webpack-loader原理

loader

loader 是导出为一个函数的 node 模块。该函数在 loader 转换资源的时候调用。给定的函数将调用 loader API，并通过 this 上下文访问。

loader配置

{
  test: /.js$/
  use: [
    {
      loader: path.resolve('path/to/loader.js'),
      options: {/* ... */}
    }
  ]
}

本地loader配置

resolveLoader: {
  modules: [
    'node_modules',
    path.resolve(__dirname, 'loaders')
  ]
}

loader用法

//返回简单结果
module.exports = function(content){
    return content
}

//返回多个值
module.exports = function(content){
    this.callback(...)
}

//同步loader
module.exports = function(content){
    this.callback(...)
}

//异步loader
module.exports = function(content){
    let callback = this.async(...)
    setTimeout(callback,1000)
}

loader 工具库

1.loader-utils 但最常用的一种工具是获取传递给 loader 的选项

2.schema-utils 用于保证 loader 选项，进行与 JSON Schema 结构一致的校验

import { getOptions } from 'loader-utils';
import validateOptions from 'schema-utils';

const schema = {
  type: 'object',
  properties: {
    test: {
      type: 'string'
    }
  }
}

export default function(source) {
  const options = getOptions(this);

  validateOptions(schema, options, 'Example Loader');

  // 对资源应用一些转换……

  return `export default ${ JSON.stringify(source) }`;
};

loader依赖

如果一个 loader 使用外部资源（例如，从文件系统读取），必须声明它。这些信息用于使缓存 loaders 无效，以及在观察模式(watch mode)下重编译。

import path from 'path';

export default function(source) {
  var callback = this.async();
  var headerPath = path.resolve('header.js');

  this.addDependency(headerPath);

  fs.readFile(headerPath, 'utf-8', function(err, header) {
    if(err) return callback(err);
    callback(null, header + "
" + source);
  });
};

模块依赖

根据模块类型，可能会有不同的模式指定依赖关系。例如在 CSS 中，使用 @import 和 url(...) 语句来声明依赖。这些依赖关系应该由模块系统解析。

可以通过以下两种方式中的一种来实现：

通过把它们转化成 require 语句。
使用 this.resolve 函数解析路径。
css-loader 是第一种方式的一个例子。它将 @import 语句替换为 require 其他样式文件，将 url(...) 替换为 require 引用文件，从而实现将依赖关系转化为 require 声明。

对于 less-loader，无法将每个 @import 转化为 require，因为所有 .less 的文件中的变量和混合跟踪必须一次编译。因此，less-loader 将 less 编译器进行了扩展，自定义路径解析逻辑。然后，利用第二种方式，通过 webpack 的 this.resolve 解析依赖。

loaderUtils.stringifyRequest(this,require.resolve('./xxx.js'))

loader API

方法名	含义
this.request	被解析出来的 request 字符串。例子："/abc/loader1.js?xyz!/abc/node_modules/loader2/index.js!/abc/resource.js?rrr"
this.loaders	所有 loader 组成的数组。它在 pitch 阶段的时候是可以写入的。
this.loaderIndex	当前 loader 在 loader 数组中的索引。
this.async	异步回调
this.callback	回调
this.data	在 pitch 阶段和正常阶段之间共享的 data 对象。
this.cacheable	默认情况下，loader 的处理结果会被标记为可缓存。调用这个方法然后传入 false，可以关闭 loader 的缓存。cacheable(flag = true: boolean)
this.context	当前处理文件所在目录
this.resource	当前处理文件完成请求路径，例如 /src/main.js?name=1
this.resourcePath	当前处理文件的路径
this.resourceQuery	查询参数部分
this.target	webpack配置中的target
this.loadModule	但 Loader 在处理一个文件时，如果依赖其它文件的处理结果才能得出当前文件的结果时,就可以通过 this.loadModule(request: string, callback: function(err, source, sourceMap, module)) 去获得 request 对应文件的处理结果
this.resolve	解析指定文件路径
this.addDependency	给当前处理文件添加依赖文件，依赖发送变化时，会重新调用loader处理该文件
this.addContextDependency	把整个目录加入到当前正在处理文件的依赖当中
this.clearDependencies	清除当前正在处理文件的所有依赖中
this.emitFile	输出一个文件
loader-utils.stringifyRequest	把绝对路径转换成相对路径
loader-utils.interpolateName	用多个占位符或一个正则表达式转换一个文件名的模块。这个模板和正则表达式被设置为查询参数，在当前loader的上下文中被称为name或者regExp

