在須要多個操做的時候,會致使多個回調函數嵌套,致使代碼不夠直觀,就是常說的回調地獄,一般經過promise來解決數組
Promise本意是承諾,在程序中的意思就是承諾我過一段時間後會給你一個結果。 何時會用到過一段時間?答案是異步操做,異步是指可能比較長時間纔有結果的才作,例如網絡請求、讀取本地文件等promise
構造函數初始化邏輯緩存
const PENDING = 'pending';//初始態
const FULFILLED = 'fulfilled';//初始態
const REJECTED = 'rejected';//初始態
let self = this;//先緩存當前promise實例
self.status = PENDING;//設置狀態
self.onResolvedCallbacks = [];//定義存放成功的回調的數組
self.onRejectedCallbacks = []; //定義存放失敗回調的數組
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executor執行器,包含兩個參數,分別是resolve 解決和reject 拒絕,new Promise這個executor就會執行bash
Promise有三個狀態:初始化狀態爲pending,成功狀態爲fulfilled,失敗狀態rejected,若是代碼一旦成功就不會走向失敗,若 一直pending 永遠不給你明確的答覆網絡
當調用如下方法的時候,若是promise狀態爲pending的話能夠轉成成功態,若是已是成功態或者失敗態了,則什麼都不作異步
function resolve(value){
if(value!=null &&value.then&&typeof value.then == 'function'){
return value.then(resolve,reject);
}
setTimeout(function(){
if(self.status == PENDING){
self.status = FULFILLED;
self.value = value;
self.onResolvedCallbacks.forEach(cb=>cb(self.value));
}
})
}
function reject(reason){ //2.1.2
setTimeout(function(){
if(self.status == PENDING){
self.status = REJECTED;
self.value = reason;
self.onRejectedCallbacks.forEach(cb=>cb(self.value));
}
});
}
}
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由於此函數執行可能會異常,因此須要捕獲,若是出錯了,須要用錯誤對象reject,若是這函數執行失敗了,則用失敗的緣由reject這個promise,須要用try...catch(e)...進行處理函數
try{
executor(resolve,reject);
}catch(e){
reject(e);
};
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Promise的解析過程ui
function resolvePromise(promise2,x,resolve,reject){
if(promise2 === x){
return reject(new TypeError('循環引用'));
}
let called = false;
if(x instanceof Promise){
if(x.status == PENDING){
x.then(function(y){
resolvePromise(promise2,y,resolve,reject);
},reject);
}else{
x.then(resolve,reject);
}
}else if(x!= null &&((typeof x=='object')||(typeof x == 'function'))){
try{
let then = x.then;
if(typeof then == 'function'){
then.call(x,function(y){
if(called)return;
called = true;
resolvePromise(promise2,y,resolve,reject)
},function(err){
if(called)return;
called = true;
reject(err);
});
}else{
resolve(x);
}
}catch(e){
if(called)return;
called = true;
reject(e);
}
}else{
resolve(x);
}
}
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then
方法就是用來指定Promise 對象的狀態改變時肯定執行的操做,resolve 時執行第一個函數(onFulfilled),reject 時執行第二個函數(onRejected) 此方法中,若是成功和失敗的回調沒有傳,則表示這個then沒有任何邏輯,只會把值日後拋this
Promise.prototype.then = function(onFulfilled,onRejected){
onFulfilled = typeof onFulfilled == 'function'?onFulfilled:function(value){return value};
onRejected = typeof onRejected == 'function'?onRejected:reason=>{throw reason};
let self = this;
let promise2;
if(self.status == FULFILLED){
return promise2 = new Promise(function(resolve,reject){
setTimeout(function(){
try{
let x =onFulfilled(self.value);
resolvePromise(promise2,x,resolve,reject);
}catch(e){
reject(e);
}
})
});
}
if(self.status == REJECTED){
return promise2 = new Promise(function(resolve,reject){
setTimeout(function(){
try{
let x =onRejected(self.value);
resolvePromise(promise2,x,resolve,reject);
}catch(e){
reject(e);
}
})
});
}
if(self.status == PENDING){
return promise2 = new Promise(function(resolve,reject){
self.onResolvedCallbacks.push(function(){
try{
let x =onFulfilled(self.value);
resolvePromise(promise2,x,resolve,reject);
}catch(e){
reject(e);
}
});
self.onRejectedCallbacks.push(function(){
try{
let x =onRejected(self.value);
resolvePromise(promise2,x,resolve,reject);
}catch(e){
reject(e);
}
});
});
}
}
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promise的鏈式調用
spa
then
可使用鏈式調用的寫法緣由在於,每一次執行該方法時老是會返回一個Promise
對象
catch
只是 promise.then(undefined, onRejected); 方法的一個別名而已。 也就是說,這個方法用來註冊當promise對象狀態變爲Rejected時的回調函數 catch原理就是隻傳失敗的回調
Promise.prototype.catch = function(onRejected){
this.then(null,onRejected);
}
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Promise.all
接收一個 promise對象的數組做爲參數,當這個數組裏的全部promise對象所有變爲resolve或reject狀態的時候,它纔會去調用 .then 方法
Promise.all = function(promises){
return new Promise(function(resolve,reject){
let done = gen(promises.length,resolve);
for(let i=0;i<promises.length;i++){
promises[i].then(function(data){
done(i,data);
},reject);
}
});
}
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Promise.race
只要有一個promise對象進入 FulFilled 或者 Rejected 狀態的話,就會繼續進行後面的處理
Promise.race = function(promises){
return new Promise(function(resolve,reject){
for(let i=0;i<promises.length;i++){
promises[i].then(resolve,reject);
}
});
}
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別人提供 給你一個方法,須要你傳入一個promise,但你只有一個普通的值,你就能夠經過這個方法把這個普通的值(string number object)轉成一個promise對象 返回一個馬上成功的promise
Promise.resolve = function(value){
return new Promise(function(resolve){
resolve(value);
});
}
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返回一個馬上失敗的promise
Promise.reject = function(reason){
return new Promise(function(resolve,reject){
reject(reason);
});
}
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