Make webpack conditionally load additional or alternative css file - css

Question

I'm trying to implement a kind of css theming in an angular 4 project. We use webpack 3 for bundling. The product is intended to be used by several companies and has to look according to their brandbooks. So we need themes.
We gonna have several builds, but we don't want to have several versions of code. All themes should remain in the same codebase. The differences are minimal: colors, icons, fonts — everything may be changed in css.
I have thought of several ways to do it, the most obvious would be to implement theming via :host-context for components and change the class of body by changing environment variable for webpack. With such method we will heve every theme inside our bundle, which is not good. Maybe there's another way?
I wonder if it is possible to have webpack load not the css file it is asked for. Instead it could look for another file by pattern, and if it exists, use that file instead of original one. Or load both files.
For example, we have a button.component.ts which imports button.component.css. If we don't tell webpack to use any theme, it works as usual. But if we do, it tries to read button.component.theme-name.css in the same directory. If that file exists, webpack imports it instead (or altogether with) the default file.
That's basically what I'm trying to do. I guess, the same mechanism would be useful for html templates in angular.
Is there a plugin to do such magic? Or maybe some sophisticated loader option? If you have another way to solve my task — feel free to drop a comment!

Answer 1

I created a loader which can append or replace the content of a loaded file with the content of its sibling which has a chosen theme's title in its name.
TL;DR
Create a file with loader.
Use it in webpack config.
Run webpack in THEME=<themeName> evironment.
theme-loader.js
const fs = require('fs');
const loaderUtils = require('loader-utils');
module.exports = function (mainData) {
const options = loaderUtils.getOptions(this);
let themeName = options.theme;
let mode = options.mode;
if (themeName) {
// default mode
if (!Object.keys(transform).includes(mode)) {
mode = 'replace';
}
// fileName.suffix.ext -> fileName.suffix.themeName.ext
const themeAssetPath = this.resourcePath.replace(/\.([^\.]*)$/, `.${themeName}.$1`);
const callback = this.async();
// for HMR to work
this.addDependency(themeAssetPath);
fs.readFile(themeAssetPath, 'utf8', (err, themeData) => {
if (!err) {
callback(null, transform[mode](mainData, themeData));
} else if (err.code === 'ENOENT') {
// don't worry! if it's not here then it's not needed
callback(null, mainData);
} else {
callback(err);
}
});
} else {
return mainData;
}
};
const transform = {
// concat theme file with main file
concat: (mainData, themeData) => mainData + '\n' + themeData,
// replace main file with theme file
replace: (mainData, themeData) => themeData
};
A piece of sample webpack.config.js to use this handmade loader:
resolveLoader: {
modules: [
paths.libs, // ./node_modules
paths.config // this is where our custom loader sits
]
},
module: {
rules: [
// component styles
{
test: /\.css$/,
include: path.join(paths.src, 'app'),
use: [
'raw-loader',
// search for a themed one and append it to main file if found
{
loader: 'theme-loader',
options: {
theme: process.env.THEME,
mode: 'concat'
}
}
]
},
// angular templates — search for a themed one and use it if found
{
test: /\.html$/,
use: ['raw-loader',
{
loader: 'theme-loader',
options: {
theme: process.env.THEME,
mode: 'replace'
}
}
]
}
]
}
For example, an app.component.css:
:host {
background: #f0f0f0;
color: #333333;
padding: 1rem 2rem;
display: flex;
flex-direction: column;
flex: 1;
justify-content: center;
}
nav {
/* ... */
/* something about nav element */
/* ... */
}
header {
/* ... */
/* pile of styles for header */
/* ... */
}
To implement dark theme we don't need to change all that flex and padding staff and maybe nav and header don't have their own background and font color settings. So we'll just have to override host element style. We create app.component.dark.css:
:host {
background: #222222;
color: #e0e0e0;
}
The we run webpack with environment variable THEME set to dark. The loader takes a request to process app.component.css, tries to load app.component.dark.css and voila! Themed css is appended to the end of resulting file. Because of cascade,
if multiple competing selectors have the same importance and specificity, … later rules will win over earlier rules (MDN).
For HTML we don't have such method. So we'll have to rewrite our template completely. Hopefully, you won't need to do it too often. I my case, I wanted to change like header and footer to fit the cutomer's branding demand.
This was my first attempt to create a webpack loader, please leave a comment if you see a problem with it.