Maintaining a reference to an element while binding event handlers in a prototype

Question

Maintaining a reference to an element while binding event handlers in a prototype

Is there a way to utilize a bound event handler in prototype while maintaining the "this" context? It seems that doing so disrupts the default binding provided by prototype for event handlers.

According to the documentation:

The handler's context (this value) is set to the extended element being observed (even if the event actually occurred on a descendent element and bubbled up).

This is exactly what I need. However, once I bind the function to my own context, is there a way to access the "element being observed" in any manner? Using e.element() may not give me the observed element if it is a child element:

initialize: function() {
     Event.observe('foo', 'click', this.addItem.bind(this));
},

...

addItem: function(e) {
   e.element() // -> clicked element, not necessarily the observed element
}

I am aware of bindAsEventListener option but it doesn't seem necessary here, and I'm unsure how to access the targeted element once the function is bound.

Additionally, I know that there's an e.currentTarget property, but does it work in IE?

javascript events prototypejs

Answer 1

Answer №1

When utilizing the .bind method, your reference to the observed element using this gets replaced. Consider the following HTML markup:

<div id="foo">
    I am the Foo
    <button id="bar">I am the Bar</button>
</div>

If you avoid using .bind, accessing the observed and firing elements can be achieved like this:

function greenEggsAndHam (evt) {
    console.log('Observed Element: %o', this);
    console.log('Clicked Element: %o', $(evt.element()));
}
Event.observe('foo', 'click', greenEggsAndHam);

However, if you need to use a class method as the event handler and require the use of .bind, you should include the observed element's identifier as an argument when binding the event for the first time, as shown below:

var Test = Class.create({
    initialize: function() {
        Event.observe('foo', 'click', this.greenEggsAndHam.bind(this, 'foo'));
    },
    greenEggsAndHam: function (el, evt) {
        console.log('Observed Element: %o', $(el));
        console.log('Clicked Element: %o', $(evt.element()));
        console.log('Class reference: %o', this);
    }
});
var T = new Test();

To maintain consistency, it is recommended to use .bindAsEventListener instead of .bind, and adjust your event handler to function (evt, el) { so that the event object comes first and the observed element second.

Answer 2

When utilizing the .bind method, your reference to the observed element using this gets replaced. Consider the following HTML markup:

<div id="foo">
    I am the Foo
    <button id="bar">I am the Bar</button>
</div>

If you avoid using .bind, accessing the observed and firing elements can be achieved like this:

function greenEggsAndHam (evt) {
    console.log('Observed Element: %o', this);
    console.log('Clicked Element: %o', $(evt.element()));
}
Event.observe('foo', 'click', greenEggsAndHam);

However, if you need to use a class method as the event handler and require the use of .bind, you should include the observed element's identifier as an argument when binding the event for the first time, as shown below:

var Test = Class.create({
    initialize: function() {
        Event.observe('foo', 'click', this.greenEggsAndHam.bind(this, 'foo'));
    },
    greenEggsAndHam: function (el, evt) {
        console.log('Observed Element: %o', $(el));
        console.log('Clicked Element: %o', $(evt.element()));
        console.log('Class reference: %o', this);
    }
});
var T = new Test();

To maintain consistency, it is recommended to use .bindAsEventListener instead of .bind, and adjust your event handler to function (evt, el) { so that the event object comes first and the observed element second.

Answer 3

Answer №2

Recently, I encountered a similar issue and found a solution by encapsulating the function within another function to perform the necessary tasks. Although there might have been some confusion in the sequence of steps, utilizing the curry method can include this in the arguments of the anonymous function, allowing for reordering as needed.

initialize: function() {
  Event.observe('foo', 'click', 
    function(e,that) { that.addItem(e,this); }.curry(this)
    );
  },
addItem: function(evt, evtElm) {
  ...
  }

Answer 4

Recently, I encountered a similar issue and found a solution by encapsulating the function within another function to perform the necessary tasks. Although there might have been some confusion in the sequence of steps, utilizing the curry method can include this in the arguments of the anonymous function, allowing for reordering as needed.

initialize: function() {
  Event.observe('foo', 'click', 
    function(e,that) { that.addItem(e,this); }.curry(this)
    );
  },
addItem: function(evt, evtElm) {
  ...
  }

Maintaining a reference to an element while binding event handlers in a prototype

Answer №1

Answer №2

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