The usage of async and await in Vue

Ichiro Qiu Shan 2020-11-13 02:57:22
usage async await vue


But first, let me say async Usage of , It's put in front of the function as a keyword , Used to indicate that a function is an asynchronous function , because async It means asynchronous , Asynchronous function means that the execution of the function will not block the execution of the following code . Write a async function

async function timeout() {

return 'hello world';
}

The grammar is very simple , It's just adding... Before the function async keyword , To show that it's asynchronous , So how do we call it ?async Functions are also functions , We usually use functions as we like , Just call it with parentheses , To show that it doesn't block the execution of the code behind it , We are async Add a sentence after the function call console.log;

async function timeout() {

return 'hello world'
}
timeout();
console.log(' Although in the back , But I'll do it first ');

Open the browser console , We see that
 Insert picture description here
async function timeout Called , But there's no output , It's not supposed to return ‘hello world’, Don't worry , Have a look timeout() What does the execution return ? Put the above timeout() Change statement to console.log(timeout())

async function timeout() {

return 'hello world'
}
console.log(timeout());
console.log(' Although in the back , But I'll do it first ');

Keep looking at the console
 Insert picture description here
original async The function returns a promise object , If you want to get promise Return value , We should use then Method , Continue to modify the code

async function timeout() {

return 'hello world'
}
timeout().then(result => {

console.log(result);
})
console.log(' Although in the back , But I'll do it first ');

Look at the console
 Insert picture description here
We've got "hello world’, meanwhile timeout There is no blocking in the execution of the following code , and What we said just now is the same .

At this time , You may have noticed in the console Promise There is one resolved, This is a async The implementation principle inside the function . If async Function returns a value , When this function is called , Internally it will call Promise.solve() Method to turn it into a promise Object as return , But if timeout Error thrown inside the function ? Then it will call Promise.reject() Return to one promise object , At this time, make a revision timeout function

async function timeout(flag) {

if (flag) {

return 'hello world'
} else {

throw 'my god, failure'
}
}
console.log(timeout(true)) // call Promise.resolve() return promise object .
console.log(timeout(false)); // call Promise.reject() return promise object .

The console is as follows :
 Insert picture description here
If an error is thrown inside a function , promise The object has a catch Method to capture .

timeout(false).catch(err => {

console.log(err)
})

async The key words are almost the same , Let's think about await keyword ,await It means waiting , So what does it wait for , What's behind it ? In fact, it can be followed by any expression , But we're going to put one back promise The expression of the object . Be careful await Keywords can only be put in async In the function

Now write a function , Let it go back to promise object , The function is to 2s And then multiply the number by 2
// 2s Then return double the value

function doubleAfter2seconds(num) {

return new Promise((resolve, reject) => {

setTimeout(() => {

resolve(2 * num)
}, 2000);
} )
}

Now write another async function , So that you can use await keyword , await What's on the back is back promise An expression of an object , So it can be written after it doubleAfter2seconds Function call

async function testResult() {

let result = await doubleAfter2seconds(30);
console.log(result);
}

testResult();
Open console ,2s after , Output 60.

Now let's look at the execution of the code , call testResult function , It encountered await, await Wait a minute , The code pauses here , No more downward execution , What is it waiting for ? Wait for the next one promise Finished executing object , And then to get promise resolve And return , After getting the return value , It continues to go down . Specific to the Our code , encounter await after , The code is suspended , wait for doubleAfter2seconds(30) completion of enforcement ,doubleAfter2seconds(30) Back to promise Start execution ,2 second after ,promise resolve 了 , And return the value of 60, At this time await To get the return value 60, And then assign it to result, The end of the pause , The code just starts to execute , perform console.log sentence .

Just this one function , We may not see async/await The role of , If we want to calculate 3 The value of the number , Then output the obtained value ?

async function testResult() {

let first = await doubleAfter2seconds(30);
let second = await doubleAfter2seconds(50);
let third = await doubleAfter2seconds(30);
console.log(first + second + third);
}

6 Seconds later , Console output 220, We can see , Writing asynchronous code is like writing synchronous code , There's no more pullback zones .

Be careful :

async and await be based on promise Of . Use async Will always return a promise object . This is important , Remember , This may be a mistake prone place you come across .
In the use of await When we pause the function , Not the whole code .
async and await It's non blocking .
You can still use Promise for example Promise.all().

版权声明
本文为[Ichiro Qiu Shan]所创,转载请带上原文链接,感谢

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