loader原理

loader-runner

runLoaders({
    resource: "/abs/path/to/file.txt?query",
    // String: Absolute path to the resource (optionally including query string)

    loaders: ["/abs/path/to/loader.js?query"],
    // String[]: Absolute paths to the loaders (optionally including query string)
    // {loader, options}[]: Absolute paths to the loaders with options object

    context: { minimize: true },
    // Additional loader context which is used as base context

    readResource: fs.readFile.bind(fs)
    // A function to read the resource
    // Must have signature function(path, function(err, buffer))

}, function(err, result) {
    // err: Error?

    // result.result: Buffer | String
    // The result

    // result.resourceBuffer: Buffer
    // The raw resource as Buffer (useful for SourceMaps)

    // result.cacheable: Bool
    // Is the result cacheable or do it require reexecution?

    // result.fileDependencies: String[]
    // An array of paths (files) on which the result depends on

    // result.contextDependencies: String[]
    // An array of paths (directories) on which the result depends on
})


function splitQuery(req) {
    var i = req.indexOf("?");
    if(i < 0) return [req, ""];
    return [req.substr(0, i), req.substr(i)];
}

function dirname(path) {
    if(path === "/") return "/";
    var i = path.lastIndexOf("/");
    var j = path.lastIndexOf("\");
    var i2 = path.indexOf("/");
    var j2 = path.indexOf("\");
    var idx = i > j ? i : j;
    var idx2 = i > j ? i2 : j2;
    if(idx < 0) return path;
    if(idx === idx2) return path.substr(0, idx + 1);
    return path.substr(0, idx);
}


//loader开始执行阶段
function processResource(options, loaderContext, callback) {
    // 将loader索引设置为最后一个loader
    loaderContext.loaderIndex = loaderContext.loaders.length - 1;

    var resourcePath = loaderContext.resourcePath
    if(resourcePath) {
        //添加文件依赖
        loaderContext.addDependency(resourcePath);
        //读取文件
        options.readResource(resourcePath, function(err, buffer) {
            if(err) return callback(err);
            //读取完成后放入options
            options.resourceBuffer = buffer;
            iterateNormalLoaders(options, loaderContext, [buffer], callback);
        });
        
    } else {
        iterateNormalLoaders(options, loaderContext, [null], callback);
    }
}

//从右往左递归执行loader
function iterateNormalLoaders(options, loaderContext, args, callback) {
    //结束条件，loader读取完毕
    if(loaderContext.loaderIndex < 0)
        return callback(null, args);
        
    var currentLoaderObject = loaderContext.loaders[loaderContext.loaderIndex];

    //迭代
    if(currentLoaderObject.normalExecuted) {
        loaderContext.loaderIndex--;
        return iterateNormalLoaders(options, loaderContext, args, callback);
    }
    
    
    var fn = currentLoaderObject.normal;
    currentLoaderObject.normalExecuted = true;

    if(!fn) {
        return iterateNormalLoaders(options, loaderContext, args, callback);
    }
    
    //转换buffer数据。如果当前loader设置了raw属性
    convertArgs(args, currentLoaderObject.raw);

    runSyncOrAsync(fn, loaderContext, args, function(err) {
        if(err) return callback(err);

        var args = Array.prototype.slice.call(arguments, 1);
        iterateNormalLoaders(options, loaderContext, args, callback);
    });

