

Then, as said, to have your application running in a Cozy with HMR, your have to disable CSPs when running the stack. For that, cozy-scripts has a dedicated config file to pass to your Docker image in cozy-scripts/stack/disableCSP.yaml. You can use this config by running your stack in another terminal like:

# in your app root directory
docker run --rm -it
  -p 8080:8080 -p 5984:5984
  -v \"$(pwd)/build\":/data/cozy-app/app
  -v \"$(pwd)/node_modules/cozy-scripts/stack/disableCSP.yaml\":/etc/cozy/cozy.yaml
  cozy/cozy-app-dev

Make HMR available in my application

For a React application

You have to use react-hot-loader inside your main App component like this:

import React from 'react'
import { hot } from 'react-hot-loader'

const App = () => (
  ...
)

export default hot(module)(App)

That’s it. Then, just use the last version of cozy-scripts to run your application with HMR.

For a Vue application

Inside your entry point index.js (in src/targets/<target>), you have to add this part related to module.hot:

import Vue from 'vue'
import store from 'lib/store'

const renderApp = function() {
  const App = require('components/App').default
  return new Vue({
    store, // inject store to all children
    el: '[role=application]',
    render: h => h(App)
  })
}

if (module.hot) {
  module.hot.accept('components/App', function() {
    renderApp()
  })
}

That’s it. Then, just use the last version of cozy-scripts to run your application with HMR.