最近需要單獨使用到koa-compose這個模組,雖然使用koa的時候大致知道中介軟體的執行流程,但是沒仔細研究過原始碼用起來還是不放心(主要是這個模組程式碼少,多的話也沒興趣去研究了)。
koa-compose看起來程式碼少,但是確實繞。閉包,遞迴,Promise。。。看了一遍腦子裡繞不清楚。看了網上幾篇解讀文章,都是針對單行程式碼做解釋,還是繞不清楚。最後只好採取一種傻瓜的方式:
koa-compose去掉一些註釋,型別校驗後,原始碼如下:
function compose (middleware) {
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)
}
}
}
}
複製程式碼
寫出如下程式碼:
var index = -1;
function compose() {
return dispatch(0)
}
function dispatch (i) {
if (i <= index) return Promise.reject(new Error('next() called multiple times'))
index = i
var fn = middleware[i]
if (i === middleware.length) fn = next
if (!fn) return Promise.resolve('fn is undefined')
try {
return Promise.resolve(fn(context, dispatch.bind(null, i + 1)));
} catch (err) {
return Promise.reject(err)
}
}
function f1(context,next){
console.log('middleware 1');
next().then(data=>console.log(data));
console.log('middleware 1');
return 'middleware 1 return';
}
function f2(context,next){
console.log('middleware 2');
next().then(data=>console.log(data));
console.log('middleware 2');
return 'middleware 2 return';
}
function f3(context,next){
console.log('middleware 3');
next().then(data=>console.log(data));
console.log('middleware 3');
return 'middleware 3 return';
}
var middleware=[
f1,f2,f3
]
var context={};
var next=function(context,next){
console.log('middleware 4');
next().then(data=>console.log(data));
console.log('middleware 4');
return 'middleware 4 return';
};
compose().then(data=>console.log(data));
複製程式碼
直接執行結果如下:
"middleware 1"
"middleware 2"
"middleware 3"
"middleware 4"
"middleware 4"
"middleware 3"
"middleware 2"
"middleware 1"
"fn is undefined"
"middleware 4 return"
"middleware 3 return"
"middleware 2 return"
"middleware 1 return"
按著程式碼執行流程一步步分析:
dispatch(0)
i==0,index==-1 i>index 往下
index=0
fn=f1
Promise.resolve(f1(context, dispatch.bind(null, 0 + 1)))
這就會執行
f1(context, dispatch.bind(null, 0 + 1))
進入到f1執行上下文
console.log('middleware 1');
輸出middleware 1
next()
其實就是呼叫
dispatch(1)
bind的功勞
遞迴開始
dispatch(1)
i==1,index==0 i>index 往下
index=1
fn=f2
Promise.resolve(f2(context, dispatch.bind(null, 1 + 1)))
這就會執行
f2(context, dispatch.bind(null, 1 + 1))
進入到f2執行上下文
console.log('middleware 2');
輸出middleware 2
next()
其實就是呼叫
dispatch(2)
接著遞迴
dispatch(2)
i==2,index==1 i>index 往下
index=2
fn=f3
Promise.resolve(f3(context, dispatch.bind(null, 2 + 1)))
這就會執行
f3(context, dispatch.bind(null, 2 + 1))
進入到f3執行上下文
console.log('middleware 3');
輸出middleware 3
next()
其實就是呼叫
dispatch(3)
接著遞迴
dispatch(3)
i==3,index==2 i>index 往下
index=3
i === middleware.length
fn=next
Promise.resolve(next(context, dispatch.bind(null, 3 + 1)))
這就會執行
next(context, dispatch.bind(null, 3 + 1))
進入到next執行上下文
console.log('middleware 4');
輸出middleware 4
next()
其實就是呼叫
dispatch(4)
接著遞迴
dispatch(4)
i==4,index==3 i>index 往下
index=4
fn=middleware[4]
fn=undefined
reuturn Promise.resolve('fn is undefined')
回到next執行上下文
console.log('middleware 4');
輸出middleware 4
return 'middleware 4 return'
Promise.resolve('middleware 4 return')
回到f3執行上下文
console.log('middleware 3');
輸出middleware 3
return 'middleware 3 return'
Promise.resolve('middleware 3 return')
回到f2執行上下文
console.log('middleware 2');
輸出middleware 2
return 'middleware 2 return'
Promise.resolve('middleware 2 return')
回到f1執行上下文
console.log('middleware 1');
輸出middleware 1
return 'middleware 1 return'
Promise.resolve('middleware 1 return')
回到全域性上下文
至此已經輸出
"middleware 1"
"middleware 2"
"middleware 3"
"middleware 4"
"middleware 4"
"middleware 3"
"middleware 2"
"middleware 1"
那麼
"fn is undefined"
"middleware 4 return"
"middleware 3 return"
"middleware 2 return"
"middleware 1 return"
怎麼來的呢
回頭看一下,每個中介軟體裡都有
next().then(data=>console.log(data));
按照之前的分析,then裡最先拿到結果的應該是next中介軟體的,而且結果就是Promise.resolve('fn is undefined')
的結果,然後分別是f4,f3,f2,f1。那麼為什麼都是最後才輸出呢?
Promise.resolve('fn is undefined').then(data=>console.log(data));
console.log('middleware 4');
複製程式碼
執行一下就清楚了
或者
setTimeout(()=>console.log('fn is undefined'),0);
console.log('middleware 4');
複製程式碼
整個呼叫過程還可以看成是這樣的:
function composeDetail(){
return Promise.resolve(
f1(context,function(){
return Promise.resolve(
f2(context,function(){
return Promise.resolve(
f3(context,function(){
return Promise.resolve(
next(context,function(){
return Promise.resolve('fn is undefined')
})
)
})
)
})
)
})
)
}
composeDetail().then(data=>console.log(data));
複製程式碼
方法雖蠢,但是compose的作用不言而喻了
最後,if (i <= index) return Promise.reject(new Error('next() called multiple times'))
這句程式碼何時回其作用呢?
一箇中介軟體裡呼叫兩次next()
,按照上面的套路走,相信很快就明白了。