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问题:

Is there a best-practice or common way in JavaScript to have class members as event handlers?

Consider the following simple example:

<head>
    <script language="javascript" type="text/javascript">

        ClickCounter = function(buttonId) {
            this._clickCount = 0;
            document.getElementById(buttonId).onclick = this.buttonClicked;
        }

        ClickCounter.prototype = {
            buttonClicked: function() {
                this._clickCount++;
                alert('the button was clicked ' + this._clickCount + ' times');
            }
        }

    </script>
</head>
<body>
    <input type="button" id="btn1" value="Click me" />
    <script language="javascript" type="text/javascript">
        var btn1counter = new ClickCounter('btn1');
    </script>
</body>

The event handler buttonClicked gets called, but the _clickCount member is inaccessible, or this points to some other object.

Any good tips/articles/resources about this kind of problems?

回答1:

ClickCounter = function(buttonId) {
    this._clickCount = 0;
    var that = this;
    document.getElementById(buttonId).onclick = function(){ that.buttonClicked() };
}

ClickCounter.prototype = {
    buttonClicked: function() {
        this._clickCount++;
        alert('the button was clicked ' + this._clickCount + ' times');
    }
}

EDIT almost 10 years later, with ES6, arrow functions and class properties

class ClickCounter  {
   count = 0;
   constructor( buttonId ){
      document.getElementById(buttonId)
          .addEventListener( "click", this.buttonClicked );
  }
   buttonClicked = e => {
     this.count += 1;
     console.log(`clicked ${this.count} times`);
   }
}

https://codepen.io/anon/pen/zaYvqq

回答2:

A function attached directly to the onclick property will have the execution context's this property pointing at the element.

When you need to an element event to run against a specific instance of an object (a la a delegate in .NET) then you'll need a closure:-

function MyClass() {this.count = 0;}
MyClass.prototype.onclickHandler = function(target)
{
   // use target when you need values from the object that had the handler attached
   this.count++;
}
MyClass.prototype.attachOnclick = function(elem)
{
    var self = this;
    elem.onclick = function() {self.onclickHandler(this); }
    elem = null; //prevents memleak
}

var o = new MyClass();
o.attachOnclick(document.getElementById('divThing'))

回答3:

I don't know why Function.prototype.bind wasn't mentioned here yet. So I'll just leave this here ;)

ClickCounter = function(buttonId) {
    this._clickCount = 0;
    document.getElementById(buttonId).onclick = this.buttonClicked.bind(this);
}

ClickCounter.prototype = {
    buttonClicked: function() {
        this._clickCount++;
        alert('the button was clicked ' + this._clickCount + ' times');
    }
}

回答4:

You can use fat-arrow syntax, which binds to the lexical scope of the function

function doIt() {
  this.f = () => {
    console.log("f called ok");
    this.g();
  }
  this.g = () => {
    console.log("g called ok");
  }
}

After that you can try

var n = new doIt();
setTimeout(n.f,1000);

You can try it on babel or if your browser supports ES6 on jsFiddle.

Unfortunately the ES6 Class -syntax does not seem to allow creating function lexically binded to this. I personally think it might as well do that. EDIT: There seems to be experimental ES7 feature to allow it.