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Detecting drag-and-drop support


From:

HTML5: Drag and Drop in Depth

with Bill Weinman

Video: Detecting drag-and-drop support

One of the first things you're going to want to do when coding for HTML5 drag and drop is detect whether or not the browser supports the API. Because the API for HTML5 drag and drop is event-based there is no specific window object method to test for in order to detect support for drag and drop. So instead we have to test for the draggable property, which should be available on every element. So we come over here to our text editor and I'm going to scroll down here and show you this whole file so that you've got that, and there it is.
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Watch the Online Video Course HTML5: Drag and Drop in Depth
1h 0m Intermediate Jul 26, 2011

Viewers: in countries Watching now:

Join Bill Weinman as he shows how to make just about any web page element draggable with a combination of JavaScript and HTML5, a technique that has increased browser support and that eliminates the need for external libraries such as jQuery. The course covers how to detect drag-and-drop support in the user's browser, code a simple event listener, use a drop zone, and even receive dropped objects without a drop zone. A simple working example game, a practical implementation of drag-and-drop that can be applied to almost any web site, is also demonstrated.

Topics include:
  • Exploring the HTML/Javascript/CSS/DOM relationship
  • Detecting drag and drop support
  • Using the HTML5 draggable and dropzone attributes and onDragStart event
  • Exploring different events
  • Receiving drops with or without a drop zone
  • Creating a simple game
Subjects:
Developer Web
Software:
HTML
Author:
Bill Weinman

Detecting drag-and-drop support

One of the first things you're going to want to do when coding for HTML5 drag and drop is detect whether or not the browser supports the API. Because the API for HTML5 drag and drop is event-based there is no specific window object method to test for in order to detect support for drag and drop. So instead we have to test for the draggable property, which should be available on every element. So we come over here to our text editor and I'm going to scroll down here and show you this whole file so that you've got that, and there it is.

It's pretty small and straight-forward. You'll notice here this function detectDragAndDrop, and this is a little bit more complicated than it otherwise would need to be. You'll notice this line down here, if ('draggable' in document.createElement('span')) return true, and so what that does is it tests if when you create an element called span, which would be a normal span element for an HTML document, if the resulting object has a property called draggable. And this ought to tell us whether or not HTML5 drag and drop is supported.

Unfortunately, Microsoft Internet Explorer, which ironically is the original implementation of this drag-and- drop API, does not support the draggable attribute in elements, and so when you create that element it doesn't have draggable in it. And so that doesn't work for Microsoft Internet Explorer. And so what we've had to do is test first if this is an Internet Explorer browser, and if it is then check the user agent string and see whether or not the version is higher than 6.0 or not. And the drag and drop is available in Internet Explorer beginning with version 6.0.

And so if it's not Internet Explorer, then we'll go ahead and do this test anyway and that'll catch all of the other ones like Chrome and Safari and Firefox and so this is just a little bit more complicated than normally we would expect. Normally I am going to say it is a bad idea to detect support for something by looking at the user agent string and seeing whether or not it's the browser you think it is. User agent strings in practice can be easily spoofed and actually are commonly spoofed, but in this case there's just no simple way to do this, so here it is.

This is the way that we need to do it for this particular future. And so I'm going to go ahead, I am going to take this detecting.html and I'm going to open it up in Chrome, and you'll see it says using HTML5 drag and drop, and if I go back here in and I open it up in Firefox it'll do the same thing. It says using HTML5 drag and drop and that will be true even in Internet Explorer. I am going to go ahead and open it in Internet Explorer and in Explorer here you see it says using HTML5 drag and drop. But then when we open it in Opera, which we know does not support this, it says this browser does not support drag and drop.

And that if you remember is a string down here. Here we're calling this detectDragAndDrop function and we have a status message that says "Using HTML5 drag and drop" if it is supported and a status message that says "This browser does not support drag and drop" where it doesn't. So detecting support for HTML5 drag and drop is a little more complicated than detecting support for most of the other HTML5 APIs. Fortunately, the good news is that HTML5 drag and drop is supported by most modern browsers.

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