WeChat applet converter loader design and implementation

小program development tutorialThe column introduces the design and implementation of loader.

The loader configuration in the configuration file

can be matched according to the configuration file Go to the rule and execute the corresponding loader.

// analyze.config.js
// 引入loader
const jsLoader = require('./lib/jsLoader')
const jsonLoader = require('./lib/jsonLoader')
const cssLoader = require('./lib/cssLoader')
const htmlLoader = require('./lib/htmlLoader')
const signLoader = require('./lib/signLoader')
const config = {
    entry: './',
    output: {
        name: 'dist',
        src: './'
    },
    module: [
        {
            test: /\.js$/,
            loader: [signLoader, jsLoader],
        },
        {
            test: /\.wxss$/,
            loader: [cssLoader],
            outputPath: (outputPath) => outputPath.replace('.wxss', '.acss')
        },
        {
            test: /\.wxml$/,
            loader: [htmlLoader],
            outputPath: (outputPath) => outputPath.replace('.wxml', '.axml')
        },
        {
            test: /\.json$/,
            loader: [jsonLoader],
        },
    ]
}
module.exports = config

Specific loader implementation
Take jsLoader as an example, receive the source code as a parameter, and return the new source code obtained after compilation

// 前几篇中封装的js转换器
const JsParser = require('./JsParser')
function loader(source) {
    
    const jsParser = new JsParser()
    let ast = jsParser.parse(source)
    ast = jsParser.astConverter(ast)
    return jsParser.astToCode(ast)
}
module.exports = loader

Different file selections Corresponding to loader

// 重写Analyze函数中的analyzeFileToLoard文件分析部分
function Analyze(filePath, outputPath){
    if (fs.statSync(filePath).isDirectory()) {
        const files = fs.readdirSync(filePath)
        files.forEach(file => {
            const currentFilePath = filePath+'/'+file
            const currentOutputPath = outputPath+'/'+file
            if(fs.statSync(currentFilePath).isDirectory()) {
                fs.mkdirSync(currentOutputPath)
                Analyze(currentFilePath, currentOutputPath)
            } else analyzeFileToLoard(currentFilePath, currentOutputPath)
        })
    } else analyzeFileToLoard(filePath, outputPath)
}

function analyzeFileToLoard(inputPath, outputPath) {
    let source = readFile(inputPath) // 读取源码
    const loaders = config.module
    loaders.forEach(loader => { // 遍历配置文件，看是否有匹配文件的loader规则
        if (loader.test.test(inputPath)) {
            // 使用loader
            source = useLoader(source, loader.loader, outputPath)
            // 输出路径处理函数
            if (loader.outputPath) outputPath = loader.outputPath(outputPath)
        }
    })
    writeFile(outputAppPath(outputPath), source) // 将处理过后的源码写入文件
}

loader filtering and execution

loader execution is in reverse order, executed from right to left. Here we first use the synchronous loader to discuss.
There is a pitch stage before the loader is executed. I feel that the naming method of pitch is not particularly suitable. I prefer to call it the filtering stage. First, execute the pitch method on the loader sequentially. If the pitch has a return value, the loader executed before the loader will no longer be executed.

function useLoader(source, loaders = []) {
    // 执行loader存储列表
    const loaderList = []
    // 递归去筛选需要执行的loader
    function loaderFilter(loaders) {
        const [firstLoader, ...ortherLoader] = loaders
        if (loaders.length === 0) return
        // 执行pitch，并将剩余的loader传入作为参数
        if (firstLoader.pitch && firstLoader.pitch(ortherLoader)) return ortherLoader
        else {
            // 将可用loader加入待执行列表
            loaderList.push(firstLoader)
            // 剩余loader作为参数 递归调用
            loaderFilter(ortherLoader)
        }
    }
    // 大概，暂时用不到。。。
    const remainLoader = loaderFilter(loaders)
    // 同步loader逆序执行
    function runLoader(source, loaderList) {
        const loader = loaderList.pop()
        let newSource = loader(source)
        if (loaderList.length > 0) return runLoader(newSource, loaderList)
        else return newSource
    }
    source = runLoader(source, loaderList)
    return source
}

Experiment
Write a signLoader to see if the loader can be executed in reverse order as we thought

function loader(source) {
let sign = `/**
* @Author： LY
*/
`
    source = sign + source
    return source
}
module.exports = loader

Result:

It’s so simple The loader part is considered completed, but writing this way can only execute some synchronous loaders, and asynchronous loaders cannot wait for execution to complete before writing.

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