}


function convertArgs(args, raw) {
    if(!raw && Buffer.isBuffer(args[0]))
        args[0] = utf8BufferToString(args[0]);
    else if(raw && typeof args[0] === "string")
        args[0] = Buffer.from(args[0], "utf-8");
}

exports.getContext = function getContext(resource) {
    var splitted = splitQuery(resource);
    return dirname(splitted[0]);
};

function createLoaderObject(loader){
    //初始化loader配置
    var obj = {
        path: null,
        query: null,
        options: null,
        ident: null,
        normal: null,
        pitch: null,
        raw: null,
        data: null,
        pitchExecuted: false,
        normalExecuted: false
    };
    
    //设置响应式属性
    Object.defineProperty(obj, "request", {
        enumerable: true,
        get: function() {
            return obj.path + obj.query;
        },
        set: function(value) {
            if(typeof value === "string") {
                var splittedRequest = splitQuery(value);
                obj.path = splittedRequest[0];
                obj.query = splittedRequest[1];
                obj.options = undefined;
                obj.ident = undefined;
            } else {
                if(!value.loader)
                    throw new Error("request should be a string or object with loader and object (" + JSON.stringify(value) + ")");
                obj.path = value.loader;
                obj.options = value.options;
                obj.ident = value.ident;
                if(obj.options === null)
                    obj.query = "";
                else if(obj.options === undefined)
                    obj.query = "";
                else if(typeof obj.options === "string")
                    obj.query = "?" + obj.options;
                else if(obj.ident)
                    obj.query = "??" + obj.ident;
                else if(typeof obj.options === "object" && obj.options.ident)
                    obj.query = "??" + obj.options.ident;
                else
                    obj.query = "?" + JSON.stringify(obj.options);
            }
        }
    });

    obj.request = loader;

    //冻结对象
    if(Object.preventExtensions) {
        Object.preventExtensions(obj);
    }
    return obj;
    
}

exports.runLoaders = function runLoaders(options, callback) {
    //options = {resource...,fn...}

    // 读取options
    var resource = options.resource || "";
    var loaders = options.loaders || [];
    var loaderContext = options.context || {};
    var readResource = options.readResource || readFile;

    //
    var splittedResource = resource && splitQuery(resource);
    var resourcePath = splittedResource ? splittedResource[0] : undefined;
    var resourceQuery = splittedResource ? splittedResource[1] : undefined;
    var contextDirectory = resourcePath ? dirname(resourcePath) : null;
    
    //执行状态
    var requestCacheable = true;
    var fileDependencies = [];
    var contextDependencies = [];
    
    //准备loader对象
    loaders = loaders.map(createLoaderObject);

    loaderContext.context = contextDirectory; //当前文件所在目录
    loaderContext.loaderIndex = 0; //从0个开始
    loaderContext.loaders = loaders; //loaders数组
    loaderContext.resourcePath = resourcePath; //当前文件所在位置
    loaderContext.resourceQuery = resourceQuery; //当前文件的？部分
    loaderContext.async = null; //异步状态
    loaderContext.callback = null; //同步状态
    loaderContext.cacheable = function cacheable(flag) { //是否设置缓存
        if(flag === false) {
            requestCacheable = false;
        }
    };
    loaderContext.dependency = loaderContext.addDependency = function addDependency(file) {
        fileDependencies.push(file);
    };//记录文件依赖
    loaderContext.addContextDependency = function addContextDependency(context) {
        contextDependencies.push(context);
    };//记录目录依赖
    loaderContext.getDependencies = function getDependencies() {
        return fileDependencies.slice();
    };//获取文件依赖
    loaderContext.getContextDependencies = function getContextDependencies() {
        return contextDependencies.slice();
    };//获取文件目录依赖
    loaderContext.clearDependencies = function clearDependencies() {
        fileDependencies.length = 0;
        contextDependencies.length = 0;
        requestCacheable = true;
    };//删除依赖
    
