koa 框架一直都保持着简洁性, 它只对 node 的 HTTP 模块进行了封装, 而在真正实际使用, 我们还需要更多地像路由这样的模块来构建我们的应用, 而 koa-router 是常用的 koa 的路由库. 这里通过解析 koa-router 的源码来达到深入学习的目的.

源码架构图

调用链路-routes()

HTTP请求调用流程

Usage

const Koa = require('koa');
const Router = require('koa-router');

const app = new Koa();
const router = new Router();

router.get('/', async (ctx, next) => {
 console.log('index');
 ctx.body = 'index';
});

app.use(router.routes()).use(router.allowedMethods());

app.listen(3000);

Router

function Router(opts) {
 if (!(this instanceof Router)) {
 return new Router(opts);
 }

 this.opts = opts || {};
 this.methods = this.opts.methods || [
 'HEAD',
 'OPTIONS',
 'GET',
 'PUT',
 'PATCH',
 'POST',
 'DELETE'
 ];

 // 存放router.param方法指定的参数的中间件
 this.params = {};
 // 存放layer实例
 this.stack = [];
};

Layer

function Layer(path, methods, middleware, opts) {
 this.opts = opts || {};
 this.name = this.opts.name || null;
 this.methods = [];
 // 存放path路径参数的一些属性，eg: /test/:str => { name: str, prefix: '/' ....}
 this.paramNames = [];
 // 存放该路由的中间件
 this.stack = Array.isArray(middleware) ? middleware : [middleware];

 methods.forEach(function(method) {
 var l = this.methods.push(method.toUpperCase());
 // 如果支持get请求，一并支持head请求
 if (this.methods[l-1] === 'GET') {
 this.methods.unshift('HEAD');
 }
 }, this);

 // ensure middleware is a function
 this.stack.forEach(function(fn) {
 var type = (typeof fn);
 if (type !== 'function') {
 throw new Error(
 methods.toString() + " `" + (this.opts.name || path) +"`: `middleware` "
 + "must be a function, not `" + type + "`"
 );
 }
 }, this);

 this.path = path;
 // 将路由转为正则表达式
 this.regexp = pathToRegExp(path, this.paramNames, this.opts);

 debug('defined route %s %s', this.methods, this.opts.prefix + this.path);
};

给Router实例挂载HTTP方法

/**
 * Create `router.verb()` methods, where *verb* is one of the HTTP verbs such
 * as `router.get()` or `router.post()`.
 *
 * Match URL patterns to callback functions or controller actions using `router.verb()`,
 * where **verb** is one of the HTTP verbs such as `router.get()` or `router.post()`.
 *
 * Additionaly, `router.all()` can be used to match against all methods.
 *
 * ```javascript
 * router
 * .get('/', (ctx, next) => {
 * ctx.body = 'Hello World!';
 * })
 * .post('/users', (ctx, next) => {
 * // ...
 * })
 * .put('/users/:id', (ctx, next) => {
 * // ...
 * })
 * .del('/users/:id', (ctx, next) => {
 * // ...
 * })
 * .all('/users/:id', (ctx, next) => {
 * // ...
 * });
 * ```
 *
 * When a route is matched, its path is available at `ctx._matchedRoute` and if named,
 * the name is available at `ctx._matchedRouteName`
 *
 * Route paths will be translated to regular expressions using
 * [path-to-regexp](https://github.com/pillarjs/path-to-regexp).
 *
 * Query strings will not be considered when matching requests.
 *
 * #### Named routes
 *
 * Routes can optionally have names. This allows generation of URLs and easy
 * renaming of URLs during development.
 *
 * ```javascript
 * router.get('user', '/users/:id', (ctx, next) => {
 * // ...
 * });
 *
 * router.url('user', 3);
 * // => "/users/3"
 * ```
 *
 * #### Multiple middleware
 *
 * Multiple middleware may be given:
 *
 * ```javascript
 * router.get(
 * '/users/:id',
 * (ctx, next) => {
 * return User.findOne(ctx.params.id).then(function(user) {
 * ctx.user = user;
 * next();
 * });
 * },
 * ctx => {
 * console.log(ctx.user);
 * // => { id: 17, name: "Alex" }
 * }
 * );
 * ```
 *
 * ### Nested routers
 *
 * Nesting routers is supported:
 *
 * ```javascript
 * var forums = new Router();
 * var posts = new Router();
 *
 * posts.get('/', (ctx, next) => {...});
 * posts.get('/:pid', (ctx, next) => {...});
 * forums.use('/forums/:fid/posts', posts.routes(), posts.allowedMethods());
 *
 * // responds to "/forums/123/posts" and "/forums/123/posts/123"
 * app.use(forums.routes());
 * ```
 *
 * #### Router prefixes
 *
 * Route paths can be prefixed at the router level:
 *
 * ```javascript
 * var router = new Router({
 * prefix: '/users'
 * });
 *
 * router.get('/', ...); // responds to "/users"
 * router.get('/:id', ...); // responds to "/users/:id"
 * ```
 *
 * #### URL parameters
 *
 * Named route parameters are captured and added to `ctx.params`.
 *
 * ```javascript
 * router.get('/:category/:title', (ctx, next) => {
 * console.log(ctx.params);
 * // => { category: 'programming', title: 'how-to-node' }
 * });
 * ```
 *
 * The [path-to-regexp](https://github.com/pillarjs/path-to-regexp) module is
 * used to convert paths to regular expressions.
 *
 * @name get|put|post|patch|delete|del
 * @memberof module:koa-router.prototype
 * @param {String} path
 * @param {Function=} middleware route middleware(s)
 * @param {Function} callback route callback
 * @returns {Router}
 */
var methods = require('methods');

