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There is an unusual feature of the event handler functions worth mentioning here, as it trips people up most frequently when writing object-oriented JavaScript, a feature that we will lean on heavily in developing Ajax clients.
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We ve got a handle on a DOM element, and assigned a callback function to the onclick property. When the DOM element receives a mouse click, the callback is invoked. However, the function context (that is, the value that variable this resolves to see appendix B for a fuller discussion of JavaScript Function objects) is assigned to the DOM node that received the event. Depending on where and how the function was originally declared, this can be very confusing. Let's explore the problem with an example. We define a class to represent a button object, which has a reference to a DOM node, a callback handler, and a value that is displayed when the button is clicked. Any instance of the button will respond in the same way to a mouse click event, and so we define the callback handler as a method of the button class. That s a sufficient spec for starters, so let s look at the code. Here s the constructor for our button:
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function Button(value,domEl){ this.domEl=domEl; this.value=value; this.domEl.onclick=this.clickHandler; }
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We go on to define an event handler as part of the Button class:
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Button.prototype.clickHandler=function(){ alert(this.value); }
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It looks straightforward enough, but it doesn t do what we want it to. The alert box will generally return a message undefined, not the value property that we passed to the constructor. Let s see why. The function clickHandler gets invoked by the browser when the DOM element is clicked, and it sets the function context to the DOM element, not the Button JavaScript object. So, this.value refers to the value property of the DOM element, not the Button object. You d never tell by looking at the declaration of the event-handler function, would you We can fix things up by passing a reference to the Button object to the DOM element, that is, by modifying our constructor like this:
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function Button(value,domEl){ this.domEl=domEl; this.value=value; this.domEl.buttonObj=this; this.domEl.onclick=this.clickHandler; }
The DOM element still doesn t have a value property, but it has a reference to the Button object, which it can use to get the value. We finish up by altering the event handler like this:
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Button.prototype.clickHandler=function(){ var buttonObj=this.buttonObj; var value=(buttonObj && buttonObj.value) buttonObj.value : "unknown value"; alert(value); }
The DOM node refers to the Button, which refers to its value property, and our event handler does what we want it to. We could have attached the value directly to the DOM node, but attaching a reference to the entire backing object allows this pattern to work easily with arbitrarily complex objects. In passing, it s worth noting that we ve implemented a mini-MVC pattern here, with the DOM element View fronting a backing object Model. That s the classic event model, then. The main shortcoming of this event model is that it allows only one event-handler function per element. In the Observer pattern that we presented in chapter 3, we noted that an observable element could have any number of observers attached to it at a given time. When writing a simple script for a web page, this is unlikely to be a serious shortcoming, but as we move toward the more complex Ajax clients, we start to feel the constraint more. We will take a closer look at this in section 4.3.3, but first, let s look at the more recent event model.
4.3.2 The W3C event model
The more flexible event model proposed by the W3C is complex. An arbitrary number of listeners can be attached to a DOM element. Further, if an action takes place in a region of the document in which several elements overlap, the event handlers of each are given an opportunity to fire and to veto further calls in the event stack, known as swallowing the event. The specification proposes that the event stack be traversed twice in total, first propagating from outermost to innermost (from the document element down) and then bubbling up again from the inside to the outside. In practice, different browsers implement different subsets of this behavior. In Mozilla-based browsers and Safari, event callbacks are attached using addEventListener() and removed by a corresponding removeEventListener(). Internet Explorer offers similar functions: attachEvent() and detachEvent(). Mike Foster s xEvent object (part of the x library see the Resources section at the end of this chapter) makes a brave attempt at creating a Fa ade (see chapter 3) across these implementations in order to provide a rich cross-browser event model. There is a further cross-browser annoyance here, as the callback handler functions defined by the user are called slightly differently. Under Mozilla browsers,
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