了解TypeScript的async,await,promise，第1篇

参考地址：https://blog.csdn.net/u012863664/article/details/77881921

先来一段Promise代码：

function timeout(ms:number){
    return new Promise((resolveCallback, rejectCallback) =>{
        resolveCallback(8888); ///此处，执行then里面的 第1个 匿名函数
        rejectCallback(7777);///此处，执行then里面的 第2个 匿名函数
    });
}

timeout(100)
.then( (value1)=>{
    console.log(`I am resolveCallback: value1:${value1}`);
}, (err)=>{
    console.log("I am rejectCallback: " + err);
});

then里面第1个参数是一个 匿名回调函数，这个匿名回调函数就对应 Promise 里面，第一个参数 resolveCallbacke.

resolveCallback是什么，取决于 then 里面定义了啥。。。

rejectCallback同理。

话不多说，先上一段代码：

了解一下 async,await 基本语法

async函数内部return语句返回的值，会成为then方法回调函数的参数

async函数内部抛出错误，会导致返回的 Promise 对象变为reject状态，

抛出的错误对象会被catch方法回调函数接收到

function getData(){
    return "syy";
}

console.log(getData());  //syy

async function getData2(){
    return "syy2";
}
console.log(getData2());  // Promise { resolved }
// 
getData2().then(data =>{console.log(data)});  //syy2

interface TEST_TYPE{
    name:string,
    age:number
}
let testA = <TEST_TYPE>{
    name:"henry",
    age:666
}

function changeA(testA:TEST_TYPE)
{
    testA.age = 777;
//    throw new Error("changeA new a error");  
    return testA;////特别注意返回的数据，我们在外部 then里面捕获...包括异常
}

async function testAsync(testA:TEST_TYPE)
{
//    throw new Error("testAsync new a error.")
    let testB = await changeA(testA);
//    testB.age = 333;
    testB.name = "明天要交作业";
    return testB;
//    return testA ;
}

const result1 = testAsync(testA);
result1
.then( retValue=>{
    console.log(retValue);
    console.log(retValue.name);
    console.log(retValue.age);},
    reject=>{
        console.log("reject. It must be some error happen");
        console.log(reject);
    }
).catch(err=>{
    console.log("catch some error.");
    console.log(err);
});

在来个例子，来理解一下 程序是如何运行的

async function sleep(interval:number){
    return new Promise(resolve=>{
      setTimeout(resolve,interval);
    })
    .then(resolve2=>{console.log("finish resolve2.")},
      reject2=>{console.log("reject2")});
}
  
async function one2XInAsync(interval:number=100){
    for(let i=1;i<=interval;i++){
        console.log("one2FiveInAsync:-----------" + i);
        if(i==3)
        {
        throw new Error("i==3 break");
        }
        await sleep(2000);
    }
}
  
one2XInAsync(20)
  .then(resolve3=>{
    console.log("resolve3");
    console.log(resolve3);
  }, reject3=>{
    console.log("reject3");
    console.error(reject3);
});

继续贴一段代码

let testNum = 1+1;
function takeLongTime(){
    return new Promise(resolve =>{
        setTimeout( ()=>{
            resolve("Hello world");
        }, 4000);
    });
}

async function test1(){
    let t1 = await takeLongTime();
    console.log(testNum);
    console.log(t1);
}
test1();

单一的 Promise 链并不能发现 async/await 的优势，但是，如果需要处理由多个 Promise 组成的 then 链的时候，优势就能体现出来了（很有意思，Promise 通过 then 链来解决多层回调的问题，现在又用 async/await 来进一步优化它）。

假设一个业务，分多个步骤完成，每个步骤都是异步的，而且依赖于上一个步骤的结果。我们仍然用 setTimeout 来模拟异步操作：

function takeLongTime(n:number){
    return new Promise(resolve=>{
        setTimeout(()=>resolve(n+1000),n);
    });
}

function step1(n:number){
    console.log(`step1 with ${n}`);
    return takeLongTime(n);
}

function step2(n:number){
    console.log(`step2 with ${n}`);
    return takeLongTime(n);
}

function step3(n:number){
    console.log(`step3 with ${n}`);
    return takeLongTime(n);
}


/**采用Promise方式实现 */
function doIt1() {
    console.time("doIt1");
    const time1 = 300;
    step1(time1)
        .then(time2 => step2(<number>time2))
        .then(time3 => step3(<number>time3))
        .then(result => {
            console.log(`result is ${result}`);
            console.timeEnd("doIt");
        });
}

doIt1();
// step1 with 300
// step2 with 1300
// step3 with 2300
// result is 3300
// doIt: 3908.552001953125ms
// doIt: 3908.640ms



async function doIt2(){
    console.time("doIt2");
    const time1 = 400;
    const time2 = await step1(time1);
    const time3 = await step2(<number>time2);
    const result = await step3(<number>time3);
    console.log(`result is ${result}`);
    console.timeEnd("doIt2");
}

doIt2();
// step1 with 400
// step2 with 1400
// step3 with 2400
// result is 3400
// doIt2: 4211.223876953125ms
// doIt2: 4212.416ms

回顾一下匿名函数

const sleep1 = (timeout:number=2000) =>{console.log("[sleep1] a non name function,with a default params value:" , timeout);};

const sleep2 = (timeout=2000) =>{console.log("[sleep2] a non name function,with a default params value:" , timeout);};

const sleep3 = () =>{
    console.log("[sleep3] a non name function,with no params.");
};
const sleep4 = () =>console.log("[sleep4] a non name function,with no params.");

sleep1();
sleep1(666);
sleep2();
sleep2(777);
sleep3();
sleep4();

[sleep1] a non name function,with a default params value: 2000

[sleep1] a non name function,with a default params value: 666

[sleep2] a non name function,with a default params value: 2000

[sleep2] a non name function,with a default params value: 777

[sleep3] a non name function,with no params.

[sleep4] a non name function,with no params.