Remove event listener that has been added using bind(this)

问题

How do I remove the click listener I bound to window in the constructor below? I need it to listen on window, and I need access to the button instance inside it.

class MyEl extends HTMLButtonElement {
  constructor() {
    super();
    this.clickCount = 0;
    window.addEventListener('click', this.clickHandler.bind(this));
  }
  
  clickHandler(e) {
    if (e.target === this) {
      this.textContent = `clicked ${++this.clickCount} times`;
      window.removeEventListener('click', this.clickHandler);
    }
  }
  
  disconnectedCallback() {
      window.removeEventListener('click', this.clickHandler);
  }
}

customElements.define('my-el', MyEl, { extends: 'button' });

<button is="my-el" type="button">Click me</button>

回答1:

It's not possible with your current implementation - every call of .bind creates a new separate function, and you can only call removeEventListener to remove a listener if the passed function is the same (===) as the one passed to addEventListener (just like .includes for arrays, or .has for Sets):

const fn = () => 'foo';
console.log(fn.bind(window) === fn.bind(window));

As a workaround, you could assign the bound function to a property of the instance:

class MyEl extends HTMLButtonElement {
  constructor() {
    super();
    this.clickCount = 0;
    this.boundListener = this.clickHandler.bind(this);
    window.addEventListener('click', this.boundListener);
  }
  
  clickHandler(e) {
    this.textContent = `clicked ${++this.clickCount} times`;
    window.removeEventListener('click', this.boundListener);
  }
}

customElements.define('my-el', MyEl, { extends: 'button' });

<button is="my-el" type="button">Click me</button>

回答2:

Create a wrapper func for your clickHandler like so.

class MyEl extends HTMLButtonElement {
  constructor() {
    super();
    this.clickCount = 0;
    this.wrapper = e => this.clickHandler.apply(this, e);
    window.addEventListener('click', this.wrapper);
  }
  
  clickHandler(e) {
    this.textContent = `clicked ${++this.clickCount} times`;
    
    window.removeEventListener('click', this.wrapper);
  }
}

customElements.define('my-el', MyEl, { extends: 'button' });

<button is="my-el" type="button">Click me</button>

回答3:

Another pattern is to keep your Listener inside the constructor.

To remove an Event Listener (no matter what pattern) you can add a 'remove' function the moment you create an Event Listener.

Since the remove function is called within the listen scope, it uses the same name and function

pseudo code:

  listen(name , func){
    window.addEventListener(name, func);
    return () => window.removeEventListener( name , func );
  }

  let remove = listen( 'click' , () => alert('BOO!') );

  //cleanup:
  remove();

Run Code Snippet below to see it being used with multiple buttons

Events bubbling UP & shadowDOM

to save you an hour once you do more with events...

Note that WebComponents (ie CustomElements with shadowDOM) need CustomEvents with the composed:true property if you want them to bubble up past its shadowDOM boundary

    new CustomEvent("check", {
      bubbles: true,
      //cancelable: false,
      composed: true       // required to break out of shadowDOM
    });

Removing added Event Listeners

Note: this example does not run on Safari, as Apple refuses to implement extending elements : extends HTMLButtonElement

class MyEl extends HTMLButtonElement {
  constructor() {
    let ME = super();// super() retuns this scope; ME makes code easier to read
    let count = 0;// you do not have to stick everything on the Element
    ME.mute = ME.listen('click' , event => {
      //this function is in constructor scope, so has access to ALL its contents
      if(event.target === ME) //because ALL click events will fire!
        ME.textContent = `clicked ${ME.id} ${++count} times`;
      //if you only want to allow N clicks per button you call ME.mute() here
    });
  }

  listen(name , func){
    window.addEventListener( name , func );
    console.log('added' , name , this.id );
    return () => { // return a Function!
      console.log( 'removeEventListener' , name , 'from' , this.id);
      this.style.opacity=.5;
      window.removeEventListener( name , func );
    }
  }
  eol(){ // End of Life
    this.parentNode.removeChild(this);
  }
  disconnectedCallback() {
      console.log('disconnectedCallback');
      this.mute();
  }
}

customElements.define('my-el', MyEl, { extends: 'button' });

button{
  width:12em;
}

<button id="One" is="my-el" type="button">Click me</button>
<button onclick="One.mute()">Mute</button> 
<button onclick="One.eol()">Delete</button> 
<br>
<button id="Two" is="my-el" type="button">Click me too</button>
<button onclick="Two.disconnectedCallback()">Mute</button> 
<button onclick="Two.eol()">Delete</button> 

Notes:

• count is not available as this.count but is available to all functions defined IN constructor scope. So it is (kinda) private, only the click function can update it.

• onclick=Two.disconnectedCallback() just as example that function does NOT remove the element.

来源：https://stackoverflow.com/questions/54921027/remove-event-listener-that-has-been-added-using-bindthis