koa2的洋葱模型实现原理
koa2的洋葱模型遵循先进后出的原则,这点和栈的特性很像。请求从最外层进去,响应从最外层出来。下面有两张概念图提供参考:
www.programman.cn
实现洋葱模型的核心代码在 koa-compose 里面:
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!');
}
return function(context, next) {
// 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, dispatch.bind(null, i + 1)));
} catch (err) {
return Promise.reject(err);
}
}
};
}
其中,compose 函数,负责处理注册的所有中间件,我们使用以下方式注册中间件(middleware)
app.use(async (ctx, next) => {
// code
await next();
// code
});
app.use(async (ctx, next) => {
// code
await next();
// code
});
那么,koa2 是如何处理注册的中间件,并实现它的洋葱模型呢?
koa2 会将注册的所有中间件函数,放在数组 middleware 中,使用 koa-compose,处理 moddleware 这个数组。
下面就是对 compose 函数的解析:
function compose(middleware) {
// 容错判断,如果middleware不是数组,或者元素不是函数,则抛异常
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!');
}
// compose函数最终返回一个闭包函数
return function(context, next) {
// last called middleware #
let index = -1;
return dispatch(0);
// 使用递归操作,将各个function作为前一个function的next参数传递过去
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, dispatch.bind(null, i + 1)));
} catch (err) {
return Promise.reject(err);
}
}
};
}