vector-im-hydrogen-web/scripts/postcss/css-url-to-variables.js

/*
Copyright 2021 The Matrix.org Foundation C.I.C.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

const valueParser = require("postcss-value-parser");

/**
 * This plugin extracts content inside url() into css variables.
 * The extracted css variables are added to the :root section.
 */
let counter;
let urlVariables;
const idToPrepend = "icon-url";

function findAndReplaceUrl(decl) {
    const value = decl.value;
    const parsed = valueParser(value);
    parsed.walk(node => {
        if (node.type !== "function" || node.value !== "url") {
            return;
        }
        const url = node.nodes[0].value;
        if (!url.match(/\.svg\?primary=.+/)) {
            return;
        }
        const variableName = `${idToPrepend}-${counter++}`;
        urlVariables.set(variableName, url);
        node.value = "var";
        node.nodes = [{ type: "word", value: `--${variableName}` }];
    });
    decl.assign({prop: decl.prop, value: parsed.toString()})
}

function addResolvedVariablesToRootSelector(root, { Rule, Declaration }) {
    const newRule = new Rule({ selector: ":root", source: root.source });
    // Add derived css variables to :root
    urlVariables.forEach((value, key) => {
        const declaration = new Declaration({ prop: `--${key}`, value: `url("${value}")`});
        newRule.append(declaration);
    });
    root.append(newRule);
}

/* *
 * @type {import('postcss').PluginCreator}
 */
module.exports = (opts = {}) => {
    urlVariables = new Map();
    counter = 0;
    return {
        postcssPlugin: "postcss-url-to-variable",

        Once(root, { Rule, Declaration }) {
            root.walkDecls(decl => findAndReplaceUrl(decl));
            if (urlVariables.size) {
                addResolvedVariablesToRootSelector(root, { Rule, Declaration });
            }
        },
    };
};

module.exports.postcss = true;