koa-compose

[hoperyy]

• compose 接收 middlewares，返回一个 thenable 对象，可以理解为 promise1

• promsie1 的状态取决于 middleware 1 的状态，

  middleware 1 的状态取决于其 next 的返回值状态，也就是 middleware 2 的状态，

  middleware 2 本身返回一个 promsie，但为了自动执行，下面代码中，next 返回了递归的 dispatch，也就是 middleware 2 等外面又包了一层 promise

• 处理下错误和 middleware 执行完的情况

function compose(middlewares) {
    let index = 0;
    const dispatch = (context) => {
        const fn = middlewares[index];

        if (!fn) {
            return Promise.resolve();
        }

        try {
            return Promise.resolve(fn(context, () => {
                index++;
                return dispatch(context);
            }));
        } catch(err) {
            return Promise.reject(err);
        }
    };

    return function(context) {
        return dispatch(context);
    };
}

const delay = (time = 1000) => {
    return new Promise(resolve => setTimeout(resolve, time));
};

const m1 = async (context, next) => {
    await delay();
    console.log(1);
    await next();
};

const m2 = async (context, next) => {
    await delay();
    console.log(2);
    await next();
};

const m3 = async (context, next) => {
    await delay();
    console.log(3);
    await delay();
    await next();
};

const run = compose([ m1, m2, m3 ]);

run({}).then(() => {
    console.log('done');
});