koa next()方法的大致思路

function App(){

}
App.prototype = {
	use(fn){
		this.fns.push(fn)
	
	},
	fns:[],
	start(){
		let i=0;
		let fns = this.fns;
		function f(){
			fns[i]("s",()=>{
				i++;
				if(fns[i]){
					f()
				}
			})
		}
		f()
		
	}
}
const app = new App()
app.use((ctx, next)=>{
	console.log('1-1')
	next()
	console.log('1-2')
})
app.use((ctx, next)=>{
	console.log('2-1')
	next()
	console.log('2-2')
})
app.use((ctx, next)=>{
	console.log('3-1')
	next()
	console.log('3-2')
})
app.start()