Loading images last with jQuery

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There are lots of ways to make your webpages faster and more responsive. YSlow is a great tool to help you find many great ways to make a particular page faster.

One of the best things you can do is reduce the number of requests (css/js/images/etc) to the server. Typically, this would mean that you would combine files – merge all of your JS and CSS (and minify while you are at it), and use CSS Sprites to combine images.

One major problem of using CSS Sprites is that it can be quite painful to maintain. Over time, if you want to add or change some of your images – you basically need to rebuild and replace the combined images and all of the CSS rules specifying coordinates. Sometimes, this makes the CSS Sprite technique unreasonable to implement.

In one such case, we had about 50 images in one application that were causing the page to take a long time to load. These images were previews of some different design choices that the user could make. The design choices themselves (and their previews) were database driven so that we can add new designs through an admin interface. So, CSS Spriteing the previews would seriously hamper that flexibility.

One other design consideration was that the previews weren’t that important – the page was fully functional and usable without the images. In fact, the designs weren’t even visible until you toggled the design menu.

There is a lazy loader plugin for jQuery already available here – but it didn’t fit our needs. Instead of skipping images in order to get the page working as soon as possible (and initiate the load once the page is usable) – it is made to skip loading offscreen images until they are scrolled into view. It might have somewhat worked for our needs – but I thought it was better to load the images as soon as possible, instead of waiting for the design menu to be expanded to initiate the load. That way, most of the time the designs would be visible by the time they open the menu – but it wouldn’t interfere with the rest of the interface.

My solution was to set the src for all of the previews to a single animated loading image – like one you can get here. Then, I set a custom attribute on the image for the real preview’s url. Finally, some jQuery code runs after the page is done loading which replaces each src attribute with the url in the custom attribute, which will load the real image.

Sample HTML:

<ul>
    <li templateid="7bcf8f23-fdd0-45c5-a429-d2ffb59e47f0" class="selected"><span>3D Dots
        Dark</span>
        <img src="/static/img/ajax-loader-small.gif" deferredsrc="/resources/7bcf8f23-fdd0-45c5-a429-d2ffb59e47f0/preview.jpg"
            class="deferredLoad" alt="3D Dots Dark" />
    </li>
    <li templateid="b1a09e28-629e-472a-966e-fc98fc269607"><span>3D Dots Lite</span>
        <img src="/static/img/ajax-loader-small.gif" deferredsrc="/resources/b1a09e28-629e-472a-966e-fc98fc269607/preview.jpg"
            class="deferredLoad" alt="3D Dots Lite" />
    </li>
    <li templateid="e121d26a-9c8f-466f-acc7-9a79d5e8cfa9"><span>Beauty</span>
        <img src="/static/img/ajax-loader-small.gif" deferredsrc="/resources/e121d26a-9c8f-466f-acc7-9a79d5e8cfa9/preview.jpg"
            class="deferredLoad" alt="Beauty" />
    </li>
    <li templateid="322e4c7a-33e7-4e05-bb72-c4076a83a3d0"><span>Black and White</span>
        <img src="/static/img/ajax-loader-small.gif" deferredsrc="/resources/322e4c7a-33e7-4e05-bb72-c4076a83a3d0/preview.jpg"
            class="deferredLoad" alt="Black and White" />
    </li>
    <li templateid="57716da9-91ef-4cf0-82f1-722d0770ad7f"><span>Blank</span>
        <img src="/static/img/ajax-loader-small.gif" deferredsrc="/resources/57716da9-91ef-4cf0-82f1-722d0770ad7f/preview.jpg"
            class="deferredLoad" alt="Blank" />
    </li>
    <li templateid="a79e1136-db47-4acd-be3e-2daf4522796d"><span>Blue Leaves</span>
        <img src="/static/img/ajax-loader-small.gif" deferredsrc="/resources/a79e1136-db47-4acd-be3e-2daf4522796d/preview.jpg"
            class="deferredLoad" alt="Blue Leaves" />
    </li>
    <li templateid="03cb737d-4da7-46d5-b4e4-5ad4b4a3aaf4"><span>Blue Open</span>
        <img src="/static/img/ajax-loader-small.gif" deferredsrc="/resources/03cb737d-4da7-46d5-b4e4-5ad4b4a3aaf4/preview.jpg"
            class="deferredLoad" alt="Blue Open" />
    </li>
    <li templateid="899dff2f-38ba-44f7-9fe2-af66e62674a4"><span>Compass</span>
        <img src="/static/img/ajax-loader-small.gif" deferredsrc="/resources/899dff2f-38ba-44f7-9fe2-af66e62674a4/preview.jpg"
            class="deferredLoad" alt="Compass" />
    </li>
</ul>

