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Monthly Archives: March 2013

Animating lists with CSS 3 transitions

Ever wanted to implement animated lists or grids, like in the following 12-second video?

This may look like a native iOS app, but this UI is all implemented with HTML, CSS, and JavaScript (see the previous post for details about my new iPhone app, Touralot, built with PhoneGap). So, how does all this slidey stuff work?

Using “transform” and “transition” in lists

The basic principles here are:

  • For animations to be smooth and hardware-accelerated, they have to be done with transform: translate3d
  • Even so, element positions should still be determined by the DOM structure (minimizing use of JavaScript)

So, consider some markup like the following:

<ul class="items">
    <li>Monday</li>
    <li>Tuesday</li>
    <li>Wednesday</li>
    ...
</ul>

The element positions are determined by the DOM structure, but not using translate3d. Now consider the following CSS rules:

.items { position: relative; }
.items li { 
    position: absolute; top: 0; left: 0;
    transition: all 0.2s ease-out;
}
 
.items li:nth-child(1)  { transform: translate3d(0, 0%, 0); }
.items li:nth-child(2)  { transform: translate3d(0, 100%, 0); }
.items li:nth-child(3)  { transform: translate3d(0, 200%, 0); }
...

As you can see, the <li> elements will now be positioned absolutely against the parent <ul>, and we use translate3d to recover the usual vertical offset based on each element’s position in the DOM. What’s more, there’s a transition rule so that, whenever translate3d changes, the browser smoothly animates the element to its new position.

[Aside: For Webkit, you’d need prefixed rules as well (-webkit-transform and -webkit-transition). Current versions of IE and Firefox don’t require the prefixes.]

What’s the result? Well, now whenever you change the set of <li> elements inside the <ul>, all the other <li> elements will smoothly animate into their new positions, without you having to write any code to tell them to do so. This works wonderfully if you’re generating your DOM through any kind of templates/binding library such as with Knockout’s foreach binding, but also works nicely just with plain old DOM operations. Try this:

The add and remove functionality doesn’t have to tell the elements to animate. They just do that naturally. For example, the code for add is just:

$(".append").click(function () {
    $("<li>New item</li>").insertAfter($(".items").children()[2]);
});

Browser quirks

Webkit (or at least Chrome) has annoying bug whereby it doesn’t account for in-flight CSS transitions when computing the scroll extents of a container. So you’ll need to do something like the following to force it to recompute the scroll container bounds at the end of the transition:

// Workaround for Webkit bug: force scroll height to be recomputed after the transition ends, not only when it starts
$(".items").on("webkitTransitionEnd", function () {
    $(this).hide().offset();
    $(this).show();
});

Also, since IE<10 doesn’t support transform or transition, you might also want to include some conditional CSS like the following to get basic non-animated behaviour on older browsers:

<!--[if lte IE 9]><style type="text/css">
    /* Old IE doesn't support CSS transform or transitions */
    .list-example .items li { position: relative; display: inline-block; }
</style><![endif]-->

It works for grids, too

With this technique, there’s nothing special about one-dimensional vertical lists. It works exactly as well for two-dimensional grids, without any changes.

Well actually, this raises the question of where all those .items li:nth-child(x) rules are coming from. So far I assumed you were writing them by hand. But how many such rules should you write? Couldn’t we generate them programmatically? Why yes, of course. Try this utility function:

function createListStyles(rulePattern, rows, cols) {
    var rules = [], index = 0;
    for (var rowIndex = 0; rowIndex < rows; rowIndex++) {
        for (var colIndex = 0; colIndex < cols; colIndex++) {
            var x = (colIndex * 100) + "%",
                y = (rowIndex * 100) + "%",
                transforms = "{ -webkit-transform: translate3d(" + x + ", " + y + ", 0); transform: translate3d(" + x + ", " + y + ", 0); }";
            rules.push(rulePattern.replace("{0}", ++index) + transforms);
        }
    }
    var headElem = document.getElementsByTagName("head")[0],
        styleElem = $("<style>").attr("type", "text/css").appendTo(headElem)[0];
    if (styleElem.styleSheet) {
        styleElem.styleSheet.cssText = rules.join("\n");
    } else {
        styleElem.textContent = rules.join("\n");
    }
}

You can specify a maximum number of elements to account for, and the number of columns desired in your grid (or pass 1 for a single-column vertical list). Of course, it would be nicer still if we could write just one rule that specified the translate3d values as a function of the element index, but I’m not aware of any way of doing that in CSS 3. Let me know if you can think of one!

Here’s the result, as a three-column grid:

Supporting arbitrary reorderings

The technique so far is great for animating all elements other than the one you’re inserting or reordering. But if you remove and insert an element in a new position, it will appear there instantly, without a transition, because it is a brand new element as far as the CSS transition logic is concerned.

So, how would it be possible to achieve something like the following, where the “reorder” button smoothly moves elements to new positions? Try it: click the random order button:

One possible technique is to override the translate3d values for each element with a snapshot of their current values, so that the elements retain their coordinates independently of their DOM order. Then, after mutating the DOM, remove your snapshot values, and then the CSS transition will kick in to move the element smoothly to its final location.

