1. app.use()
app.use(async (ctx, next) => {}); // 將中間件放入中間件數(shù)組
源碼：
use(fn) {
if (typeof fn !== 'function') throw new TypeError('middleware must be a function!');
if (isGeneratorFunction(fn)) {
deprecate('Support for generators will be removed in v3. ' +
'See the documentation for examples of how to convert old middleware ' +
'https://github.com/koajs/koa/blob/master/docs/migration.md');
fn = convert(fn);
}
debug('use %s', fn._name || fn.name || '-');
this.middleware.push(fn); // 將中間件放入中間件數(shù)組
return this;
}
2. app.listen()
module.exports = app.listen(config.port, '0.0.0.0', () => {
console.log('進(jìn)入app');
});
源碼：
listen(...args) {
debug('listen');
// this.callback()為connection listener, 當(dāng)客戶端連接上服務(wù)器時會觸發(fā)該函數(shù)贴见，即handleRequest
const server = http.createServer(this.callback());
return server.listen(...args);
}
/**
* Return a request handler callback
* for node's native http server.
*
* @return {Function}
* @api public
*/
callback() {
// 將中間件數(shù)組傳入koa-compose, 得到一個返回函數(shù)瘩蚪，將該函數(shù)傳入this.handleRequest
const fn = compose(this.middleware);
if (!this.listeners('error').length) this.on('error', this.onerror);
const handleRequest = (req, res) => {
const ctx = this.createContext(req, res);
return this.handleRequest(ctx, fn);
};
return handleRequest;
}
/**
* Handle request in callback.
*
* @api private
*/
handleRequest(ctx, fnMiddleware) {
const res = ctx.res;
res.statusCode = 404;
const onerror = err => ctx.onerror(err);
const handleResponse = () => respond(ctx);
onFinished(res, onerror);
// fnMiddleware(ctx) 返回promise
/*
return Promise.resolve(fn(context, function next () {
return dispatch(i + 1)
}))
首先執(zhí)行第一個中間件函數(shù)，傳入context和next函數(shù)汉匙，next返回也是返回promise
*/
return fnMiddleware(ctx).then(handleResponse).catch(onerror);
}
3. koa-compose
function compose (middleware) {
if (!Array.isArray(middleware)) throw new TypeError('Middleware stack must be an array!')
for (const fn of middleware) {
if (typeof fn !== 'function') throw new TypeError('Middleware must be composed of functions!')
}
/**
* @param {Object} context
* @return {Promise}
* @api public
*/
return function (context, next) {
// next 為undefined
// last called middleware #
let index = -1
return dispatch(0)
function dispatch (i) {
if (i <= index) return Promise.reject(new Error('next() called multiple times'))
index = i
let fn = middleware[i]
if (i === middleware.length) fn = next
if (!fn) return Promise.resolve()
try {
return Promise.resolve(fn(context, function next () {
return dispatch(i + 1)
}))
} catch (err) {
return Promise.reject(err)
}
}
}
}
總結(jié)
整個流程為：
app.use函數(shù)將中間件放入中間件數(shù)組拱烁。當(dāng)app接收到http請求時，會執(zhí)行connection listener函數(shù)噩翠，這個函數(shù)中會去執(zhí)行koa-compose返回的函數(shù)(return dispatch(0))邻梆，此時會執(zhí)行第一個中間件函數(shù)，傳入content和next函數(shù)绎秒，每個next函數(shù)也會返回一個promise浦妄。在中間件函數(shù)中await next()，即執(zhí)行下一個中間件函數(shù)(return dispatch(i + 1))见芹，直到所有的中間件都執(zhí)行結(jié)束后剂娄，將undefined賦值給fn(if (i === middleware.length) fn = next)。