Sample javascript:

$(function(){
        $("img.deferredLoad").each(function() {
            var $this = $(this);
            $this.attr("src", $this.attr("deferredSrc")).removeClass("deferredLoad");
        });
});
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Getting the width of a hidden element with jQuery using width()

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UPDATE #3: It needs to be noted that the fix introduced in jQuery 1.4.4 is only for the width() and height() methods.  If you need inner/outer dimensions the method below still needs to be used.  I have updated the method to return height/width, outer height/width, and inner height/width.  There is an optional parameter to include margins in the outer dimension calculations.  Thanks Ryan and Fred for the heads up.

UPDATE #2: jQuery 1.4.4 was released today (11/11/2010) and included in the release was an update to the width() and height() methods.z  Each method will now return the correct dimension of the element if it is within a hidden element.  For further information, you can view the bug report.

UPDATE #1: Based on the feedback in the comments regarding the use of the swap method, I am updating this post with a solution from Ryan Wheale.  He created a function to return the dimensions of an element that is hidden or nested within 1 more hidden elements.  Here is the code that he posted below in his comment:

//Optional parameter includeMargin is used when calculating outer dimensions
(function($) {
$.fn.getHiddenDimensions = function(includeMargin) {
    var $item = this,
        props = { position: 'absolute', visibility: 'hidden', display: 'block' },
        dim = { width:0, height:0, innerWidth: 0, innerHeight: 0, outerWidth: 0, outerHeight: 0 },
        $hiddenParents = $item.parents().andSelf().not(':visible'),
        includeMargin = (includeMargin == null)? false : includeMargin;

    var oldProps = [];
    $hiddenParents.each(function() {
        var old = {};

        for ( var name in props ) {
            old[ name ] = this.style[ name ];
            this.style[ name ] = props[ name ];
        }

        oldProps.push(old);
    });

    dim.width = $item.width();
    dim.outerWidth = $item.outerWidth(includeMargin);
    dim.innerWidth = $item.innerWidth();
    dim.height = $item.height();
    dim.innerHeight = $item.innerHeight();
    dim.outerHeight = $item.outerHeight(includeMargin);

    $hiddenParents.each(function(i) {
        var old = oldProps[i];
        for ( var name in props ) {
            this.style[ name ] = old[ name ];
        }
    });

    return dim;
}
}(jQuery));

This basically performs the same operations as the swap method.  This is safer to use in case the swap method is removed from the jQuery core.

I have tested this in multiple cases and each time the correct results were returned.

Thanks Ryan.

————————————————————————————————————————————-

Original post

I recently ran into a problem with jQuery’s width().  The problem is with a visible element that is inside a hidden element will return a value of 0 instead of its’ actual calculated width.  After messing around with it for a little bit I finally came up with a solution.  The method I used involved adding some CSS properties to the hidden element.  The CSS properties involved are position, visibility, and display.

//$HiddenItem is the element that is wrapping the element that you want the width of
//$Item is the element you want the width of

$HiddenItem.css({
    position: "absolute",
    visibility: "hidden",
    display: "block"
})
$Item.width();

$HiddenItem.css({
    position: "",
    visibility: "",
    display: ""
})

After setting the above CSS properties on the element, you can then call width() and the correct value will be returned. After you call the width() method you should clear the properties in order to return the element to the way it was.

Setting the properties to an empty string is probably not the best way to do it though. What if there was a position value already set? Using this method would clear out that initial values of the CSS properties.

I found the swap() method to be handy in this situation. I found this method while looking through the jQuery source code.

