.31-浅析webpack源码之doResolve事件流(3)

　　放个流程图：

　　这里也放一下request对象内容，这节完事后如下(把vue-cli的package.json也复制过来了)：

/*{ context: { issuer: '', compiler: undefined },path: 'd:\\workspace\\doc',request: './input.js',query: '',module: false,directory: false,file: false,descriptionFilePath: 'd:\\workspace\\doc\\package.json',descriptionFileData: *package.json内容*,descriptionFileRoot: 'd:\\workspace\\doc',relativePath: '.',__innerRequest_request: './input.js',__innerRequest_relativePath: '.',__innerRequest: './input.js' }
*/

　　上一节看到这：

// 调用的是callback()
function innerCallback(err, result) {if (arguments.length > 0) {if (err) return callback(err);if (result) return callback(null, result);return callback();}runAfter();
}

　　这里接下来会调用runAfter方法，之前有讲解过这个，简单讲就是触发after-type的事件流，这里的type为parsed-resolve，即触发after-parsed-resolve事件流。

　　来源如下：

plugins.push(new NextPlugin("after-parsed-resolve", "described-resolve"));

　　这个插件就非常简单了：

NextPlugin.prototype.apply = function(resolver) {var target = this.target;resolver.plugin(this.source, function(request, callback) {resolver.doResolve(target, request, null, callback);});
};

　　直接调用doResolve方法触发下一个target的事件流，比起有描述文件的情况，这里的区别就是request少了几个参数，触发下一个事件流时没有message。

　　刚发现事件流的名字代表着某阶段，此时代表描述文件解析完毕。

　　接下来的事件流来源于以下几个插件：

// described-resolve
alias.forEach(function(item) {plugins.push(new AliasPlugin("described-resolve", item, "resolve"));
});
plugins.push(new ConcordModulesPlugin("described-resolve", {}, "resolve"));
aliasFields.forEach(function(item) {plugins.push(new AliasFieldPlugin("described-resolve", item, "resolve"));
});
plugins.push(new ModuleKindPlugin("after-described-resolve", "raw-module"));
plugins.push(new JoinRequestPlugin("after-described-resolve", "relative"));

AliasPlugin　　

　　先从第一个开始看，alias参数引用vue-cli的代码，这里的alias在上面的第二部分进行了转换(具体可参考28节)。

　　数组的元素作为参数传入了AliasPlugin插件中，源码如下：

/*
源配置alias: {'vue$': 'vue/dist/vue.esm.js','@': '../src'}
转换后为alias:[{name: 'vue',onlyModule: true,alias: 'vue/dist/vue.esm.js'},{name: '@',onlyModule: false,alias: '../src'           }]
*/
AliasPlugin.prototype.apply = function(resolver) {var target = this.target;var name = this.name;var alias = this.alias;var onlyModule = this.onlyModule;resolver.plugin(this.source, function(request, callback) {var innerRequest = request.request;if (!innerRequest) return callback();// 两个元素传进来并不满足if条件 跳过// startsWith可参考ES6的新方法http://es6.ruanyifeng.com/#docs/string#includes-startsWith-endsWithif (innerRequest === name || (!onlyModule && startsWith(innerRequest, name + "/"))) {if (innerRequest !== alias && !startsWith(innerRequest, alias + "/")) {var newRequestStr = alias + innerRequest.substr(name.length);var obj = Object.assign({}, request, {request: newRequestStr});return resolver.doResolve(target, obj, "aliased with mapping '" + name + "': '" + alias + "' to '" + newRequestStr + "'", createInnerCallback(function(err, result) {if (arguments.length > 0) return callback(err, result);// don't allow other aliasing or raw requestcallback(null, null);}, callback));}}return callback();});
};

　　不太懂这里的处理是干什么，反正两个元素传进来都没有满足if条件，跳过。

ConcordModulesPlugin

　　described-resolve事件流还没有完，所以callback执行后只是记数，下一个插件源码如下：

ConcordModulesPlugin.prototype.apply = function(resolver) {var target = this.target;resolver.plugin(this.source, function(request, callback) {// 获取的还是'./input.js'var innerRequest = getInnerRequest(resolver, request);if (!innerRequest) return callback();// request.descriptionFileData就是配置文件package.json中的内容var concordField = DescriptionFileUtils.getField(request.descriptionFileData, "concord");// 找不到该属性直接返回if (!concordField) return callback();// 下面的都不用跑了var data = concord.matchModule(request.context, concordField, innerRequest);if (data === innerRequest) return callback();if (data === undefined) return callback();if (data === false) {var ignoreObj = Object.assign({}, request, {path: false});return callback(null, ignoreObj);}var obj = Object.assign({}, request, {path: request.descriptionFileRoot,request: data});resolver.doResolve(target, obj, "aliased from description file " + request.descriptionFilePath + " with mapping '" + innerRequest + "' to '" + data + "'", createInnerCallback(function(err, result) {if (arguments.length > 0) return callback(err, result);// Don't allow other aliasing or raw requestcallback(null, null);}, callback));});
};

