Talking about several methods of handling JavaScript asynchronous operation

Source: Internet
Author: User

This article tags:    javascript native JavaScript advantage javascript asynchronous JS asynchronous Operation callback function TensorFlow REST server Introduction

JS Asynchronous Operation , is already a commonplace topic, the article on this topic casually Google can see a lot of. So why do I have to write this stuff? In recent work, in order to write a relatively complex set of plugins, you need to handle a variety of asynchronous operations. But for the sake of size and compatibility, it is not intended to introduce any pollyfill, even Babel is not allowed to use, which means that it can only be handled in ES5 way. The way to use callbacks is very bad for decoupling, so I found another way to deal with this problem. The problem is done, but also triggered some of their own thinking: the processing of JS asynchronous operation, there are some ways?

First, the callback function

The legendary "Callback Hell" is from the callback function. And the callback function is the most basic method of handling JS asynchronous operation. Let's look at a simple example:

First define three functions:

function fn1 () {  console.log (' Function 1 ')}function fn2 () {  setTimeout (() = {    Console.log (' Function 2 ')  },}function Fn3 () {  Console.log (' function 3 ')}

fn2This can be seen as an asynchronous function that is deferred for 500 milliseconds. Now I want to be able to execute in turn, fn1 fn2 fn3 . To ensure that fn3 in the final execution, we can take it as fn2 a callback function:

function fn2 (f) {  setTimeout (() = {    Console.log (' Function 2 ')    f ()  }, +)}fn2 (FN3)

As can be seen, fn2 and fn3 fully coupled, if there are multiple similar functions, it is likely that fn1(fn2(fn3(fn4(...)))) this situation will occur. Back to the evils of hell I will not repeat, I believe you must have their own understanding of the reader.

Second, event Publish/Subscribe

The Publish/subscribe model is also one of many design patterns, which is exactly how you can handle asynchronous operations fairly gracefully under ES5. What is a publish/subscribe? In the example above, fn1 fn2 fn3 It can be seen as the publisher of an event, and as long as it executes, an event is published. At this point, we can subscribe to the event and process the events in bulk, including their sequencing. Here we add a method for the message subscriber based on the example in the previous section (for simplicity, the code uses the ES6 notation):

Class Asyncfunarr {  constructor (... arr) {    This.funcarr = [... arr]  }  Next () {    CONST FN = This.funcar R.shift ()    if (typeof fn = = = ' function ') fn ()  }  Run () { ()  }}const Asyncfunarr = new ASYNCFU Narr (FN1, fn2, Fn3)

Then, fn1 fn2 within,, fn3 call its next() method:

function fn1 () {  console.log (' Function 1 ') ()}function fn2 () {  setTimeout (() = { C17/>console.log (' function 2 ') ()  },}function Fn3 () {  Console.log (' function 3 ') ()}//output =>//function 1//function 2//function 3

As you can see, the processing order of the functions equals AsyncFunArr the order of the arguments passed in. To AsyncFunArr maintain an array internally, each invocation of the next() method will sequentially roll out an object saved by the array and execute it, which is a common method I use in my actual work.

Third, Promise

Using promise, there is no need to write a single message subscriber function, only the asynchronous function will return a promise. And look at the example:

function fn1 () {  console.log (' Function 1 ')}function fn2 () {  return new Promise ((Resolve, reject) + = {    set Timeout (() = {      Console.log (' Function 2 ')      Resolve ()    }  )})}function Fn3 () {  Console.log (' Function 3 ')}

Similarly, we define three functions, which fn2 are asynchronous functions that return promise, and now we want to execute them sequentially, just as follows:

FN1 () fn2 (). then (() = {fn3 ()})//output =>//function 1//function 2//function 3

There are two major differences between the use of promise and callbacks, the first one is decoupled from each other, fn2 and the fn3 second is that the function is nested revision for chained invocation, both semantically and in a way that is more friendly to the developer.

Iv. Generator

If the use of promise can be linked to the callback, then the generator method can eliminate the large heap of promise feature methods, such as a lot of then() .

function fn1 () {  Console.log (' function 1 ')}function fn2 () {  setTimeout (() = {    Console.log (' function 2 ') ) ()  },}function Fn3 () {  Console.log (' function 3 ')}function* Asyncfunarr (... fn) {  fn[0] ( )  yield fn[1] ()  fn[2] ()}const af = Asyncfunarr (fn1, fn2, Fn3) ()//output =>//function 1//function 2// Function 3

In this example, the generator function asyncFunArr() accepts a list of functions to be executed fn , and the asynchronous function is yield executed. Within an asynchronous function, by activating the next step of the generator function.

So roughly, it looks like the Publish/subscribe pattern is very similar, all by calling a method inside an asynchronous function to tell the subscriber to do the next step. But this approach is still not elegant, for example, if there are multiple asynchronous functions, then the generator function must be rewritten, and at the level of semantics is also a bit less intuitive.

Five, elegant async/await

Using the latest version of node is natively supported async/await , and can be used in older browsers through a variety of pollyfill. So why is it the async/await most elegant way to say it? And look at the code:

function fn1 () {  console.log (' Function 1 ')}function fn2 () {  return new Promise ((Resolve, reject) + = {    set Timeout (() = {      Console.log (' Function 2 ')      Resolve ()    }  )})}function Fn3 () {  Console.log (' Function 3 ')}async function Asyncfunarr () {  fn1 ()  await fn2 ()  fn3 ()}asyncfunarr ()//Output =>// function 1//function 2//function 3

Did you find that when you define an asynchronous function, its content is exactly the same as fn2 when you used promise in the previous article? Then look at the execution function asyncFunArr() , which is very similar in the way it executes and when you use generator.

Asynchronous operations return promise, requiring sequential execution only to await the corresponding function, which is semantically friendly and easy to maintain code-just return to promise and await it. There is no need to maintain internal execution like generator yield .

Vi. the end of the

As an induction and summary, the content of this article is a lot of knowledge points from the experience of others, but added some of their own understanding and experience. Do not seek popular science in others, but seek to accumulate as individuals. I hope readers can put forward the revised comments and learn to progress together!

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