 // A method for quickly swapping in/out CSS properties to get correct calculations swap: function( elem, options, callback ) {      var old = {};       // Remember the old values, and insert the new ones      for ( var name in options ) {           old[ name ] = elem.style[ name ];           elem.style[ name ] = options[ name ];      }       callback.call( elem );       // Revert the old values      for ( var name in options ){          elem.style[ name ] = old[ name ];      } } 

By using the swap method, the old CSS properties will be remembered and reapplied after finding the width of the element. The swap method takes in 3 parameters:

  1. The element that you would like to swap the CSS properties on
  2. CSS key/value pairs that you want to change
  3. A callback function to call after the properties are set

To rewrite the above to use the swap method I would do the following:

var props = { position: "absolute", visibility: "hidden", display: "block" };
var itemWidth = 0;

$.swap($HiddenItem[0], props, function(){
     itemWidth = $Item.width();
});

//Use itemWidth

I coded up a small example on jsbin. Here is the link http://jsbin.com/ofine3/2.

$HiddenItem.width();

jQuery Custom Selector for selecting elements by exact text :textEquals

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I needed a way to find labels based on the text that they contained.  I thought about using :contains() for this, but in this particular case the text items I was searching on were names and I could have similar names that :contains() could incorrectly match on.  For instance, if I was searching for “Banks, Tim” and there was an item with the text “Banks, Timothy” I would get both items returned.  This is not the behavior I was looking for.

I decided to write a little custom selector to match on exact text.  Here is the code for the custom selector:

$.expr[':'].textEquals = function(a, i, m) {
return $(a).text().match("^" + m[3] + "$");
};

What is happening here is I am using a regular expression to test if the start and end of the element’s text matches the string passed in.  Now I could search for the name “Banks, Tim” on a label element like this:

$("label:textEquals('Banks, Tim')");

Make Table Rows Sortable Using jQuery UI Sortable

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So you want to make table rows sortable using jQuery UI? Luckily, the Sortable interaction does most of the work for you.

But there’s a catch: one problem that I ran into when implementing this (with UI version 1.7) was the cell widths of the row would collapse once I started dragging it.

Suppose you have a table of data, like this one:

<table id="sort" class="grid" title="Kurt Vonnegut novels">
	<thead>
		<tr><th>Year</th><th>Title</th><th>Grade</th></tr>
	</thead>
	<tbody>
		<tr><td>1969</td><td>Slaughterhouse-Five</td><td>A+</td></tr>
		<tr><td>1952</td><td>Player Piano</td><td>B</td></tr>
		<tr><td>1963</td><td>Cat's Cradle</td><td>A+</td></tr>
		<tr><td>1973</td><td>Breakfast of Champions</td><td>C</td></tr>
		<tr><td>1965</td><td>God Bless You, Mr. Rosewater</td><td>A</td></tr>
	</tbody>
</table>

Your first attempt to make it sortable might look like this:

$("#sort tbody").sortable().disableSelection();

And it actually works, but there is a bit of a problem. The cell widths seem to be collapsing once you start dragging a row (notice how close the “C” cell is to the “Breakfast of Champions” cell). It looks like this:

Sortable row collapsed widths

The problem has to do with the helper object. The helper object is basically the DOM element that follows the cursor during the drag event. When it is created by default, the cells collapse to the size of the content inside of them.

You can specify a function that returns a jQuery object to create a custom helper object. By creating a function that will keep the cell widths consistent, this problem can be fixed.

// Return a helper with preserved width of cells
var fixHelper = function(e, ui) {
	ui.children().each(function() {
		$(this).width($(this).width());
	});
	return ui;
};

$("#sort tbody").sortable({
	helper: fixHelper
}).disableSelection();

Now it works as expected:
Sortable row fixed

Keep Original Variable State Between Event Binding and Execution – JavaScript

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Also known as: Binding Events Inside of a Loop with jQuery

Sometimes I want to create a closure that accesses a variable in the state that it was in when the closure was created. I don’t want to know what the current value is, I want to know what the value was when I created the function. I may want to do this when binding an event to a DOM object, or when executing a function inside of setTimeout().

The Problem

Here is an example of the problem using the jQuery library (consider what will happen when you click on any of the li elements):

	$(function() {
		$("body").append("<ul id='container'></ul>");
		
		for (var i = 0; i < 5; i++)
		{
			var $item = $("<li />").text("Item " + i);
			$("#container").append($item);
			
			$item.click(function() {
				alert("You clicked number " + i);  // always "You clicked number 5"
			});
		}
	});

The event handler will always use the final value of the variable “i”, which will be 5 in this case. One way to fix this problem is to create a function and pass “i” as a formal parameter:

Solution 1 – Traditional Function

	function bindItem($item, ind) {
		$item.click(function() {
			alert("You clicked number " + ind);  // Works as expected
		});
	}

	$(function() {
		$("body").append("<ul id='container'></ul>");

		for (var i = 0; i < 5; i++)
		{
			var $item = $("<li />").text("Item " + i);
			$("#container").append($item);
			bindItem($item, i);
		}
	});

Since this passes the variable as a parameter to a function, the variable “ind” will always remain the same, even if “i” changes after the function finishes execution. The function bindItem() creates a new scope. Any variables (passed as parameters or declared inside the function) will reserve their own space in memory, and will not be affected by the state of the variable ‘i’. Thanks to reddit user Mych for correcting me.

