egg源码浅析一npm init egg --type=simple

要egg文档最开始的时候，有这样的几条命令：

我们推荐直接使用脚手架，只需几条简单指令，即可快速生成项目:
$ mkdir egg-example && cd egg-example
$ npm init egg --type=simple
$ npm i

其中的 npm init egg --type=simple 命令为什么能够生成egg项目的基本构架呢？

一、npm init命令

根据 https://www.npmjs.cn/cli/init/ 官网的解释:

npm init <initializer> can be used to set up a new or existing npm package.

initializer in this case is an npm package named create-<initializer>, which will be installed by npx, and then have its main bin executed.

　按照上面的解释，npm init egg 相当于 npx create-egg 并且执行create-egg的bin.

二、npx命令

　　npx相当于npm，但npx会把node_modules/.bin/目录加入环境变量.

　　这里讲怎么用：http://www.ruanyifeng.com/blog/2019/02/npx.html

也就是说，npm init egg 相当于执行 npx create-egg，而npx create-egg命令会下载create-egg库，再执行里面的bin(package.json里的bin字段)。

　　另外，npx create-egg 会在当前目录/node_modules目录下查找有没有create-egg，没有就会下载到临时目录，最后删除。

三、下载create-egg库

　　create-egg的目录结构如下，package.json里没有main，却有bin字段，bin字段值是./bin/create-egg.js。当安装create-egg的时，会在当前安装目录下的node_modules/.bin/目录放置create-egg作为可执行文件。

　　create-egg.js文件里只有一行代码：

console.log('--------i am create-egg');
require('egg-init/bin/egg-init');

　　再来看egg-init库。

四、egg-init

#!/usr/bin/env node
'use strict';
const co = require('co');
const Command = require('..');

co(function* () {
  console.log('process.cwd:', process.cwd());
  console.log('process.argv.slice:', process.argv.slice(2));
  // process.cwd: /Users/demon/Desktop/test/createeggtest
  // process.argv.slice: [ '--type=simple' ]
  yield new Command().run(process.cwd(), process.argv.slice(2));
}).catch(err => {
  console.error(err.stack);
  process.exit(1);
});

process.cwd是命令执行的所有文件夹，argv.slice(2)之后是--type=simple字符串。

之后，向 https://registry.npmjs.org/egg-init-config/latest请求，获取响应：

{
    "name":"egg-init-config",
    "version":"1.5.0",
    "description":"egg init boilerplate config",
    "config":{
        "boilerplate":{
            "simple":{
                "package":"egg-boilerplate-simple",
                "description":"Simple egg app boilerplate"
            },
            "microservice":{
                "package":"egg-boilerplate-microservice",
                "description":"Microservice app boilerplate based on egg"
            },
            "sequelize":{
                "package":"egg-boilerplate-sequelize",
                "description":"egg app with sequelize"
            },
            "ts":{
                "package":"egg-boilerplate-ts",
                "description":"Simple egg && typescript app boilerplate"
            },
            "empty":{
                "package":"egg-boilerplate-empty",
                "description":"Empty egg app boilerplate"
            },
            "plugin":{
                "package":"egg-boilerplate-plugin",
                "description":"egg plugin boilerplate"
            },
            "framework":{
                "package":"egg-boilerplate-framework",
                "description":"egg framework boilerplate"
            }
        }
    },
    ...
}

根据得到的响应，匹配--type=simple，得到package:

"simple":{
    "package":"egg-boilerplate-simple",
    "description":"Simple egg app boilerplate"
},

得到egg-boilerplate-simple，再次向 https://registry.npmjs.org/egg-boilerplate-simple/latest请求，

{
  "name": "egg-boilerplate-simple",
  "version": "3.3.1",
 　...
  "dist": {
    "integrity": "sha512-zj8ES0n7lAXGMWornOOY1n0Q+skX0wA9hjl40QsTa3SS5FcExVoEWQ52kCeT+bnvNKEL8fiOD9S2J4J9wDjbKg==",
    "shasum": "c56c87c1f2b705a9247b0903958aac76d141b2c6",
    "tarball": "https://registry.npmjs.org/egg-boilerplate-simple/-/egg-boilerplate-simple-3.3.1.tgz",
    "fileCount": 31,
    "unpackedSize": 19461,
    "npm-signature": "-----BEGIN PGP SIGNATURE-----
Version: OpenPGP.js v3.0.4
Comment: https://openpgpjs.org
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=Q83Q
-----END PGP SIGNATURE-----
"
  },
　　...
}

拿到dist.tarball字段的值，下载到临时目录下的egg-init-boilerplate目录并解压:

const saveDir = path.join(os.tmpdir(), 'egg-init-boilerplate');

yield compressing.tgz.uncompress(response.res, saveDir);

之后，进入saveDir/package/boilerplate目录，boilerplate目录内容如下

之后就是把这个目录的内容拷贝到目标目录下，当然会把_前缀去掉：

const to = path.join(targetDir, this.replaceTemplate(this.fileMapping[file] || file, locals));
const content = fs.readFileSync(from);
this.log('write to %s', to);

// check if content is a text file
const result = isTextOrBinary.isTextSync(from, content)
   ? this.replaceTemplate(content.toString('utf8'), locals)
   : content;

mkdirp.sync(path.dirname(to));
fs.writeFileSync(to, result);

这就是npm init egg --type=simple内容的由来了。