methods.forEach(function (method) {
 Router.prototype[method] = function (name, path, middleware) {
 var middleware;

 // 如果指定了路由name属性
 if (typeof path === 'string' || path instanceof RegExp) {
 middleware = Array.prototype.slice.call(arguments, 2);
 } else {
 middleware = Array.prototype.slice.call(arguments, 1);
 path = name;
 name = null;
 }

 // 路由注册
 this.register(path, [method], middleware, {
 name: name
 });

 return this;
 };
});

Router.prototype.register

/**
 * Create and register a route.
 *
 * @param {String} path Path string.
 * @param {Array.<String>} methods Array of HTTP verbs.
 * @param {Function} middleware Multiple middleware also accepted.
 * @returns {Layer}
 * @private
 */
Router.prototype.register = function (path, methods, middleware, opts) {
 opts = opts || {};

 var router = this;
 // layer实例数组，初始为空数组
 var stack = this.stack;

 // support array of paths
 if (Array.isArray(path)) {
 // 如果是多路径，递归注册路由
 path.forEach(function (p) {
 router.register.call(router, p, methods, middleware, opts);
 });

 return this;
 }

 // create route
 var route = new Layer(path, methods, middleware, {
 end: opts.end === false ? opts.end : true,
 name: opts.name,
 sensitive: opts.sensitive || this.opts.sensitive || false,
 strict: opts.strict || this.opts.strict || false,
 prefix: opts.prefix || this.opts.prefix || "",
 ignoreCaptures: opts.ignoreCaptures
 });

 // 设置前置路由
 if (this.opts.prefix) {
 route.setPrefix(this.opts.prefix);
 }

 // add parameter middleware
 Object.keys(this.params).forEach(function (param) {
 // 将router中this.params维护的参数中间件挂载到layer实例中
 route.param(param, this.params[param]);
 }, this);

 // 所有layer实例存放在router的stack属性中
 stack.push(route);

 return route;
};

Router.prototype.match

/**
 * Match given `path` and return corresponding routes.
 *
 * @param {String} path
 * @param {String} method
 * @returns {Object.<path, pathAndMethod>} returns layers that matched path and
 * path and method.
 * @private
 */
Router.prototype.match = function (path, method) {
 // layer实例组成的数组
 var layers = this.stack;
 var layer;
 var matched = {
 path: [],
 pathAndMethod: [],
 route: false
 };

 for (var len = layers.length, i = 0; i < len; i++) {
 layer = layers[i];

 debug('test %s %s', layer.path, layer.regexp);

 // 1.匹配路由
 if (layer.match(path)) {
 matched.path.push(layer);