This is what I wanted in the first place, but we may not want to have a named function here – if we used a closure we would still have access to variables in the same scope (like $item) without having to pass them as parameters to bindItem().

This does the trick:

Solution 2 – Self Executing Function

	$(function() {
		$("body").append("<ul id='container'></ul>");
		
		for (var i = 0; i < 5; i++)
		{
			var $item = $("<li />").text("Item " + i);
			$("#container").append($item);
			
			(function() { // Closure here here instead of "bindItem()"
				var ind = i;
				$item.click(function() {
					alert("You clicked number " + ind); // Works as expected
				});
			})(); // Execute immediately
		}
	});

This is a trick I first saw in the jQuery plugin development tutorial, in the custom alias section.

Declaring the closure creates a new scope in the loop. Any variables (such as ‘ind’) passed as parameters or declared inside of this scope will reserve a new space in memory that is not a reference to ‘i’.

The extra parentheses at the end of a block like this “(function() {…})()” cause the closure to execute immediately .

These are just a couple ways to solve the problem of variable state changing between event binding and execution. Closures are a powerful language feature that makes JavaScript very flexible, but when execution of them is being delayed via a timeout or event handler, it is important to keep track of the variables that they are closed over.

Accessible Custom AJAX and .NET

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One general rule in making an accessible web application is that you shouldn’t change content of the page with javascript. This is because screen readers have a tough time monitoring the page and notifying the user of dynamic changes. Usually, the reason you would use Ajax is exactly that – do something on the server, and then update some small part of the page without causing the whole page to refresh. That means if you have an “ajaxy” page and you care about accessibility, you have to provide a non-ajax interface as a fallback for screen readers.

Ajax.NET, Microsoft’s library for Ajax, makes this fallback easy to implement. Microsoft has you define your AJAX properties in an abstraction layer – the page XAML – which means that the framework can decide to not render the AJAX code to certain browsers (and screen readers) that will not support it, and instead use the standard postback method.

The problem with Ajax.NET is that the communication can be bloated (mostly because of the abstraction layer, it sends more post values than you might need – like the encrypted viewstate value), which negates many of the benefits of an Ajax solution. I really wanted to roll my own ajax communications to make them as lightweight as possible.

My solution was to write the page in a standard .NET postback manner, and then use a user-defined property that would allow javascript to replace the postback in javascript/jQuery with an Ajax call.

Here’s my code:

$(function() {
            if (serverVars.uiVersion != "accessible") { // use js/ajax for checking/unchecking where possible
                var $todos = $("#chkToDo");
               $todos.removeAttr("onclick"); // remove postback
                $todos.click(
                    function() {
                        //some stuff altering the document, and an ajax call to report the change to the db
                    }
                );
            }

This works great, although you need to be careful about your server-side events. In my case, I had an OnCheckChanged event to handle the postback/accessible mode. Even though checking/unchecking the box no longer fired an autopostback – ASP.NET will still fire the checkchanged event if you postback later for some other reason (e.g. – a linkbutton elsewhere on the page) after the checked status had changed. So, if a user had changed the state of a checkbox, then clicked another link button on the page – instead of sending the user to the intended page,my app just refreshed the whole page(because my CheckChanged event redirected to reload the page – which caused it to skip the ‘click’ event of the linkbutton). Once I realized this was happening, it was easy enough to fix – I just needed to only run the event logic if the user was in accessibility mode. I spent a little time running in circles on that one though, at first I thought my client side document changes were causing a ViewState validation error on the server.

