The source code

    /**
     * @class Ext.dd.DragSource
     * @extends Ext.dd.DDProxy
     * A simple class that provides the basic implementation needed to make any element draggable.
     * @constructor
     * @param {Mixed} el The container element
     * @param {Object} config
     */
    Ext.dd.DragSource = function(el, config){
        this.el = Ext.get(el);
        if(!this.dragData){
            this.dragData = {};
        }

        Ext.apply(this, config);

        if(!this.proxy){
            this.proxy = new Ext.dd.StatusProxy();
        }
        Ext.dd.DragSource.superclass.constructor.call(this, this.el.dom, this.ddGroup || this.group,
              {dragElId : this.proxy.id, resizeFrame: false, isTarget: false, scroll: this.scroll === true});

        this.dragging = false;
    };

    Ext.extend(Ext.dd.DragSource, Ext.dd.DDProxy, {
        /**
         * @cfg {String} ddGroup
         * A named drag drop group to which this object belongs.  If a group is specified, then this object will only
         * interact with other drag drop objects in the same group (defaults to undefined).
         */
        /**
         * @cfg {String} dropAllowed
         * The CSS class returned to the drag source when drop is allowed (defaults to "x-dd-drop-ok").
         */
        dropAllowed : "x-dd-drop-ok",
        /**
         * @cfg {String} dropNotAllowed
         * The CSS class returned to the drag source when drop is not allowed (defaults to "x-dd-drop-nodrop").
         */
        dropNotAllowed : "x-dd-drop-nodrop",

        /**
         * Returns the data object associated with this drag source
         * @return {Object} data An object containing arbitrary data
         */
        getDragData : function(e){
            return this.dragData;
        },

        // private
        onDragEnter : function(e, id){
            var target = Ext.dd.DragDropMgr.getDDById(id);
            this.cachedTarget = target;
            if(this.beforeDragEnter(target, e, id) !== false){
                if(target.isNotifyTarget){
                    var status = target.notifyEnter(this, e, this.dragData);
                    this.proxy.setStatus(status);
                }else{
                    this.proxy.setStatus(this.dropAllowed);
                }

                if(this.afterDragEnter){
                    /**
                     * An empty function by default, but provided so that you can perform a custom action
                     * when the dragged item enters the drop target by providing an implementation.
                     * @param {Ext.dd.DragDrop} target The drop target
                     * @param {Event} e The event object
                     * @param {String} id The id of the dragged element
                     * @method afterDragEnter
                     */
                    this.afterDragEnter(target, e, id);
                }
            }
        },

        /**
         * An empty function by default, but provided so that you can perform a custom action
         * before the dragged item enters the drop target and optionally cancel the onDragEnter.
         * @param {Ext.dd.DragDrop} target The drop target
         * @param {Event} e The event object
         * @param {String} id The id of the dragged element
         * @return {Boolean} isValid True if the drag event is valid, else false to cancel
         */
        beforeDragEnter : function(target, e, id){
            return true;
        },

        // private
        alignElWithMouse: function() {
            Ext.dd.DragSource.superclass.alignElWithMouse.apply(this, arguments);
            this.proxy.sync();
        },

        // private
        onDragOver : function(e, id){
            var target = this.cachedTarget || Ext.dd.DragDropMgr.getDDById(id);
            if(this.beforeDragOver(target, e, id) !== false){
                if(target.isNotifyTarget){
                    var status = target.notifyOver(this, e, this.dragData);
                    this.proxy.setStatus(status);
                }

                if(this.afterDragOver){
                    /**
                     * An empty function by default, but provided so that you can perform a custom action
                     * while the dragged item is over the drop target by providing an implementation.
                     * @param {Ext.dd.DragDrop} target The drop target
                     * @param {Event} e The event object
                     * @param {String} id The id of the dragged element
                     * @method afterDragOver
                     */
                    this.afterDragOver(target, e, id);
                }
            }
        },

        /**
         * An empty function by default, but provided so that you can perform a custom action
         * while the dragged item is over the drop target and optionally cancel the onDragOver.
         * @param {Ext.dd.DragDrop} target The drop target
         * @param {Event} e The event object
         * @param {String} id The id of the dragged element
         * @return {Boolean} isValid True if the drag event is valid, else false to cancel
         */
        beforeDragOver : function(target, e, id){
            return true;
        },

        // private
        onDragOut : function(e, id){
            var target = this.cachedTarget || Ext.dd.DragDropMgr.getDDById(id);
            if(this.beforeDragOut(target, e, id) !== false){
                if(target.isNotifyTarget){
                    target.notifyOut(this, e, this.dragData);
                }
                this.proxy.reset();
                if(this.afterDragOut){
                    /**
                     * An empty function by default, but provided so that you can perform a custom action
                     * after the dragged item is dragged out of the target without dropping.
                     * @param {Ext.dd.DragDrop} target The drop target
                     * @param {Event} e The event object
                     * @param {String} id The id of the dragged element
                     * @method afterDragOut
                     */
                    this.afterDragOut(target, e, id);
                }
            }
            this.cachedTarget = null;
        },

        /**
         * An empty function by default, but provided so that you can perform a custom action before the dragged
         * item is dragged out of the target without dropping, and optionally cancel the onDragOut.
         * @param {Ext.dd.DragDrop} target The drop target
         * @param {Event} e The event object
         * @param {String} id The id of the dragged element
         * @return {Boolean} isValid True if the drag event is valid, else false to cancel
         */
        beforeDragOut : function(target, e, id){
            return true;
        },