    //设置响应属性，获取resource自动添加query,设置时自动解析
    Object.defineProperty(loaderContext, "resource", {
        enumerable: true,
        get: function() {
            if(loaderContext.resourcePath === undefined)
                return undefined;
            return loaderContext.resourcePath + loaderContext.resourceQuery;
        },
        set: function(value) {
            var splittedResource = value && splitQuery(value);
            loaderContext.resourcePath = splittedResource ? splittedResource[0] : undefined;
            loaderContext.resourceQuery = splittedResource ? splittedResource[1] : undefined;
        }
    });
    Object.defineProperty(loaderContext, "request", {
        enumerable: true,
        get: function() {
            return loaderContext.loaders.map(function(o) {
                return o.request;
            }).concat(loaderContext.resource || "").join("!");
        }
    });
    Object.defineProperty(loaderContext, "remainingRequest", {
        enumerable: true,
        get: function() {
            if(loaderContext.loaderIndex >= loaderContext.loaders.length - 1 && !loaderContext.resource)
                return "";
            return loaderContext.loaders.slice(loaderContext.loaderIndex + 1).map(function(o) {
                return o.request;
            }).concat(loaderContext.resource || "").join("!");
        }
    });
    Object.defineProperty(loaderContext, "currentRequest", {
        enumerable: true,
        get: function() {
            return loaderContext.loaders.slice(loaderContext.loaderIndex).map(function(o) {
                return o.request;
            }).concat(loaderContext.resource || "").join("!");
        }
    });
    Object.defineProperty(loaderContext, "previousRequest", {
        enumerable: true,
        get: function() {
            return loaderContext.loaders.slice(0, loaderContext.loaderIndex).map(function(o) {
                return o.request;
            }).join("!");
        }
    });
    Object.defineProperty(loaderContext, "query", {
        enumerable: true,
        get: function() {
            var entry = loaderContext.loaders[loaderContext.loaderIndex];
            return entry.options && typeof entry.options === "object" ? entry.options : entry.query;
        }
    });
    Object.defineProperty(loaderContext, "data", {
        enumerable: true,
        get: function() {
            return loaderContext.loaders[loaderContext.loaderIndex].data;
        }
    });
    
    // 完成loader上下文
    //冻结对象
    if(Object.preventExtensions) {
        Object.preventExtensions(loaderContext);
    }

    var processOptions = {
        resourceBuffer: null,
        readResource: readResource
    };
    
    //进入loaderPitching阶段
    iteratePitchingLoaders(processOptions, loaderContext, function(err, result) {
        if(err) {
            return callback(err, {
                cacheable: requestCacheable,
                fileDependencies: fileDependencies,
                contextDependencies: contextDependencies
            });
        }
        callback(null, {
            result: result,
            resourceBuffer: processOptions.resourceBuffer,
            cacheable: requestCacheable,
            fileDependencies: fileDependencies,
            contextDependencies: contextDependencies
        });
    });
}

//进入loaderPitch阶段
function iteratePitchingLoaders(options, loaderContext, callback) {
    // 在最后一个loader之后终止
    if(loaderContext.loaderIndex >= loaderContext.loaders.length)
        //开始递归解析依赖
        return processResource(options, loaderContext, callback);

    var currentLoaderObject = loaderContext.loaders[loaderContext.loaderIndex];

    // 迭代
    if(currentLoaderObject.pitchExecuted) {
        loaderContext.loaderIndex++;
        return iteratePitchingLoaders(options, loaderContext, callback);
    }
    