Extending jQuery to Select ASP Controls

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If you have worked with JavaScript in an ASP.NET Web Forms environment, you almost certainly have been frustrated that this markup:

	<asp:TextBox runat="server" ID="txtPhoneNumber" />

renders out as something like:

	<input type="text" id="ctl00_ctl00_ctl00_main_Content_txtPhoneNumber"
		name="ctl00$ctl00$ctl00$main$Content$txtPhoneNumber" />

The fastest and easiest way to get a reference to a DOM element in JavaScript is using the ID attribute and document.getElementById(). Unfortunately, the ID attribute generated by the server is unpredictable and based on the server naming containers. There are a couple ways of that I have previously dealt with that problem, but both have problems.

Old Solutions

  1. Accessing the ClientID of the server control

    If you use inline code inside <% %> tags, you can access the ClientID directly from the .aspx page.

    	var goodID = '<% txtPhoneNumber.ClientID %>';  // = 'ctl00_ctl00_ctl00_main_HeaderTabs_txtPhoneNumber'
    	var badID = 'txtPhoneNumber'; // The text box does not have this ID, it will not work
    	var uglyID = 'ctl00_ctl00_ctl00_main_Content_txtPhoneNumber'; // DO NOT hardcode the generated ID into your code!
    

    This is not ideal because you cannot reference the ClientID from outside of the page, so you cannot keep your JavaScript in external files.

  2. Setting attributes on control and accessing with jQuery selectors

    jQuery has an excellent selector API that can easily grab an element if you know attributes on it. So, if there were a couple controls defined as:

    	<asp:TextBox runat="server" ID="txtPhoneNumber" CssClass="txtPhoneNumber" />
    	<asp:TextBox runat="server" ID="txtAddress" ClientID="txtAddress" />
    

    You could access them with jQuery:

    	$(".txtPhoneNumber").keyup(...);   // This works
    	$("[ClientID='txtAddress']").keyup(...);   // This works
    
    	$("#txtPhoneNumber").keyup(...);   // This still DOESN'T work
    	$("#txtAddress").keyup(...);   // This still DOESN'T work
    

    This is not ideal because it requires adding extra attributes onto any server control that you want to access with JavaScript.

Original jQuery Solution

I first happened upon a solution to the same problem over on John Sheehan’s blog. This looked promising, but did not work the current latest release of jQuery (1.3). Another contributer to this blog, Tim Banks, updated the code in to work with the newer version.

However, I found an error in Internet Explorer and a more reliable way to get the ClientID would be to use the jQuery attribute selector and match based on an “id” attribute that ends with the Server ID. So, I wrote a JavaScript function called $asp that returned a jQuery collection. This function worked well and was implemented in the latest Foliotek release.

	function $asp(serverID) {
		return $("[id$='" + serverID+ "']");
	}

	// Once this function is included, you can call it and get back a jQuery collection
	$asp("txtPhoneNumber").keyup(...);

Updated jQuery and Sizzle Solution

It seemed like it would be better if the solution actually extended the selector engine rather than using a function to select objects. This would fit better into a jQuery development paradigm, allow more complex selectors, and also allow the selector to be used in other library functions, like “filter” or “find”. Also, it would be nice to be able to use the tag name in the selector to give a performance improvement, since getElementsByTagName is a fast operation that will narrow the element collection.

So, I returned to the selector, fixed the IE bug and made sure it worked with the now latest version of jQuery (1.3.2). This short function extends the Sizzle selector engine to return an element that has an ID that ends with the ID that is passed in the parenthesis after the “:asp()” selector.

	// Include this function before you use any selectors that rely on it
	jQuery.expr[':'].asp = function(elem, i, match) {
		return (elem.id && elem.id.match(match[3] + "$"));
	};

	// Now all of these are valid selectors
	// They show why this method has more functionality than the previous $asp() function.
	$(":asp(txtPhoneNumber)")
	$("input:asp(txtPhoneNumber):visible")
	$(":asp(txtPhoneNumber), :asp(txtAddress)")
	$("ul:asp(listTodos) li")
	$("#content").find("ul:asp(listTodos)")

This function allows access to server controls without adding additional markup and without the JavaScript existing on the .aspx page.


Note, there is a potential limitation if you had one control with the ID=”txtPhoneNumber” and another with ID=”mytxtPhoneNumber”. Both elements end with “txtPhoneNumber” and the selector would not necessarily return the correct value. This solution is not perfect in that sense, but the benefits it provides over other methods (cleaner markup and ability to use external JavaScript files) make it a good alternative.