 // 2.匹配http请求方法
 if (layer.methods.length === 0 || ~layer.methods.indexOf(method)) {
 matched.pathAndMethod.push(layer);
 // 3.指定了http请求方法，判定为路由匹配成功
 if (layer.methods.length) matched.route = true;
 }
 }
 }

 return matched;
};

Router.prototype.routes

/**
 * Returns router middleware which dispatches a route matching the request.
 *
 * @returns {Function}
 */
Router.prototype.routes = Router.prototype.middleware = function () {
 var router = this;

 var dispatch = function dispatch(ctx, next) {
 debug('%s %s', ctx.method, ctx.path);

 // 请求路由
 var path = router.opts.routerPath || ctx.routerPath || ctx.path;
 // 将注册路由和请求的路由进行匹配
 var matched = router.match(path, ctx.method);
 var layerChain, layer, i;

 if (ctx.matched) {
 ctx.matched.push.apply(ctx.matched, matched.path);
 } else {
 ctx.matched = matched.path;
 }

 ctx.router = router;

 // route属性是三次匹配的结果，表示最终是否匹配成功
 if (!matched.route) return next();

 // 同时满足路由匹配和http请求方法的layer数组
 var matchedLayers = matched.pathAndMethod
 // 匹配多个路由时认为最后一个是匹配有效的路由
 var mostSpecificLayer = matchedLayers[matchedLayers.length - 1]
 ctx._matchedRoute = mostSpecificLayer.path;
 if (mostSpecificLayer.name) {
 ctx._matchedRouteName = mostSpecificLayer.name;
 }

 // 将匹配的路由reduce为一个数组
 layerChain = matchedLayers.reduce(function(memo, layer) {
 // 执行注册路由中间件之前，对context中的一些参数进行设置
 memo.push(function(ctx, next) {
 // :path/XXX 捕获的路径
 ctx.captures = layer.captures(path, ctx.captures);
 // 捕获的路径上的参数, { key: value }
 ctx.params = layer.params(path, ctx.captures, ctx.params);
 // 路由名称
 ctx.routerName = layer.name;
 return next();
 });
 // 返回路由中间件的数组
 return memo.concat(layer.stack);
 }, []);

 // 处理为promise对象
 return compose(layerChain)(ctx, next);
 };

 dispatch.router = this;

 return dispatch;
};

Router.prototype.allowedMethod

/**
 * Returns separate middleware for responding to `OPTIONS` requests with
 * an `Allow` header containing the allowed methods, as well as responding
 * with `405 Method Not Allowed` and `501 Not Implemented` as appropriate.
 *
 * @example
 *
 * ```javascript
 * var Koa = require('koa');
 * var Router = require('koa-router');
 *
 * var app = new Koa();
 * var router = new Router();
 *
 * app.use(router.routes());
 * app.use(router.allowedMethods());
 * ```
 *
 * **Example with [Boom](https://github.com/hapijs/boom)**
 *
 * ```javascript
 * var Koa = require('koa');
 * var Router = require('koa-router');
 * var Boom = require('boom');
 *
 * var app = new Koa();
 * var router = new Router();
 *
 * app.use(router.routes());
 * app.use(router.allowedMethods({
 * throw: true,
 * notImplemented: () => new Boom.notImplemented(),
 * methodNotAllowed: () => new Boom.methodNotAllowed()
 * }));
 * ```
 *
 * @param {Object=} options
 * @param {Boolean=} options.throw throw error instead of setting status and header
 * @param {Function=} options.notImplemented throw the returned value in place of the default NotImplemented error
 * @param {Function=} options.methodNotAllowed throw the returned value in place of the default MethodNotAllowed error
 * @returns {Function}
 */
Router.prototype.allowedMethods = function (options) {
 options = options || {};
 var implemented = this.methods;

 return function allowedMethods(ctx, next) {
 // 所有中间件执行完之后执行allowedMethod方法
 return next().then(function() {
 var allowed = {};

