Use of abortcontroller

Today I will introduce a useful JavaScript api AbortController

AbortController What is it?

AbortController Interface represents a controller object , Allows you to abort one or more as needed Web request . You can use AbortController.AbortController() Constructor creates a new AbortController. Use AbortSignal Objects can be completed with DOM Requested communication

This api Simply put, it can provide us with the ability to terminate a fetch request

A termination fetch Requested demo as follows ：

fetchButton.onclick = async () => {
  const controller = new AbortController();

  //  Click on abort button Implementation termination fetch request 
  abortButton.onclick = () => controller.abort();

  try {
    const r = await fetch('/json', { signal: controller.signal });
    const json = await r.json();
  } catch (e) {
  //  If fetch If the request is terminated, an AbortError Error of 
    const isUserAbort = (e.name === 'AbortError');
  }
};

Copy

After early termination, this request is made in network In the panel status Is shown as canceled

In the absence of AbortController This api Before , We can't let the browser terminate a request in advance . And with this api after , The browser can terminate the request in advance, thus saving some user bandwidth . besides , This api It can also bring us some new development models

Controller and Signal

The following instantiates a AbortController, its signal Attribute is a AbortSignal

const controller = new AbortController();
const { signal } = controller;

Copy

• controller It can be done by controller.abort() To terminate its corresponding signal
• signal Itself cannot be terminated directly . It can be passed to some function calls such as fetch Or directly monitor signal The state of change （ Can pass signal.aborted see signal The state of or monitoring it abort event ）

The actual use

Termination in normal objects

Some old DOM api It is not supported AbortSignal. for example WebScocket Only one is provided close Method to close when we don't need it . If you want to use AbortSignal Can be similar to the following packaging

function abortableSocket(url, signal) {
  const w = new WebSocket(url);

  if (signal.aborted) {
    w.close();  // signal Close immediately after the termination websocket
  }
  signal.addEventListener('abort', () => w.close());

  return w;
}

Copy

This is also very simple to use , But it's important to note that if signal If it has been terminated, it will not trigger abort event , We need to make a judgment first signal Has terminated

Remove event monitoring

We often need to be in js In dealing with dom Monitoring and unloading of . However, the following example will not take effect when the event listening and unloading incoming functions are not the same reference

window.addEventListener('resize', () => doSomething());

//  Will not enter into force 
window.removeEventListener('resize', () => doSomething());

Copy

Therefore, we often need some additional code to maintain the consistency of the callback function reference . And with AbortSignal Then we can have a new way to realize

const controller = new AbortController();
const { signal } = controller;

window.addEventListener('resize', () => doSomething(), { signal });

controller.abort();

Copy

because addEventListener Can also receive signal Attribute . We only need to call controller.abort(), This controller Of signal The related event listener passed will be unloaded automatically

Constructor mode

stay JavaScript We may need to manage very complex lifecycles in objects , Such as WebSocket. We need to start and then execute a series of logic and then terminate . Maybe we will write the following code

const someObject = new SomeObject();
someObject.start();

//  After performing some operations 
someObject.stop();

Copy

It can also be done through AbortSignal To implement

const controller = new AbortController();
const { signal } = controller;

const someObject = new someObject(signal);

//  After performing some operations 
controller.abort();

Copy

1. This makes it very clear that this object can only be executed once , Only from the beginning to the end , But not vice versa . If you want to use it again after it is terminated, you need to create an object again
2. You can share one in many places signal. We don't need to hold multiple SomeObject Example . Just call controller.abort(), these SomeObject All instances of can be terminated
3. If SomeObject There are also internal calls like fetch And so on api Just put this signal Keep delivering , be fetch Can also be terminated together

Here is an example . Shows two signal Usage of . Pass to built-in apifetch And inspection signal State to perform some actions

export class SomeObject {
  constructor(signal) {
    this.signal = signal;

    //  Perform some operations, such as sending a request 
    const p = fetch('/json', { signal });
  }

  doComplexOperation() {
    if (this.signal.aborted) {
      throw new Error(`thing stopped`);
    }
    for (let i = 0; i < 1_000_000; ++i) {
      //  Perform complex operations 
    }
  }
}

Copy

react hook Asynchronous calls in

We usually go to useEffect Some asynchronism in api call . With the help of signal Next time useEffect Call again api Will terminate the previous call

function FooComponent({ something }) {
  useEffect(() => {
    const controller = new AbortController();
    const { signal } = controller;

    const p = (async () => {
      const j = await fetch(url + something, { signal });
    })();

    return () => controller.abort();
  }, [something]);

  return <>...<>;
}

Copy

You can also encapsulate a useEffectAsync Of hook

function useEffectAsync(cb,dependence) {
   const controller = new AbortController();
   const { signal } = controller;
   useEffect(() => {
     cb(signal);
     return () => controller.abort();
   },dependence)
}

Copy

Some useful AbortSignal Method

These methods may not be implemented yet

• AbortSignal.timeout(ms): Create a that terminates after a given time AbortSignal

function abortTimeout(ms) {
  const controller = new AbortController();
  setTimeout(() => controller.abort(), ms);
  return controller.signal;
}

Copy

• AbortSignal.any(signals)： Create a AbortSignal, If any of the incoming signal It's over , This one returns signal It will also be terminated

function abortAny(signals) {
  const controller = new AbortController();
  signals.forEach((signal) => {
    if (signal.aborted) {
      controller.abort();
    } else {
      signal.addEventListener('abort', () => controller.abort());
    }
  });
  return controller.signal;
}

Copy

• AbortSignal.throwIfAborted()： If signal Itself has ended , Calling this method will throw an execution abort(reason) When the specified reason abnormal ; Otherwise, it will be executed silently

  if (signal.aborted) {
    throw new Error(...);
  }
  // becomes
  signal.throwIfAborted();

Copy

This method is not easy at present polyfill, But it can be realized through the following tool functions

function throwIfSignalAborted(signal) {
  if (signal.aborted) {
    throw new Error(...);
  }
}

Copy

Reference resources

https://whistlr.info/2022/abortcontroller-is-your-friend/