.28-浅析webpack源码之compiler.resolvers

compiler.resolvers.normal = ResolverFactory.createResolver(Object.assign({
    fileSystem: compiler.inputFileSystem
}, options.resolve));
compiler.resolvers.context = ResolverFactory.createResolver(Object.assign({
    fileSystem: compiler.inputFileSystem,
    resolveToContext: true
}, options.resolve));
compiler.resolvers.loader = ResolverFactory.createResolver(Object.assign({
    fileSystem: compiler.inputFileSystem
}, options.resolveLoader));

/*
    "resolve": {
        "unsafeCache": true,
        "modules": ["node_modules"],
        "extensions": [".js", ".json"],
        "mainFiles": ["index"],
        "aliasFields": ["browser"],
        "mainFields": ["browser", "module", "main"],
        "cacheWithContext": false
    },
*/
compiler.resolvers.normal = ResolverFactory.createResolver(Object.assign({
    fileSystem: compiler.inputFileSystem
}, options.resolve));

exports.createResolver = function(options) {
    //// OPTIONS ////

    // A list of directories to resolve modules from, can be absolute path or folder name
    // 模块文件夹的目录或者文件夹名称
    var modules = options.modules || ["node_modules"];

    // A list of description files to read from
    // 描述配置文件名
    var descriptionFiles = options.descriptionFiles || ["package.json"];

    // A list of additional resolve plugins which should be applied
    // The slice is there to create a copy, because otherwise pushing into plugins
    // changes the original options.plugins array, causing duplicate plugins
    // 额外的插件
    var plugins = (options.plugins && options.plugins.slice()) || [];

    // A list of main fields in description files
    // 不知道干啥的
    var mainFields = options.mainFields || ["main"];

    // A list of alias fields in description files
    // 不知道干啥的
    var aliasFields = options.aliasFields || [];

    // A list of main files in directories
    // 模块主入口文件名
    var mainFiles = options.mainFiles || ["index"];

    // A list of extensions which should be tried for files
    // 默认的文件扩展名
    var extensions = options.extensions || [".js", ".json", ".node"];

    // Enforce that a extension from extensions must be used
    var enforceExtension = options.enforceExtension || false;

    // A list of module extensions which should be tried for modules
    var moduleExtensions = options.moduleExtensions || [];

    // Enforce that a extension from moduleExtensions must be used
    var enforceModuleExtension = options.enforceModuleExtension || false;

    // A list of module alias configurations or an object which maps key to value
    // 别名
    var alias = options.alias || [];

    // ...还有一些其他奇奇怪怪的属性

    //// options processing ////

    // ...第二部分
};

exports.createResolver = function(options) {
    //// OPTIONS ////

    // ...第一部分

    //// options processing ////

    if (!resolver) {
        // useSyncFileSystemCalls默认为undefined
        resolver = new Resolver(useSyncFileSystemCalls ? new SyncAsyncFileSystemDecorator(fileSystem) : fileSystem);
    }
    // 数组包装
    extensions = [].concat(extensions);
    moduleExtensions = [].concat(moduleExtensions);
    // 返回[['node_modules']]
    modules = mergeFilteredToArray([].concat(modules), function(item) {
        return !isAbsolutePath(item);
    });
    // 不懂这个参数干啥的
    // 返回一个对象数组
    mainFields = mainFields.map(function(item) {
        if (typeof item === "string") {
            item = {
                name: item,
                forceRelative: true
            };
        }
        return item;
    });
    // 处理别名
    if (typeof alias === "object" && !Array.isArray(alias)) { /**/ }

    // 不知道什么东西
    if (unsafeCache && typeof unsafeCache !== "object") {
        unsafeCache = {};
    }

    //// pipeline ////

    // ...第三部分
};

/*
    alias: {
        'vue$': 'vue/dist/vue.esm.js',
        '@': '../src'
    }
*/
/*
    alias:[
        {
            name: 'vue',
            onlyModule: true,
            alias: 'vue/dist/vue.esm.js'
        },
        {
            name: '@',
            onlyModule: false,
            alias: '../src'           
        }
    ]
*/
if (typeof alias === "object" && !Array.isArray(alias)) {
    alias = Object.keys(alias).map(function(key) {
        var onlyModule = false;
        var obj = alias[key];
        // 测试是否以$结尾
        if (/\$$/.test(key)) {
            onlyModule = true;
            key = key.substr(0, key.length - 1);
        }
        // alias的值是否为字符串
        if (typeof obj === "string") {
            obj = {
                alias: obj
            };
        }
        obj = Object.assign({
            name: key,
            onlyModule: onlyModule
        }, obj);
        return obj;
    });
}

exports.createResolver = function(options) {
    //// OPTIONS ////

    // ...第一部分

    //// options processing ////

    // ...第二部分

    //// pipeline ////

    // resolve
    if (unsafeCache) {
        plugins.push(new UnsafeCachePlugin("resolve", cachePredicate, unsafeCache, cacheWithContext, "new-resolve"));
        plugins.push(new ParsePlugin("new-resolve", "parsed-resolve"));
    } else {
        plugins.push(new ParsePlugin("resolve", "parsed-resolve"));
    }

    // ...无穷多的if + plugins.push(...)

    //// RESOLVER ////

    plugins.forEach(function(plugin) {
        resolver.apply(plugin);
    });
    return resolver;
};

// described-resolve
alias.forEach(function(item) {
    plugins.push(new AliasPlugin("described-resolve", item, "resolve"));
});

class AliasPlugin {
    constructor(source, options, target) {
        this.source = source;
        this.name = options.name;
        this.alias = options.alias;
        this.onlyModule = options.onlyModule;
        this.target = target;
    }
    apply(resolver) {
        var target = this.target;
        var name = this.name;
        var alias = this.alias;
        var onlyModule = this.onlyModule;
        resolver.plugin(this.source, function(request, callback) { /**/ });
    }
}

plugins.forEach(function(plugin) {
    resolver.apply(plugin);
});