手擼一個Promise
阿新 • • 發佈:2020-12-08
技術標籤:JavaScriptjs
/*
要做的事情(總結):
1.promise 是一個類 在執行和這個類的時候需要傳遞一個執行器 執行器會立即執行
2.promise 有三種狀態 成功(fulfilled) 失敗(rejected) 等待(pending)
pending==>fulfilled
pending==>rejected 狀態發生改變不能進行更改
3. resolve 和 reject 函式是用來更改狀態的
resolve:成功
reject:失敗
4.then 方法內部做的事情就是判斷狀態 狀態成功呼叫成功回撥 如果失敗呼叫失敗回撥 (被定義在原型物件)
5、then 成功回撥的引數 表示成功之後的值 then失敗回撥中的引數 表示失敗後的原因
*/
const PENDING = "pending"; //等待
const FULFILLED = "fulfilled"; //成功
const REJECTED = "rejected"; //失敗
class MyPromise {
constructor(executor) {
try {
executor(this.resolve, this.reject);
} catch (error) {
this.reject(error)
}
}
//狀態
status = PENDING; //預設等待
//成功後的值
value = undefined;
//失敗後的原因
reason = undefined;
//成功回撥
successCallback = [];
//失敗回撥
failCallback = [];
resolve = value => {
// 如果狀態不是等待 阻止向下執行
if (this.status !== PENDING) return;
//狀態成功
this. status = FULFILLED;
// 儲存成功後的值
this.value = value;
//判斷成功回撥是否存在 如果存在呼叫
// this.successCallback && this.successCallback(this.value)
while (this.successCallback.length) this.successCallback.shift()()
}
reject = reason => {
if (this.status !== PENDING) return;
//狀態失敗
this.status = REJECTED;
// 儲存失敗後的原因
this.reason = reason;
//判斷失敗回撥是否存在 如果存在呼叫
// this.failCallback && this.failCallback(this.reason)
while (this.failCallback.length) this.failCallback.shift()()
}
then(successCallback, failCallback) {
successCallback = successCallback ? successCallback : value => value;
failCallback = failCallback ? failCallback : reason => { throw reason }
let promise1 = new MyPromise((resolve, reject) => {
// 判斷狀態
if (this.status === FULFILLED) { //成功
setTimeout(() => {
try {
let x = successCallback(this.value)
//判斷x 是普通值還是promise物件
// 如果是普通值 直接呼叫resolve
// 如果是promise物件檢視物件返回結果
// 再根據promise物件返回的結果決定是呼叫resolve還是reject
resolvePromise(promise1, x, resolve, reject);
} catch (error) {
reject(error)
}
}, 0)
} else if (this.status === REJECTED) { //失敗
setTimeout(() => {
try {
let x = failCallback(this.reason)
//判斷x 是普通值還是promise物件
// 如果是普通值 直接呼叫resolve
// 如果是promise物件檢視物件返回結果
// 再根據promise物件返回的結果決定是呼叫resolve還是reject
resolvePromise(promise1, x, resolve, reject);
} catch (error) {
reject(error)
}
}, 0)
} else {
//等待
//儲存成功回撥 失敗回撥
this.successCallback.push(() => {
setTimeout(() => {
try {
let x = successCallback(this.value)
//判斷x 是普通值還是promise物件
// 如果是普通值 直接呼叫resolve
// 如果是promise物件檢視物件返回結果
// 再根據promise物件返回的結果決定是呼叫resolve還是reject
resolvePromise(promise1, x, resolve, reject);
} catch (error) {
reject(error)
}
}, 0)
});
this.failCallback.push(() => {
setTimeout(() => {
try {
let x = failCallback(this.reason)
//判斷x 是普通值還是promise物件
// 如果是普通值 直接呼叫resolve
// 如果是promise物件檢視物件返回結果
// 再根據promise物件返回的結果決定是呼叫resolve還是reject
resolvePromise(promise1, x, resolve, reject);
} catch (error) {
reject(error)
}
}, 0)
});
}
})
return promise1;
}
finally(callback){
return this.then(value=>{
return MyPromise.resolve(callback()).then(()=>value)
},reason=>{
return MyPromise.resolve(callback()).then(()=>{throw reason})
})
}
catch(failCallback){
return this.then(undefined,failCallback);
}
static all(array){
let result = [];
let index = 0;
return new MyPromise((resolve,reject)=>{
function addData(key,value){
result[key] = value;
index++;
if(index==array.lengt){
resolve(result)
}
}
for(let i = 0;i<array.length;i++){
let current = array[i]
if(current instanceof MyPromise){
//promise
current.then(value=>addData(i,value),reason=>reject(reason))
}else{
//普通值
addData(i,array[i])
}
}
})
}
static resolve(value){
if(value instanceof MyPromise) return value;
return new MyPromise(resolve=>resolve(value));
}
}
function resolvePromise(promise1, x, resolve, reject) {
if (promise1 === x) {
return reject(new TypeError("promise不能迴圈呼叫自己"))
}
if (x instanceof MyPromise) {
//是promise物件
// x.then(value=>resolve(value),reason=>reject(reason))
x.then(resolve, reject);
} else {
//是普通值
resolve(x);
}
}
module.exports = MyPromise;