hacks.mozilla.org

This post is from Les Orchard, who works on Mozilla’s web development team.

Introduction

Drag and drop is one of the most fundamental interactions afforded by graphical user interfaces. In one gesture, it allows users to pair the selection of an object with the execution of an action, often including a second object in the operation. It’s a simple yet powerful UI concept used to support copying, list reordering, deletion (ala the Trash / Recycle Bin), and even the creation of link relationships.

Since it’s so fundamental, offering drag and drop in web applications has been a no-brainer ever since browsers first offered mouse events in DHTML. But, although mousedown, mousemove, and mouseup made it possible, the implementation has been limited to the bounds of the browser window. Additionally, since these events refer only to the object being dragged, there’s a challenge to find the subject of the drop when the interaction is completed.

Of course, that doesn’t prevent most modern JavaScript frameworks from abstracting away most of the problems and throwing in some flourishes while they’re at it. But, wouldn’t it be nice if browsers offered first-class support for drag and drop, and maybe even extended it beyond the window sandbox?

As it turns out, this very wish is answered by the HTML 5 specification section on new drag-and-drop events, and Firefox 3.5 includes an implementation of those events.

If you want to jump straight to the code, I’ve put together some simple demos of the new events.

I’ve even scratched an itch of my own and built the beginnings of an outline editor, where every draggable element is also a drop target—of which there could be dozens to hundreds in a complex document, something that gave me some minor hair-tearing moments in the past while trying to make do with plain old mouse events.

And, all the above can be downloaded or cloned from a GitHub repository I’ve created expecially for this article.

The New Drag and Drop Events

So, with no further ado, here are the new drag and drop events, in roughly the order you might expect to see them fired:

dragstart

A drag has been initiated, with the dragged element as the event target.

drag

The mouse has moved, with the dragged element as the event target.

dragenter

The dragged element has been moved into a drop listener, with the drop listener element as the event target.

dragover

The dragged element has been moved over a drop listener, with the drop listener element as the event target. Since the default behavior is to cancel drops, returning false or calling preventDefault() in the event handler indicates that a drop is allowed here.

dragleave

The dragged element has been moved out of a drop listener, with the drop listener element as the event target.

drop

The dragged element has been successfully dropped on a drop listener, with the drop listener element as the event target.

dragend

A drag has been ended, successfully or not, with the dragged element as the event target.

Like the mouse events of yore, listeners can be attached to elements using addEventListener() directly or by way of your favorite JS library.

Consider the following example using jQuery, also available as a live demo:

    <div id="newschool">
        <div class="dragme">Drag me!</div>
        <div class="drophere">Drop here!</div>
    </div>
 
    <script type="text/javascript">
        $(document).ready(function() {
            $('#newschool .dragme')
                .attr('draggable', 'true')
                .bind('dragstart', function(ev) {
                    var dt = ev.originalEvent.dataTransfer;
                    dt.setData("Text", "Dropped in zone!");
                    return true;
                })
                .bind('dragend', function(ev) {
                    return false;
                });
            $('#newschool .drophere')
                .bind('dragenter', function(ev) {
                    $(ev.target).addClass('dragover');
                    return false;
                })
                .bind('dragleave', function(ev) {
                    $(ev.target).removeClass('dragover');
                    return false;
                })
                .bind('dragover', function(ev) {
                    return false;
                })
                .bind('drop', function(ev) {
                    var dt = ev.originalEvent.dataTransfer;
                    alert(dt.getData('Text'));
                    return false;
                });
        });
    </script>

Thanks to the new events and jQuery, this example is both short and simple—but it packs in a lot of functionality, as the rest of this article will explain.

Before moving on, there are at least three things about the above code that are worth mentioning:

• Drop targets are enabled by virtue of having listeners for drop events. But, per the HTML 5 spec, draggable elements need an attribute of draggable="true", set either in markup or in JavaScript.

  Thus, $('#newschool .dragme').attr('draggable', 'true').

• The original DOM event (as opposed to jQuery’s event wrapper) offers a property called dataTransfer. Beyond just manipulating elements, the new drag and drop events accomodate the transmission of user-definable data during the course of the interaction.
• Since these are first-class events, you can apply the technique of Event Delegation.

  What’s that? Well, imagine you have a list of 1000 list items—as part of a deeply-nested outline document, for instance. Rather than needing to attach listeners or otherwise fiddle with all 1000 items, simply attach a listener to the parent node (eg. the <ul> element) and all events from the children will propagate up to the single parent listener. As a bonus, all new child elements added after page load will enjoy the same benefits.

  Check out this demo, and the associated JS code to see more about these events and Event Delegation.

Using dataTransfer

As mentioned in the last section, the new drag and drop events let you send data along with a dragged element. But, it’s even better than that: Your drop targets can receive data transferred by content objects dragged into the window from other browser windows, and even other applications.

Since the example is a bit longer, check out the live demo and associated code to get an idea of what’s possible with dataTransfer.

In a nutshell, the stars of this show are the setData() and getData() methods of the dataTransfer property exposed by the Event object.