    // 加载loader module
    loadLoader(currentLoaderObject, function(err) {
        if(err) return callback(err);
        var fn = currentLoaderObject.pitch;
        //记录pitch执行状态
        currentLoaderObject.pitchExecuted = true;
        //没有pitch方法就执行下一个
        if(!fn) return iteratePitchingLoaders(options, loaderContext, callback);
        //执行pitch方法
        runSyncOrAsync(
            fn,
            loaderContext, [loaderContext.remainingRequest, loaderContext.previousRequest, currentLoaderObject.data = {}],
            function(err) {
                if(err) return callback(err);
                var args = Array.prototype.slice.call(arguments, 1);
                // Determine whether to continue the pitching process based on
                // argument values (as opposed to argument presence) in order
                // to support synchronous and asynchronous usages.
                var hasArg = args.some(function(value) {
                    return value !== undefined;
                });
                //根据有无返回值执行对象loader,如果有返回值就执行normalloader,不执行后面的pitch了
                if(hasArg) {
                    loaderContext.loaderIndex--;
                    iterateNormalLoaders(options, loaderContext, args, callback);
                } else {
                    iteratePitchingLoaders(options, loaderContext, callback);
                }
            }
        );
    });
}

//运行异步或同步loader
function runSyncOrAsync(fn, context, args, callback) {
    //设置初始状态
    var isSync = true;
    var isDone = false;
    var isError = false; // 内部错误
    var reportedError = false;
    
    //挂载loader异步方法
    context.async = function async() {
        if(isDone) {
            if(reportedError) return; // ignore
            throw new Error("async(): The callback was already called.");
        }
        isSync = false;
        return innerCallback;
    };
    //挂载loader同步方法
    var innerCallback = context.callback = function() {
        if(isDone) {
            if(reportedError) return; // ignore
            throw new Error("callback(): The callback was already called.");
        }
        isDone = true;
        isSync = false;
        try {
            callback.apply(null, arguments);
        } catch(e) {
            isError = true;
            throw e;
        }
    };
    
    try {
        var result = (function LOADER_EXECUTION() {
            return fn.apply(context, args);
        }());
        if(isSync) {
            isDone = true;
            if(result === undefined)
                return callback();
            if(result && typeof result === "object" && typeof result.then === "function") {
                return result.catch(callback).then(function(r) {
                    callback(null, r);
                });
            }
            return callback(null, result);
        }
    } catch(e) {
        if(isError) throw e;
        if(isDone) {
            // loader is already "done", so we cannot use the callback function
            // for better debugging we print the error on the console
            if(typeof e === "object" && e.stack) console.error(e.stack);
            else console.error(e);
            return;
        }
        isDone = true;
        reportedError = true;
        callback(e);
    }

}

//loaderLoader.js
module.exports = function loadLoader(loader, callback) {
    //加载loader，并且拿到loader设置的pitch与raw属性
    if(typeof System === "object" && typeof System.import === "function") {
        System.import(loader.path).catch(callback).then(function(module) {
            loader.normal = typeof module === "function" ? module : module.default;
            loader.pitch = module.pitch;
            loader.raw = module.raw;
            if(typeof loader.normal !== "function" && typeof loader.pitch !== "function")
                throw new Error("Module '" + loader.path + "' is not a loader (must have normal or pitch function)");
            callback();
        });
    } else {
        try {
            var module = require(loader.path);
        } catch(e) {
            // it is possible for node to choke on a require if the FD descriptor
            // limit has been reached. give it a chance to recover.
            if(e instanceof Error && e.code === "EMFILE") {
                var retry = loadLoader.bind(null, loader, callback);
                if(typeof setImmediate === "function") {
                    // node >= 0.9.0
                    return setImmediate(retry);
                } else {
                    // node < 0.9.0
                    return process.nextTick(retry);
                }
            }
            return callback(e);
        }
        if(typeof module !== "function" && typeof module !== "object")
            throw new Error("Module '" + loader.path + "' is not a loader (export function or es6 module))");
        loader.normal = typeof module === "function" ? module : module.default;
        loader.pitch = module.pitch;
        loader.raw = module.raw;
        if(typeof loader.normal !== "function" && typeof loader.pitch !== "function")
            throw new Error("Module '" + loader.path + "' is not a loader (must have normal or pitch function)");
        callback();
    }
};