关于promise一些总结

用法

function loadData () {
return new Promise(resolve => {
//执行一些操作
resolve();
});
}

loadData().then(() => {
//执行完以后的操作
//可能再返回一个promise
})

源码分析

function Promise (fn) {
var state = pendding, //状态，分为pendding , fullfilled, rejected
callbacks = [],
value; //返回的参数值

//定义then 方法
this.then = function () {
//链式结构
return new Promise (
handel({
onFullFilled: onFullFilled || null,
onRejected: onRejected || null,
resolve: resolve,
reject: reject
})
）
}
//处理函数, 处理不同状态时处理
function handel (callback) {
if (state === ‘pedding’) {
callbacks.push(callback);
return;
}

var handCb = state === 'fullfilled' ? callback.onFullFilled : callback.onRejected;

if (handCb === null) {
handCb = state === ‘fullfilled’ ? callback.resolve: callback.onRejected;
handCb (value);
return;
}
try ｛
let tempRet = handCb(value);
resolve(tempRet );
｝ catch (e) {
reject(e);
}
}

//resolve函数变状态
function resolve (value){
if (value && ( isObject(value) || isPromise(value))){
var then = value.then;
if (isPromise(then){
then.call(value, resolve, reject);
}
}

value = value;
state = 'fullfilled';
excute();

}

function isObject (string){
return Object.prototype.toString.call(string) === ‘[ object object]’;
}

function isPromise (string) {
return Object.prototype.toString(string) === ‘[object promise]’;
}

//reject函数变状态
function reject (reason){
value =reason;
state = ‘rejected’;
excute();
}

fucntion excute () {
setTimeout(() => {
callbacks.forEach(callbak => {
handle(callback);
})
}, 0)
}

fn(resolve);
}