- Promise 就是一个类 在执行这个类的时候 需要传递一个执行器进去 执行器会立即执行
- Promise 中有三种状态 分别为 成功 fulfilled 失败 rejected 等待 pending
pending -> fulfilled
pending -> rejected
一旦状态确定就不可更改
- resolve和reject函数是用来更改状态的
resolve: fulfilled
reject: rejected
- then方法内部做的事情就判断状态 如果状态是成功 调用成功的回调函数 如果状态是失败 调用失败回调函数 then方法是被定义在原型对象中的
- then成功回调有一个参数 表示成功之后的值 then失败回调有一个参数 表示失败后的原因
- 同一个promise对象下面的then方法是可以被调用多次的
- then方法是可以被链式调用的, 后面then方法的回调函数拿到值的是上一个then方法的回调函数的返回值
const PENDING = 'pending';
const FULFILLED = 'fulfilled';
const REJECTED = 'rejected';
class MyPromise {
constructor (executor) {
try {
executor(this.resolve, this.reject)
} catch (e) {
this.reject(e);
}
}
status = PENDING;
value = undefined;
reason = undefined;
successCallback = [];
failCallback = [];
resolve = value => {
if (this.status !== PENDING) return;
this.status = FULFILLED;
this.value = value;
while(this.successCallback.length) this.successCallback.shift()()
}
reject = reason => {
if (this.status !== PENDING) return;
this.status = REJECTED;
this.reason = reason;
while(this.failCallback.length) this.failCallback.shift()()
}
then (successCallback, failCallback) {
successCallback = successCallback ? successCallback : value => value;
failCallback = failCallback ? failCallback: reason => { throw reason };
let promsie2 = new MyPromise((resolve, reject) => {
if (this.status === FULFILLED) {
setTimeout(() => {
try {
let x = successCallback(this.value);
resolvePromise(promsie2, x, resolve, reject)
}catch (e) {
reject(e);
}
}, 0)
}else if (this.status === REJECTED) {
setTimeout(() => {
try {
let x = failCallback(this.reason);
resolvePromise(promsie2, x, resolve, reject)
}catch (e) {
reject(e);
}
}, 0)
} else {
this.successCallback.push(() => {
setTimeout(() => {
try {
let x = successCallback(this.value);
resolvePromise(promsie2, x, resolve, reject)
}catch (e) {
reject(e);
}
}, 0)
});
this.failCallback.push(() => {
setTimeout(() => {
try {
let x = failCallback(this.reason);
resolvePromise(promsie2, x, resolve, reject)
}catch (e) {
reject(e);
}
}, 0)
});
}
});
return promsie2;
}
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.length) {
resolve(result);
}
}
for (let i = 0; i < array.length; i++) {
let current = array[i];
if (current instanceof MyPromise) {
current.then(value => addData(i, value), reason => reject(reason))
}else {
addData(i, array[i]);
}
}
})
}
static resolve (value) {
if (value instanceof MyPromise) return value;
return new MyPromise(resolve => resolve(value));
}
}
function resolvePromise (promsie2, x, resolve, reject) {
if (promsie2 === x) {
return reject(new TypeError('Chaining cycle detected for promise #<Promise>'))
}
if (x instanceof MyPromise) {
x.then(resolve, reject);
} else {
resolve(x);
}
}
module.exports = MyPromise;