Here’s are a couple of handle jQuery utility functions:

(function () {
    var stylesToSnapshot = ["transform", "-webkit-transform"];
 
    $.fn.snapshotStyles = function () {
        if (window.getComputedStyle) {
            $(this).each(function () {
                for (var i = 0; i < stylesToSnapshot.length; i++)
                    this.style[stylesToSnapshot[i]] = getComputedStyle(this)[stylesToSnapshot[i]];
            });
        }
        return this;
    };
 
    $.fn.releaseSnapshot = function () {
        $(this).each(function () {
            this.offsetHeight; // Force position to be recomputed before transition starts
            for (var i = 0; i < stylesToSnapshot.length; i++)
                this.style[stylesToSnapshot[i]] = "";
        });
    };
})();

Now you can achieve the random reordering thing as follows:

$(".reorder").click(function () {
    $(".items li")
        .snapshotStyles()
        .tsort({ order: "rand" })
        .releaseSnapshot();
});

This works just the same with two-dimensional grids as with vertical lists.

For convenience, I used the tinysort library here (I only just learned about it this morning – it’s very neat), which can also sort the elements in a meaningful way, not only randomly. But it would work exactly the same if you write manual code to reorder the elements.

Supporting drag-and-drop reordering

Getting to this point may have seemed complicated, but it has some great benefits. Since the animation and positioning is controlled entirely by CSS, it composes beautifully with many other techniques for modifying the DOM. For example, if a drag-drop library shuffles the DOM elements, then they will now animate.

As an example, we can throw in jQuery UI’s sortable mechanism to enable drag-drop reordering. But let’s make it slick, and also animate the “dropping” phase of the operation, where the element you dragged moves from wherever you’re holding it into its final position.

Here’s how you can do that:

$("ul.items").sortable({
    start: function (event, ui) {
        // Temporarily move the dragged item to the end of the list so that it doesn't offset the items
        // below it (jQuery UI adds a 'placeholder' element which creates the desired offset during dragging)
        $(ui.item).appendTo(this).addClass("dragging");
    },
    stop: function (event, ui) {
        // jQuery UI instantly moves the element to its final position, but we want it to transition there.
        // So, first convert the final top/left position into a translate3d style override
        var newTranslation = "translate3d(" + ui.position.left + "px, " + ui.position.top + "px, 0)";
        $(ui.item).css("-webkit-transform", newTranslation)
                    .css("transform", newTranslation);
        // ... then remove that override within a snapshot so that it transitions.
        $(ui.item).snapshotStyles().removeClass("dragging").releaseSnapshot();
    }
});

Here’s the result. Be sure to try this in a desktop browser, not a phone/tablet, because jQuery UI sortable doesn’t handle touch events by default. Also if you’re on IE, be sure to use IE10+ or you won’t see animations.

Weird flicker in Chrome? It only happens when the example is in this iframe. Try the “Edit in JSFiddle” link to see it without.

… and here’s a two-dimensional grid, with the same drag-drop code:

If you want better support for phones and tables, there are various ways of upgrading jQuery UI “sortable” to respond to touch events. Or write your own.


Touralot: an iOS app built with PhoneGap, Knockout, and Azure Mobile Services

As a weekend project for the past few months, I’ve been building my first real mobile app: Touralot. Finally, it’s done, and it’s been approved on the iOS App Store!

Touralot lets you publish your own web-based guides to your favourite places to go and see. Maybe you’re off on vacation and would like to build a record of cool places you see, add photos, ratings, descriptions, plot them on a map, and then publish it all to the web. Or maybe you’re at home and are particularly knowledgeable about local restaurants, kids’ play centres, or whatever, and want to share your expertise.

This video gives a sense of the user experience:

Video looks blurry? Use a different browser, or open the video directly.

If you’re on an iPhone, you can get it now from the App Store (free).

The technology

Touralot is a Cordova/PhoneGap app, with the UI powered by Knockout.js, and a backend data service provided by Windows Azure Mobile Services (and a couple of other Azure facilities). It works entirely offline, except of course at the point where the user wants to log in and publish their stuff:

Screen Shot 2013-03-13 at 12.12.01

Why did I use HTML/JS and PhoneGap instead of native code (e.g., Objective-C)?

  • I wanted to see how far I could push HTML/JS to match the quality experience that people expect from native apps. This includes:
    • Standardised UI elements that respond intuitively to touch, drag/drop, and swipe gestures
    • Butter-smooth hardware-accelerated transitions
    • Retina-sharp professional artwork
    • Offline/syncing support

    Assess the result for yourself – I don’t imagine that it would even occur to normal users, at least on reasonably up-to-date devices (iPhone 4S/5), that this isn’t native.

  • This app can now be made cross-platform with relative ease. There’ll be some restyling effort involved, since Android and Windows Phone have different UI conventions, but hopefully it will mainly just be CSS changes.
  • I don’t know much Objective-C…

Coming up

Over the next few weeks I’m planning a series of blog posts to describe many of the techniques I used and challenges I went through while developing this app, along with recommendations and tips for PhoneGap development. Some of these techniques are well known, some might even be original…