KOA中间件实现原理

//基本原理
var empty=(function *(){})();
    //中间件3
    var mid2=function *(){
    console.log("2:before yield");
    yield empty;
    console.log("2:after yield");
}
//中间件2
var mid1=function *(){
    console.log("1:before yield");
    yield *(mid2());
    console.log("1:after yield");
}
//中间件1
var start=(function *(){
    console.log("0:before yield");
    yield *(mid1());
    console.log("0:after yield");
})();
while(!(start.next().done)){}

结果
0:before yield
1:before yield
2:before yield
2:after yield
1:after yield
0:after yield

---

//将数组里面的generator函数compose在一起
var mids=[
function *(next){
    console.log("0:before yield");
    yield 0;
    yield *next;
    yield 1;
    console.log("0:after yield");
 
},
function *(next){
    console.log("1:before yield");
    yield 2;
    yield *next;
    console.log("1:after yield");
    yield 3;
},
function *(next){
    console.log("2:before yield");
    yield 4;
    yield *next;
    yield 5;
    console.log("2:after yield");
}
];
 
var next = (function* (){})(),i=mids.length;
/* while (i--) {   next = mids[i].call(null, next);}}*/
for(;i>0;i--){  next=mids[i-1](next);}
var ret;
while(!(ret=next.next()).done){console.log(ret.value);} 

结果
0:before yield
0
1:before yield
2
2:before yield
4
5
2:after yield
1:after yield
3
1
0:after yield

//递归,使用next而非*next跳转到下一个中间件

var mids=[
function *(next){
    console.log("0:before yield");
    yield 0;
    yield next;
    yield 1;
    console.log("0:after yield");
 },
function *(next){
    console.log("1:before yield");
    yield 2;
    yield next;
    console.log("1:after yield");
    yield 3;
},
function *(next){
    console.log("2:before yield");
    yield 4;
    yield next;
    yield 5;
    console.log("2:after yield");
}
];
var next = (function* (){})(),i=mids.length;
for(;i>0;i--){  next=mids[i-1](next);}
 
function nextGenerator(generatorObj){
    var value,item;
    nextVal();
function nextVal(){
    item=generatorObj.next();
    if(item.done) {return;}
    value=item.value;
    if(isGenerator(value)){
    nextGenerator(value);
}else console.log(value);
    nextVal();
}
function isGenerator(obj) {
    return typeof obj.next=='function' && typeof obj.throw=='function';
}
}
nextGenerator(next);

结果

0:before yield
0
1:before yield
2
2:before yield
4
5
2:after yield
1:after yield
3
1
0:after yield

//第二个中间件不跳转
var mids=[

function *(next){
    console.log("0:before yield");
    yield 0;
    yield next;
    yield 1;
    console.log("0:after yield");
},
function *(next){
    console.log("1:before yield");
    yield 2;
    //yield next; 
    console.log("1:after yield");
    yield 3;
},
function *(next){
    console.log("2:before yield");
    yield 4;
    yield next;
    yield 5;
    console.log("2:after yield");
}
];

结果

0:before yield
0
1:before yield
2
1:after yield
3
1
0:after yield

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