Philip Walton Philip Walton - 1 month ago 14
Javascript Question

How to detect the dragleave event in Firefox when dragging outside the window

Firefox doesn't properly trigger the dragleave event when dragging outside of the window:

https://bugzilla.mozilla.org/show_bug.cgi?id=665704

https://bugzilla.mozilla.org/show_bug.cgi?id=656164

I'm trying to develop a workaround for this (which I know is possible because Gmail is doing it), but the only thing I can come up with seems really hackish.

One way of knowing when dragging outside the window has occurred it to wait for the

dragover
event to stop firing (because
dragover
fires constantly during a drag and drop operation). Here's how I'm doing that:

var timeout;

function dragleaveFunctionality() {
// do stuff
}

function firefoxTimeoutHack() {
clearTimeout(timeout);
timeout = setTimeout(dragleaveFunctionality, 200);
}

$(document).on('dragover', firefoxTimeoutHack);


This code is essentially creating and clearing a timeout over and over again. The 200 millisecond timeout will not be reached unless the
dragover
event stops firing.

While this works, I don't like the idea of using a timeout for this purpose. It feels wrong. It also means there's a slight lag before the "dropzone" styling goes away.

The other idea I had was to detect when the mouse leaves the window, but the normal ways of doing that don't seem to work during drag and drop operations.

Does anyone out there have a better way of doing this?

UPDATE:

Here's the code I am using:

<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Drag and Drop Issue</title>
<script src="http://code.jquery.com/jquery.js"></script>
</head>
<body>
Open up the console and look at what number is reporting when dragging files in and out of the window. The number should always be 0 when leaving the window, but in Firefox it's not.
<script type="text/javascript">
$(function() {
var counter = 0;
$(document).on('dragenter', function(e) {
counter += 1;
console.log(counter, e.target);
});
$(document).on('dragleave', function(e) {
counter -= 1;
console.log(counter, e.target);
});
});
</script>
</body>
</html>

Answer

I've found a solution. The problem was not so much that the dragleave event wasn't firing; rather, the dragenter event was firing twice when first dragging a file into the window (and additionally sometimes when dragging over certain elements). My original solution was to use a counter to track when the final dragleave event was occuring, but the double firing of dragenter events was messing up the count. (Why couldn't I just listen for dragleave you ask? Well, because dragleave functions very similarly to mouseout in that it fires not only when leaving the element but also when entering a child element. Thus, when dragleave fires, your mouse may very well still be within the bounds of the original element.)

The solution I came up with was to keep track of which elements dragenter and dragleave had been triggered on. Since events propagate up to the document, listening for dragenter and dragleave on a particular element will capture not only events on that element but also events on its children.

So, I created a jQuery collection $() to keep track of what events were fired on what elements. I added the event.target to the collection whenever dragenter was fired, and I removed event.target from the collection whenever dragleave happened. The idea was that if the collection were empty it would mean I had actually left the original element because if I were entering a child element instead, at least one element (the child) would still be in the jQuery collection. Lastly, when the drop event is fired, I want to reset the collection to empty, so it's ready to go when the next dragenter event occurs.

jQuery also saves a lot of extra work because it automatically does duplicate checking, so event.target doesn't get added twice, even when Firefox was incorrectly double-invoking dragenter.

Phew, anyway, here's a basic version of the code I ended up using. I've put it into a simple jQuery plugin if anyone else is interested in using it. Basically, you call .draghover on any element, and draghoverstart is triggered when first dragging into the element, and draghoverend is triggered once the drag has actually left it.

// The plugin code
$.fn.draghover = function(options) {
  return this.each(function() {

    var collection = $(),
        self = $(this);

    self.on('dragenter', function(e) {
      if (collection.length === 0) {
        self.trigger('draghoverstart');
      }
      collection = collection.add(e.target);
    });

    self.on('dragleave drop', function(e) {
      collection = collection.not(e.target);
      if (collection.length === 0) {
        self.trigger('draghoverend');
      }
    });
  });
};

// Now that we have a plugin, we can listen for the new events 
$(window).draghover().on({
  'draghoverstart': function() {
    console.log('A file has been dragged into the window.');
  },
  'draghoverend': function() {
    console.log('A file has been dragged out of window.');
  }
});
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