 // 没有响应状态码或者响应了404
 if (!ctx.status || ctx.status === 404) {
 // 在match方法中，匹配的路由的layer实例对象组成的数组
 ctx.matched.forEach(function (route) {
 route.methods.forEach(function (method) {
 // 把匹配的路由的http方法保存起来，认为是允许的http请求方法
 allowed[method] = method;
 });
 });

 var allowedArr = Object.keys(allowed);

 // 如果该方法在router实例的methods中不存在
 if (!~implemented.indexOf(ctx.method)) {
 // 如果在初始化router时配置了throw属性为true
 if (options.throw) {
 var notImplementedThrowable;
 if (typeof options.notImplemented === 'function') {
 // 指定了报错函数
 notImplementedThrowable = options.notImplemented(); // set whatever the user returns from their function
 } else {
 // 没有指定则抛出http异常
 notImplementedThrowable = new HttpError.NotImplemented();
 }
 throw notImplementedThrowable;
 } else {
 // 没有配置throw则响应501
 ctx.status = 501;
 // 设置响应头中的allow字段，返回允许的http方法
 ctx.set('Allow', allowedArr.join(', '));
 }
 } else if (allowedArr.length) {
 if (ctx.method === 'OPTIONS') {
 // 如果是OPTIONS请求，则认为是请求成功，响应200，并根据OPTIONS请求约定返回允许的http方法
 ctx.status = 200;
 ctx.body = '';
 ctx.set('Allow', allowedArr.join(', '));
 } else if (!allowed[ctx.method]) {
 // 如果请求方法在router实例的methods中存在，但是在匹配的路由中该http方法不存在
 if (options.throw) {
 var notAllowedThrowable;
 if (typeof options.methodNotAllowed === 'function') {
 notAllowedThrowable = options.methodNotAllowed(); // set whatever the user returns from their function
 } else {
 notAllowedThrowable = new HttpError.MethodNotAllowed();
 }
 throw notAllowedThrowable;
 } else {
 // 响应405 http请求方法错误
 ctx.status = 405;
 ctx.set('Allow', allowedArr.join(', '));
 }
 }
 }
 }
 });
 };
};

Router.prototype.use

/**
 * Use given middleware.
 *
 * Middleware run in the order they are defined by `.use()`. They are invoked
 * sequentially, requests start at the first middleware and work their way
 * "down" the middleware stack.
 *
 * @example
 *
 * ```javascript
 * // session middleware will run before authorize
 * router
 * .use(session())
 * .use(authorize());
 *
 * // use middleware only with given path
 * router.use('/users', userAuth());
 *
 * // or with an array of paths
 * router.use(['/users', '/admin'], userAuth());
 *
 * app.use(router.routes());
 * ```
 *
 * @param {String=} path
 * @param {Function} middleware
 * @param {Function=} ...
 * @returns {Router}
 */
Router.prototype.use = function () {
 var router = this;
 var middleware = Array.prototype.slice.call(arguments);
 var path;

 // support array of paths
 // 如果第一个参数是一个数组，且数组中元素为字符串
 if (Array.isArray(middleware[0]) && typeof middleware[0][0] === 'string') {
 // 递归调用use方法
 middleware[0].forEach(function (p) {
 router.use.apply(router, [p].concat(middleware.slice(1)));
 });

 return this;
 }

 var hasPath = typeof middleware[0] === 'string';
 if (hasPath) {
 path = middleware.shift();
 }

 middleware.forEach(function (m) {
 // 如果这个中间件是由router.routes()方法返回的dispatch中间件，即这是一个嵌套的路由
 if (m.router) {
 // 遍历router.stack属性中所有的layer
 m.router.stack.forEach(function (nestedLayer) {
 // 被嵌套的路由需要以父路由path为前缀
 if (path) nestedLayer.setPrefix(path);
 // 如果父路由有指定前缀，被嵌套的路由需要把这个前缀再加上
 if (router.opts.prefix) nestedLayer.setPrefix(router.opts.prefix);
 router.stack.push(nestedLayer);
 });

 if (router.params) {
 Object.keys(router.params).forEach(function (key) {
 m.router.param(key, router.params[key]);
 });
 }
 } else {
 router.register(path || '(.*)', [], m, { end: false, ignoreCaptures: !hasPath });
 }
 });

 return this;
};