code: function (evt) {
          var propagate = true;
          this.dragging = false;
          if (this.checkModifiers(evt) &&
                 (OpenLayers.Event.isLeftClick(evt) ||
                  OpenLayers.Event.isSingleTouch(evt))) {
              this.started = true;
              this.start = evt.xy;
              this.last = evt.xy;
              OpenLayers.Element.addClass(
                  this.map.viewPortDiv, "olDragDown"
              );
              this.down(evt);
              this.callback("down", [evt.xy]);
  
              // prevent document dragging
              OpenLayers.Event.preventDefault(evt);
  
              if(!this.oldOnselectstart) {
                  this.oldOnselectstart = document.onselectstart ?
                      document.onselectstart : OpenLayers.Function.True;
              }
              document.onselectstart = OpenLayers.Function.False;
  
              propagate = !this.stopDown;
          } else {
              this.started = false;
              this.start = null;
              this.last = null;
          }
          return propagate;
      }
docs: Method: dragstart
  This private method is factorized from mousedown and touchstart methods
  
  Parameters:
  evt - {Event} The event
  
  Returns:
  {Boolean} Let the event propagate.