The source code

    /**
     * @class Ext.tree.TreeDragZone
     * @extends Ext.dd.DragZone
     * @constructor
     * @param {String/HTMLElement/Element} tree The {@link Ext.tree.TreePanel} for which to enable dragging
     * @param {Object} config
     */
    if(Ext.dd.DragZone){
    Ext.tree.TreeDragZone = function(tree, config){
        Ext.tree.TreeDragZone.superclass.constructor.call(this, tree.innerCt, config);
        /**
        * The TreePanel for this drag zone
        * @type Ext.tree.TreePanel
        * @property
        */
        this.tree = tree;
    };

    Ext.extend(Ext.tree.TreeDragZone, Ext.dd.DragZone, {
        /**
         * @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 'TreeDD').
         */
        ddGroup : "TreeDD",

        // private
        onBeforeDrag : function(data, e){
            var n = data.node;
            return n && n.draggable && !n.disabled;
        },

        // private
        onInitDrag : function(e){
            var data = this.dragData;
            this.tree.getSelectionModel().select(data.node);
            this.tree.eventModel.disable();
            this.proxy.update("");
            data.node.ui.appendDDGhost(this.proxy.ghost.dom);
            this.tree.fireEvent("startdrag", this.tree, data.node, e);
        },

        // private
        getRepairXY : function(e, data){
            return data.node.ui.getDDRepairXY();
        },

        // private
        onEndDrag : function(data, e){
            this.tree.eventModel.enable.defer(100, this.tree.eventModel);
            this.tree.fireEvent("enddrag", this.tree, data.node, e);
        },

        // private
        onValidDrop : function(dd, e, id){
            this.tree.fireEvent("dragdrop", this.tree, this.dragData.node, dd, e);
            this.hideProxy();
        },

        // private
        beforeInvalidDrop : function(e, id){
            // this scrolls the original position back into view
            var sm = this.tree.getSelectionModel();
            sm.clearSelections();
            sm.select(this.dragData.node);
        },

        // private
        afterRepair : function(){
            if (Ext.enableFx && this.tree.hlDrop) {
                Ext.Element.fly(this.dragData.ddel).highlight(this.hlColor || "c3daf9");
            }
            this.dragging = false;
        }
    });
    }