Dragging & Resizing CSS Transformed Elements

Question

If for instance, we set a -vendor-transform: rotate(40deg) css attribute on a rectangle <div>, all the sudden dragging and resizing becomes very weird and flawed.

Here is an example with a simple jQueryUI: http://jsfiddle.net/Ja4dY/1/

You will notice, that if you drag or resize that rectangle when transformed, it will jump up or down and the cursor will not remain in the correct place. In my real code I'm using custom code for resizing and dragging, however I encountered the same problems.

Well, of course the "problem" is that the direction of an Element will change. So left can be right, top gets bottom and something inbetween and the Javascript code still handles each direction as it would be not transformed.

So, the question: How can we compensate transformed / rotated Elements ?

Any good resources / books / blogs are also very welcome.

Answer 1

You can get the current transformation matrix that is applied to an element by using getComputedStyle(). You can use this to transform the current mouse position to its position in transformed space and see whether the click/drag events are within the element boundary and/or corners. Good resources for this:

http://www.useragentman.com/blog/2011/01/07/css3-matrix-transform-for-the-mathematically-challenged/

http://www.eleqtriq.com/2010/05/css-3d-matrix-transformations/

BTW, as you're experiencing, this is non-trivial to code. We had to do it for Sencha Animator, and it was a beast.

Answer 2

The problem is that functions that make elements draggable, wether using jQuery UI or not, relies heavily on the native getBoundingClientRect() function to figure out the position of the element etc.

When applying CSS3 transforms, like rotation, the values of getBoundingClientRect() or the equalent jQuery offset() function used in jQuery UI no longer works as expected, and the position of the mouse pointer gets messed up because the size of the element is suddenly wrong after it has been rotated.

To fix it you need to add some sort of helper function that recalculates the values, and there is a monkey patch available for this that works with jQuery UI's draggable.

It's hard to say anything about how to make the same patch work for custom code, but you'll probably have to integrate it in your custom function somehow, and it will take some coding on your part, and it's even harder to come up with something that works as a helper function out of the box for custom code one has not seen, and be aware that it is rather involved doing these calculations, see the code below :

function monkeyPatch_mouseStart() {
     var oldFn = $.ui.draggable.prototype._mouseStart ;
     $.ui.draggable.prototype._mouseStart = function(event) {

            var o = this.options;

           function getViewOffset(node) {
              var x = 0, y = 0, win = node.ownerDocument.defaultView || window;
              if (node) addOffset(node);
              return { left: x, top: y };

              function getStyle(node) {
                return node.currentStyle || // IE
                       win.getComputedStyle(node, '');
              }

              function addOffset(node) {
                var p = node.offsetParent, style, X, Y;
                x += parseInt(node.offsetLeft, 10) || 0;
                y += parseInt(node.offsetTop, 10) || 0;

                if (p) {
                  x -= parseInt(p.scrollLeft, 10) || 0;
                  y -= parseInt(p.scrollTop, 10) || 0;

                  if (p.nodeType == 1) {
                    var parentStyle = getStyle(p)
                      , localName   = p.localName
                      , parent      = node.parentNode;
                    if (parentStyle.position != 'static') {
                      x += parseInt(parentStyle.borderLeftWidth, 10) || 0;
                      y += parseInt(parentStyle.borderTopWidth, 10) || 0;

                      if (localName == 'TABLE') {
                        x += parseInt(parentStyle.paddingLeft, 10) || 0;
                        y += parseInt(parentStyle.paddingTop, 10) || 0;
                      }
                      else if (localName == 'BODY') {
                        style = getStyle(node);
                        x += parseInt(style.marginLeft, 10) || 0;
                        y += parseInt(style.marginTop, 10) || 0;
                      }
                    }
                    else if (localName == 'BODY') {
                      x += parseInt(parentStyle.borderLeftWidth, 10) || 0;
                      y += parseInt(parentStyle.borderTopWidth, 10) || 0;
                    }

                    while (p != parent) {
                      x -= parseInt(parent.scrollLeft, 10) || 0;
                      y -= parseInt(parent.scrollTop, 10) || 0;
                      parent = parent.parentNode;
                    }
                    addOffset(p);
                  }
                }
                else {
                  if (node.localName == 'BODY') {
                    style = getStyle(node);
                    x += parseInt(style.borderLeftWidth, 10) || 0;
                    y += parseInt(style.borderTopWidth, 10) || 0;

                    var htmlStyle = getStyle(node.parentNode);
                    x -= parseInt(htmlStyle.paddingLeft, 10) || 0;
                    y -= parseInt(htmlStyle.paddingTop, 10) || 0;
                  }

                  if ((X = node.scrollLeft)) x += parseInt(X, 10) || 0;
                  if ((Y = node.scrollTop))  y += parseInt(Y, 10) || 0;
                }
              }
            }

                this.helper = this._createHelper(event);
                this._cacheHelperProportions();

                if($.ui.ddmanager)
                    $.ui.ddmanager.current = this;

                this._cacheMargins();

                this.cssPosition = this.helper.css("position");
                this.scrollParent = this.helper.scrollParent();

            this.offset = this.positionAbs = getViewOffset(this.element[0]);
                this.offset = {
                    top: this.offset.top - this.margins.top,
                    left: this.offset.left - this.margins.left
                };

                $.extend(this.offset, {
                    click: {
                        left: event.pageX - this.offset.left,
                        top: event.pageY - this.offset.top
                    },
                    parent: this._getParentOffset(),
                    relative: this._getRelativeOffset()
                });

                this.originalPosition = this.position = this._generatePosition(event);
                this.originalPageX = event.pageX;
                this.originalPageY = event.pageY;

                (o.cursorAt && this._adjustOffsetFromHelper(o.cursorAt));

                if(o.containment)
                    this._setContainment();

                if(this._trigger("start", event) === false) {
                    this._clear();
                    return false;
                }

                this._cacheHelperProportions();

                if ($.ui.ddmanager && !o.dropBehaviour)
                    $.ui.ddmanager.prepareOffsets(this, event);

                this.helper.addClass("ui-draggable-dragging");
                this._mouseDrag(event, true);

                if ( $.ui.ddmanager ) $.ui.ddmanager.dragStart(this, event);
                return true;
     };
 }
monkeyPatch_mouseStart();

And here's a FIDDLE showing it working as expected with jQuery UI's draggable and resizeable !