The setData() method is typically called in the dragstart listener, loading dataTransfer up with one or more strings of content with associated recommended content types.

For illustration, here’s a quick snippet from the example code:

    var dt = ev.originalEvent.dataTransfer;    
    dt.setData('text/plain', $('#logo').parent().text());
    dt.setData('text/html', $('#logo').parent().html());
    dt.setData('text/uri-list', $('#logo')[0].src);

On the other end, getData() allows you to query for content by type (eg. text/html followed by text/plain). This, in turn, allows you to decide on acceptable content types at the time of the drop event or even during dragover to offer feedback for unacceptable types during the drag.

Here’s another example from the receiving end of the example code:

    var dt = ev.originalEvent.dataTransfer;    
    $('.content_url .content').text(dt.getData('text/uri-list'));
    $('.content_text .content').text(dt.getData('text/plain'));
    $('.content_html .content').html(dt.getData('text/html'));

Where dataTransfer really shines, though, is that it allows your drop targets to receive content from sources outside your defined draggable elements and even from outside the browser altogether. Firefox accepts such drags, and attempts to populate dataTransfer with appropriate content types extracted from the external object.

Thus, you could select some text in a word processor window and drop it into one of your elements, and at least expect to find it available as text/plain content.

You can also select content in another browser window, and expect to see text/html appear in your events. Check out the outline editing demo and see what happens when you try dragging various elements (eg. images, tables, and lists) and highlighted content from other windows onto the items there.

Using Drag Feedback Images

An important aspect of the drag and drop interaction is a representation of the thing being dragged. By default in Firefox, this is a “ghost” image of the dragged element itself. But, the dataTransfer property of the original Event object exposes the method setDragImage() for use in customizing this representation.

There’s a live demo of this feature, as well as associated JS code available. The gist, however, is sketched out in these code snippets:

    var dt = ev.originalEvent.dataTransfer;    
 
    dt.setDragImage( $('#feedback_image h2')[0], 0, 0);
 
    dt.setDragImage( $('#logo')[0], 32, 32); 
 
    var canvas = document.createElement("canvas");
    canvas.width = canvas.height = 50;
 
    var ctx = canvas.getContext("2d");
    ctx.lineWidth = 8;
    ctx.moveTo(25,0);
    ctx.lineTo(50, 50);
    ctx.lineTo(0, 50);
    ctx.lineTo(25, 0);
    ctx.stroke();
 
    dt.setDragImage(canvas, 25, 25);

You can supply a DOM node as the first parameter to setDragImage(), which includes everything from text to images to <canvas> elements. The second two parameters indicate at what left and top offset the mouse should appear in the image while dragging.

For example, since the #logo image is 64×64, the parameters in the second setDragImage() method places the mouse right in the center of the image. On the other hand, the first call positions the feedback image such that the mouse rests in the upper left corner.

Using Drop Effects

As mentioned at the start of this article, the drag and drop interaction has been used to support actions such as copying, moving, and linking. Accordingly, the HTML 5 specification accomodates these operations in the form of the effectAllowed and dropEffect properties exposed by the Event object.

For a quick fix, check out the a live demo of this feature, as well as the associated JS code.

The basic idea is that the dragstart event listener can set a value for effectAllowed like so:

    var dt = ev.originalEvent.dataTransfer;
    switch (ev.target.id) {
        case 'effectdrag0': dt.effectAllowed = 'copy'; break;
        case 'effectdrag1': dt.effectAllowed = 'move'; break;
        case 'effectdrag2': dt.effectAllowed = 'link'; break;
        case 'effectdrag3': dt.effectAllowed = 'all'; break;
        case 'effectdrag4': dt.effectAllowed = 'none'; break;
    }

The choices available for this property include the following:

none

no operation is permitted

copy

copy only

move

move only

link

link only

copyMove

copy or move only

copyLink

copy or link only

linkMove

link or move only

all

copy, move, or link

On the other end, the dragover event listener can set the value of the dropEffect property to indicate the expected effect invoked on a successful drop. If the value does not match up with effectAllowed, the drop will be considered cancelled on completion.

In the a live demo, you should be able to see that only elements with matching effects can be dropped into the appropriate drop zones. This is accomplished with code like the follwoing:

    var dt = ev.originalEvent.dataTransfer;
    switch (ev.target.id) {
        case 'effectdrop0': dt.dropEffect = 'copy'; break;
        case 'effectdrop1': dt.dropEffect = 'move'; break;
        case 'effectdrop2': dt.dropEffect = 'link'; break;
        case 'effectdrop3': dt.dropEffect = 'all'; break;
        case 'effectdrop4': dt.dropEffect = 'none'; break;
    }

Although the OS itself can provide some feedback, you can also use these properties to update your own visible feedback, both on the dragged element and on the drop zone itself.

Conclusion

The new first-class drag and drop events in HTML5 and Firefox make supporting this form of UI interaction simple, concise, and powerful in the browser. But beyond the new simplicity of these events, the ability to transfer content between applications opens brand new avenues for web-based applications and collaboration with desktop software in general.