　　这里有两个工具方法：getInnerRequest、getFiled，第一个获取request的inner属性，代码如下：

module.exports = function getInnerRequest(resolver, request) {// 第一次进来是没有这些属性的if (typeof request.__innerRequest === "string" &&request.__innerRequest_request === request.request &&request.__innerRequest_relativePath === request.relativePath)return request.__innerRequest;var innerRequest;// './input.js'if (request.request) {innerRequest = request.request;// 尝试获取relativePath属性进行拼接if (/^\.\.?\//.test(innerRequest) && request.relativePath) {innerRequest = resolver.join(request.relativePath, innerRequest);}} else {innerRequest = request.relativePath;}// 属性添加request.__innerRequest_request = request.request;request.__innerRequest_relativePath = request.relativePath;return request.__innerRequest = innerRequest;
};

　　总的来说就是尝试获取__innerRequest属性，但是初次进来是没有的，所以会在后面进行添加，最后返回的仍然是'./input.js'。

　　第二个方法就比较简单了，只是从之前读取的package.json对象查询对应的字段，代码如下：

// content为package.json配置对象
function getField(content, field) {if (!content) return undefined;// 数组及单key模式if (Array.isArray(field)) {var current = content;for (var j = 0; j < field.length; j++) {if (current === null || typeof current !== "object") {current = null;break;}current = current[field[j]];}if (typeof current === "object") {return current;}} else {if (typeof content[field] === "object") {return content[field];}}
}

　　代码非常简单，这里就不讲了。

　　常规情况下，没人会去设置concord属性吧，在vue-cli我也没看到，这里先跳过。

AliasFieldPlugin

　　接下来是这个不知道干啥的插件，处理的是resolve.aliasFields参数，默认参数及插件源码如下：

// "aliasFields": ["browser"],
AliasFieldPlugin.prototype.apply = function(resolver) {var target = this.target;var field = this.field;resolver.plugin(this.source, function(request, callback) {if (!request.descriptionFileData) return callback();// 一样的var innerRequest = getInnerRequest(resolver, request);if (!innerRequest) return callback();// filed => browservar fieldData = DescriptionFileUtils.getField(request.descriptionFileData, field);if (typeof fieldData !== "object") {if (callback.log) callback.log("Field '" + field + "' doesn't contain a valid alias configuration");return callback();}var data1 = fieldData[innerRequest];var data2 = fieldData[innerRequest.replace(/^\.\//, "")];var data = typeof data1 !== "undefined" ? data1 : data2;if (data === innerRequest) return callback();if (data === undefined) return callback();if (data === false) {var ignoreObj = Object.assign({}, request, {path: false});return callback(null, ignoreObj);}var obj = Object.assign({}, request, {path: request.descriptionFileRoot,request: data});resolver.doResolve(target, obj, "aliased from description file " + request.descriptionFilePath + " with mapping '" + innerRequest + "' to '" + data + "'", createInnerCallback(function(err, result) {if (arguments.length > 0) return callback(err, result);// Don't allow other aliasing or raw requestcallback(null, null);}, callback));});
};

　　开头跟之前那个是一样的，也是调用getField从package.json中获取对应的配置，但是这个默认的browser我在vue-cli也找不到，暂时跳过。

　　正常处理完described-resolve事件流，继续执行runafter触发after-described-resolve事件流，来源如下：

plugins.push(new ModuleKindPlugin("after-described-resolve", "raw-module"));
plugins.push(new JoinRequestPlugin("after-described-resolve", "relative"));

ModuleKindPlugin

ModuleKindPlugin.prototype.apply = function(resolver) {var target = this.target;resolver.plugin(this.source, function(request, callback) {// 判断module属性if (!request.module) return callback();var obj = Object.assign({}, request);// 删除module属性delete obj.module;// 直接触发下一个事件流resolver.doResolve(target, obj, "resolve as module", createInnerCallback(function(err, result) {if (arguments.length > 0) return callback(err, result);// Don't allow other alternativescallback(null, null);}, callback));});
};

　　这里的处理十分简单，判断request对象是否是module，是则直接触发下一个事件流。

　　而在第一次时进来的是入口文件，module属性为false，所以这里会跳过，后面处理module再回来讲。

JoinRequestPlugin

JoinRequestPlugin.prototype.apply = function(resolver) {var target = this.target;resolver.plugin(this.source, function(request, callback) {var obj = Object.assign({}, request, {// request.path => d:\\workspace\\doc// request.request => ./input.js// 在join方法中会被拼接成d:\workspace\doc\.\input.js// 最后格式化返回d:\workspace\doc\input.js
            path: resolver.join(request.path, request.request),// undefinedrelativePath: request.relativePath && resolver.join(request.relativePath, request.request),request: undefined});resolver.doResolve(target, obj, null, callback);});
};