Callback function of JavaScript asynchronous operation

Ten years in the world 2021-01-21 16:38:46
callback function javascript asynchronous operation


This paper attempts to stand at the beginning of learning asynchronous From a programming point of view to explain what is Callback function .

Synchronous and asynchronous

Introducing Callback function Before , Let's start with two concepts Sync and asynchronous .

Sync Behavior usually refers to the sequence of code execution from top to bottom , The following code is always executed after the previous code is executed .

Sync Examples of operations are as follows :

let a, b;
function foo() {
a = 1;
}
foo();
b = a + 1;
console.log(b); //2

Because the code is executed in sequence , Call the function first foo();, After execution b = a + 1;, So in the implementation b = a + 1; when ,a The value of must have been determined as 1 了 , Ready to use , Variable b The final result is 2.

asynchronous Behavior means that the code is not executed in order , The following code is not always executed after the previous code is executed .

asynchronous Examples of operations are as follows :

let a, b;
function foo() {
a = 1;
}
setTimeout(foo, 1000); //1 Call... In seconds foo()
b = a + 1;
console.log(b); //NaN

There is no explicit call foo(), Instead, change the function name foo Pass to setTimeout,JavaScript Running at 1 It will be called automatically in seconds foo().

First, the code is still executed in sequence , When executed setTimeout(foo, 1000); when ,JavaScript The main thread finds that this is a task to be executed asynchronously , Will be foo() Put in Task queue Then proceed with the following synchronization code b = a + 1;, When b = a + 1; completion of enforcement , All synchronization code is executed , here JavaScript The main thread goes to Task queue Take out the tasks that need to be performed to perform , That is to say 1 Seconds later foo().

because b = a + 1; Precede foo() perform , So after this asynchronous operation , Variable b The final result is NaN.

Callback function

It introduces Sync and asynchronous The concept of , Let's see what it is Callback function .

stay asynchronous In the operation example ,setTimeout(foo, 1000); In this code foo It can be called Callback function . So-called Callback function , Is put into by the main thread Task queue The code in , This code is usually in function units , And wait until all Sync The code will not be executed until it is executed .

So the top asynchronous Operation example program if you want to get and Sync Operate the same result , It's going to have to be like this :

let a, b;
function foo() {
a = 1;
b = a + 1;
console.log(b); //2
}
setTimeout(foo, 1000); //1 Call... In seconds foo()

Because calculation b The value of depends on a Value , and a = 1; Is in Callback function Implemented in , That is to say, all Sync The code will not be executed until it is executed Callback function Medium asynchronous Code , therefore b = a + 1; Also move to Callback function in . Callback function The code in is also executed sequentially , therefore b = a + 1; The sentence should be put in a = 1; After statement . So you can make variables b The final result is 2.

Callback function application

stay JavaScript Programming ,AJAX Operation can be said to be very common asynchronous The operation scene has changed . If you don't understand AJAX The concept of , Then I strongly suggest you learn .

Here is a use of jQuery send out AJAX Examples of requests :

function handleSuccess(res) {
console.log(res);
}
function handleError(xhr) {
console.log(xhr.status);
}
$.ajax({
type: 'GET',
url: 'http://localhost:3000/',
success: handleSuccess,
error: handleError
});

among success The registration request was successful Callback function handleSuccess,error The registration request failed Callback function handleError.

When AJAX The request succeeded and received a response ,JavaScript The runtime will automatically call handleSuccess, If the request fails, call handleError.

Use here Callback function Purpose , Just for asynchronous Processing request results . Because send AJAX Request is a time-consuming operation , If Sync Handle , It will greatly reduce JavaScript Operational efficiency . When AJAX After the request is sent , We don't know when to get a response , So using asynchronous The way you program , Register the code that processes the response as Callback function .

You can find , Callback function and asynchronous Interwovenness , Callback function The concept of "I" will only appear in asynchronous Programming , Sync There is no need for programming Callback function Of .

P.S.

Right JavaScript asynchronous The description of execution mechanism is not accurate , To understand JavaScript asynchronous The implementation mechanism must learn Event Loop. This article does not want to introduce too many complex concepts , I just want beginners to learn asynchronous Programmers can quickly understand what Callback function .

If you are one JavaScript developer , So that's the end of this article . If you happen to be one, too Python developer , You can continue to watch .

Yes, I have Python For reptiles , I must have heard of Scrapy This very famous crawler framework , You may be in Scrapy Project spider I've seen the following similar code in the file :

class DoubanSpider(scrapy.Spider):
name = 'douban'
allowed_domains = ['movie.douban.com']
def start_requests(self):
for i in range(0, 10):
url = f'https://movie.douban.com/top250?start={i * 25}'
yield scrapy.Request(url=url, callback=self.parse)

among callback=self.parse Will be self.parse register as Callback function . because Scrapy It's a asynchronous The crawler frame , When scrapy.Request After the request is made , The program doesn't block , And when the request succeeds and gets a response ,Scrapy Will automatically call our registered Callback function self.parse, This is related to jQuery Of AJAX cut from the same cloth .

In use Python When programming , I spend most of my time writing Sync Code , So many beginners in the first contact asynchronous I'm not used to programming . When you get confused in learning a programming language , Maybe inspiration can be found in other programming languages .

The starting address : https://jianghushinian.cn/
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