        // private
        onDragDrop : function(e, id){
            var target = this.cachedTarget || Ext.dd.DragDropMgr.getDDById(id);
            if(this.beforeDragDrop(target, e, id) !== false){
                if(target.isNotifyTarget){
                    if(target.notifyDrop(this, e, this.dragData)){ // valid drop?
                        this.onValidDrop(target, e, id);
                    }else{
                        this.onInvalidDrop(target, e, id);
                    }
                }else{
                    this.onValidDrop(target, e, id);
                }

                if(this.afterDragDrop){
                    /**
                     * An empty function by default, but provided so that you can perform a custom action
                     * after a valid drag drop has occurred by providing an implementation.
                     * @param {Ext.dd.DragDrop} target The drop target
                     * @param {Event} e The event object
                     * @param {String} id The id of the dropped element
                     * @method afterDragDrop
                     */
                    this.afterDragDrop(target, e, id);
                }
            }
            delete this.cachedTarget;
        },

        /**
         * An empty function by default, but provided so that you can perform a custom action before the dragged
         * item is dropped onto the target and optionally cancel the onDragDrop.
         * @param {Ext.dd.DragDrop} target The drop target
         * @param {Event} e The event object
         * @param {String} id The id of the dragged element
         * @return {Boolean} isValid True if the drag drop event is valid, else false to cancel
         */
        beforeDragDrop : function(target, e, id){
            return true;
        },

        // private
        onValidDrop : function(target, e, id){
            this.hideProxy();
            if(this.afterValidDrop){
                /**
                 * An empty function by default, but provided so that you can perform a custom action
                 * after a valid drop has occurred by providing an implementation.
                 * @param {Object} target The target DD
                 * @param {Event} e The event object
                 * @param {String} id The id of the dropped element
                 * @method afterValidDrop
                 */
                this.afterValidDrop(target, e, id);
            }
        },

        // private
        getRepairXY : function(e, data){
            return this.el.getXY();
        },

        // private
        onInvalidDrop : function(target, e, id){
            this.beforeInvalidDrop(target, e, id);
            if(this.cachedTarget){
                if(this.cachedTarget.isNotifyTarget){
                    this.cachedTarget.notifyOut(this, e, this.dragData);
                }
                this.cacheTarget = null;
            }
            this.proxy.repair(this.getRepairXY(e, this.dragData), this.afterRepair, this);

            if(this.afterInvalidDrop){
                /**
                 * An empty function by default, but provided so that you can perform a custom action
                 * after an invalid drop has occurred by providing an implementation.
                 * @param {Event} e The event object
                 * @param {String} id The id of the dropped element
                 * @method afterInvalidDrop
                 */
                this.afterInvalidDrop(e, id);
            }
        },

        // private
        afterRepair : function(){
            if(Ext.enableFx){
                this.el.highlight(this.hlColor || "c3daf9");
            }
            this.dragging = false;
        },

        /**
         * An empty function by default, but provided so that you can perform a custom action after an invalid
         * drop has occurred.
         * @param {Ext.dd.DragDrop} target The drop target
         * @param {Event} e The event object
         * @param {String} id The id of the dragged element
         * @return {Boolean} isValid True if the invalid drop should proceed, else false to cancel
         */
        beforeInvalidDrop : function(target, e, id){
            return true;
        },

        // private
        handleMouseDown : function(e){
            if(this.dragging) {
                return;
            }
            var data = this.getDragData(e);
            if(data && this.onBeforeDrag(data, e) !== false){
                this.dragData = data;
                this.proxy.stop();
                Ext.dd.DragSource.superclass.handleMouseDown.apply(this, arguments);
            }
        },

        /**
         * An empty function by default, but provided so that you can perform a custom action before the initial
         * drag event begins and optionally cancel it.
         * @param {Object} data An object containing arbitrary data to be shared with drop targets
         * @param {Event} e The event object
         * @return {Boolean} isValid True if the drag event is valid, else false to cancel
         */
        onBeforeDrag : function(data, e){
            return true;
        },

        /**
         * An empty function by default, but provided so that you can perform a custom action once the initial
         * drag event has begun.  The drag cannot be canceled from this function.
         * @param {Number} x The x position of the click on the dragged object
         * @param {Number} y The y position of the click on the dragged object
         */
        onStartDrag : Ext.emptyFn,

        // private override
        startDrag : function(x, y){
            this.proxy.reset();
            this.dragging = true;
            this.proxy.update("");
            this.onInitDrag(x, y);
            this.proxy.show();
        },

        // private
        onInitDrag : function(x, y){
            var clone = this.el.dom.cloneNode(true);
            clone.id = Ext.id(); // prevent duplicate ids
            this.proxy.update(clone);
            this.onStartDrag(x, y);
            return true;
        },

        /**
         * Returns the drag source's underlying {@link Ext.dd.StatusProxy}
         * @return {Ext.dd.StatusProxy} proxy The StatusProxy
         */
        getProxy : function(){
            return this.proxy;
        },

        /**
         * Hides the drag source's {@link Ext.dd.StatusProxy}
         */
        hideProxy : function(){
            this.proxy.hide();
            this.proxy.reset(true);
            this.dragging = false;
        },

        // private
        triggerCacheRefresh : function(){
            Ext.dd.DDM.refreshCache(this.groups);
        },

        // private - override to prevent hiding
        b4EndDrag: function(e) {
        },

        // private - override to prevent moving
        endDrag : function(e){
            this.onEndDrag(this.dragData, e);
        },

        // private
        onEndDrag : function(data, e){
        },

        // private - pin to cursor
        autoOffset : function(x, y) {
            this.setDelta(-12, -20);
        },

        destroy: function(){
            Ext.dd.DragSource.superclass.destroy.call(this);
            Ext.destroy(this.proxy);
        }
    });