Interrupt operation: abortcontroller learning notes

Front end interview usually likes to ask ：

• Please cancel the handwriting function with a delay ,axios What is the principle of the cancel function ?

• How to interrupt a request fetch Principle analysis and application of ？

In my opinion 《 Use AbortController End fetch request 》, I thought it was very detailed , So refine your notes ：

AbortController Background introduction

In the current browser , There are two main ways to send a request ：XMLHttpRequest and fetch.XMLHttpRequest This interface has existed in browsers for a long time ,fetch It is ES2015 Features introduced .

XMLHttpRequest Can be terminated halfway through the request （abortable）. for instance

let xhr = new XMLHttpRequest();
xhr.method = 'GET';
xhr.url = 'https://slowmo.glitch.me/5000';
xhr.open(method, url, true);
xhr.send();

// Abort the request at a later stage
abortButton.addEventListener('click', function() {
  xhr.abort();
});

fetch Termination requests are not supported at the beginning of the import .Github The first time stay  2015 It ends in fetch The proposal requested issue  appear . stay fetch There are many solutions to this problem beyond the specification , image  cancelable-promises  And others  hacks.

finally , General purpose AbortController and AbortSignal API Come out . The API stay  DOM standard   In the definition of , Not defined in the language specification .

What is? AbortController

AbortController It's a DOM API.MDN The introduction to it is AbortController Interface represents a controller object , Allows one or more... To be terminated as needed Web request .

AbortController Can be used in fetch and addEventListener, Used to discard requests and listeners respectively .

See the official website for details ：

https://developer.mozilla.org/zh-CN/docs/Web/API/AbortController/AbortController

"AbortController" | Can I use... Support tables for HTML5, CSS3, etc

DOM file   There is a passage in ：

although Promise No built-in termination algorithm is provided （aborting mechanism）, But many use them API Need to terminate semantics .AbortController Provide a abort() Methods to support these requirements , This method is used to switch the corresponding AbortSignal Object state . Those who want to support the termination function API Acceptable AbortSignal object , And determine the execution process based on its status .

AbortController It consists of two parts ：abort-signal and abort-controller, The architecture is as follows ：

The red part is what we need to focus on , Because they will be directly reflected in practical application

AbortSignal Source code interpretation

AbortSignal The source code parsing

...
export default class AbortSignal extends EventTarget<Events, EventAttributes> {
    /**
     *  from abortedFlags Get the current AbortSignal example aborted state 
     */
    public get aborted(): boolean {
        const aborted = abortedFlags.get(this)
        if (typeof aborted !== "boolean") {
            throw new TypeError(
                `Expected 'this' to be an 'AbortSignal' object, but got ${
                    this === null ? "null" : typeof this
                }`,
            )
        }
        return aborted
    }
}
//  Set up abort Custom events 
defineEventAttribute(AbortSignal.prototype, "abort")

...

/**
 *  Create a AbortSinal example , And set up aborted Status as false, Deposit in abortedFlags in , Simultaneous binding abort Event properties 
 */
export function createAbortSignal(): AbortSignal {
    const signal = Object.create(AbortSignal.prototype)
    EventTarget.call(signal)
    abortedFlags.set(signal, false)
    return signal
}

/**
 *  Set up AbortSinal example aborted Status as true, Trigger at the same time abort Listen for event callbacks 
 */
export function abortSignal(signal: AbortSignal): void {
    if (abortedFlags.get(signal) !== false) {
        return
    }

    abortedFlags.set(signal, true)
    signal.dispatchEvent<"abort">({ type: "abort" })
}
...

1. AbortSignal Inherit EventTarget（ Third party dependency package , The function is to give instances to listen to custom events , It will help you solve compatibility problems addEventListener/onXX/dispatchEvent）,AbortSignal The custom abort Monitoring events

2. AbortSignal.aborted()： Get whether the current instance has been started abort Monitoring events .

3. abortedFlags：map type , Whether the for storing each instance has been started abort Monitoring events , The default is false（createAbortSignal Set when creating an instance ）, call abortSignal Function will be set to true

4. createAbortSignal()： Building functions , The initialization instance object is false, binding abort Monitoring events （ You need to set it yourself abort Listen for callback events ）

5. abortSignal(instance)： Set the current instance state to ture, Trigger at the same time abort Listen for callback events

AbortController The source code parsing

...
export default class AbortController {
    /**
     *  Constructors , Create a AbortSignal Instance and save it to signals in 
     */
    public constructor() {
        signals.set(this, createAbortSignal())
    }

    /**
     *  from signals Get the current AbortSignal example 
     */
    public get signal(): AbortSignal {
        return getSignal(this)
    }

    /**
     *  First from signals Get the current AbortSignal example , Then set the instance aborted Status as true, Trigger abort Listen for callback events 
     */
    public abort(): void {
        abortSignal(getSignal(this))
    }
}
...

1. AbortController Constructor will call createAbortSignal establish AbortSignal Instance and save it into a Map Type of signals in .

2. abort() Method will call abortSignal function , The parameter passed in is from signals Out of AbortSignal example

3. signal() The function of the method is from signals Remove from AbortSignal example

Reference source code ：

1. EventTarget Source portal

2. AbortController Source portal

3. fetch Source portal

AbortController addEventListener

as everyone knows , if necessary removeEventListenr(type, callback), its callback It has to be with addEventListener Is the same function reference , In some business scenarios , We don't want to write more functions. We can use signal To control .

for example , Set a listener when clicking the mouse , Monitor the mouse movement in the listener , Release the mouse to close the listener ：

  document.addEventListener('mousedown', callback);
  document.addEventListener('mouseup', callback2);
  function callback (e) {
      document.addEventListener('mousemove',  callback3);
   }

  function callback2 (e) {
     document.removeEventListener('mousemove', callback3);
  }
  
  function callback3(event) {}

If it is rewritten into AbortController How to write it? ？

    const controller = new AbortController();
    function callback (e) {
      document.addEventListener('mousemove',  (e) => {
          
      },{
           signal: controller.signal  
      });
   }
    document.addEventListener('mousedown', callback);
    document.addEventListener('mouseup', controller.abort);

Advanced

Each request , Will recreate a AbortSignal Instance ？

answer ： Yes

signals and abortedFlags All are Map type , Each request creates an instance , As time goes on and requests increase , How to prevent cache avalanche ？

answer ：signals and abortedFlags Exactly WeakMap type , and WeakMap Follow Map It's going to make a difference ,WeakMap The key of can only be a reference to an object , When the garbage collection mechanism is executed , Will detect WeakMap Whether the key of is referenced , If not quoted , The key pair will be deleted , And automatically recycle , To prevent cache avalanche problems .

AbortSignal How to have the ability to listen to events ？

answer ： It doesn't have event handling capabilities by itself , It inherits a EventTarget Class to enable it to listen and handle events

Reference article ：

An interruptible request fetch Principle analysis and application of   An interruptible request fetch Principle analysis and application of - You know

 [ translate ] Use AbortController End fetch request  [ translate ] Use AbortController End fetch request - Nuggets

An interruptible request fetch Principle analysis and application of （ Previous notes ） https://github.com/ctq123/blogs/issues/9

AbortController Use scenarios to explore  AbortController Use scenarios to explore - Simple books

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