Flex dataGrid 中TAB键 定位有关问题

<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
                layout="absolute"
                height="700"
                verticalScrollPolicy="on" creationComplete="init()">
    <mx:Script>
        <![CDATA[
            import mx.controls.ComboBox;
            import mx.controls.Alert;
            import mx.events.*;
            import mx.events.AdvancedDataGridEvent;
            import mx.collections.ArrayCollection;
            import mx.controls.dataGridClasses.DataGridColumn;

            [Bindable]
            private var data1:ArrayCollection=new ArrayCollection([{xh: 1}, {xh: 2}, {xh: 3}, {xh: 4}, {xh: 5}, {xh: 6}]);
            
            private function init():void
            {
                dg1.addEventListener(FocusEvent.KEY_FOCUS_CHANGE,keyFocusChangeHandler);
            }

            private function itemEditBeginHandler(event:AdvancedDataGridEvent):void
            {
                var rowIndex:int=event.rowIndex;
                var colIndex:int=event.columnIndex;
                var record:Object=data1.getItemAt(rowIndex);

                if (colIndex == 2)
                {
                    event.stopImmediatePropagation();
                }

            }
            private function keydownDataGrid(event:KeyboardEvent):void
            {
                try{
                    var position:Object = dg1.editedItemPosition;
                    var offsetRow:int = 0;
                    var offsetCol:int = 0;
                    //tab 
                    if(event.keyCode == Keyboard.TAB){
                        if (event.shiftKey) {
                            offsetCol = -1;
                        } else {
                             offsetCol = 1;
                        }
                    } else if (event.keyCode == Keyboard.UP){
                        offsetRow = -1;
                    } else if (event.keyCode == Keyboard.DOWN){
                        offsetRow = 1;
                    } else if (event.keyCode == Keyboard.LEFT){
                        offsetCol = -1;
                    } else if (event.keyCode == Keyboard.RIGHT){
                        offsetCol = 1;
                    } else {
                        return;
                    }
                    dg1.editedItemPosition = findNextEditableCell(position,offsetRow,offsetCol);
                } catch(e:Error) {
                    trace(e.message);
                }
            }

              private function findNextEditableCell(v:Object,offsetRow:int=0,offsetCol:int=0):Object
            {
                if (v == null || 
                    (offsetRow == 0 && offsetCol == 0) ||
                    (offsetRow != 0 && offsetCol != 0)) {
                    return null;
                }
                var loopCount:int = 0;
                var dataEditable:Boolean = false;
                while (!dataEditable && (loopCount < 2)) {
                    if (offsetCol != 0) {
                        if ((v.columnIndex + offsetCol) < dg1.columns.length &&
                            (v.columnIndex + offsetCol) >= 0) {
                             v.columnIndex = v.columnIndex + offsetCol;
                        } else {
                            if ((v.rowIndex + offsetCol) < dg1.dataProvider.length &&
                                (v.rowIndex + offsetCol) >= 0) {
                                
                                v.rowIndex = v.rowIndex + offsetCol;
                                if (offsetCol > 0) {
                                    v.columnIndex = 0;
                                } else {
                                    v.columnIndex = dg1.columns.length -1;
                                }
                            } else {
                                if (offsetCol > 0) {
                                    v.columnIndex = 0;
                                    v.rowIndex = 0;
                                } else {
                                    v.columnIndex = dg1.columns.length -1;
                                    v.rowIndex = dg1.dataProvider.length -1;
                                }
                                loopCount ++;
                            }
                        }
                    } else {
                        if ((v.rowIndex + offsetRow) < dg1.dataProvider.length &&
                            (v.rowIndex + offsetRow) >= 0) {
                             v.rowIndex = v.rowIndex + offsetRow;
                        } else {
                            if (offsetRow > 0) {
                                v.rowIndex = 0;
                            } else {
                                v.rowIndex = dg1.dataProvider.length -1;
                            }
                            loopCount ++;
                        }
                    }
                    if (dg1.columns[v.columnIndex] != null &&
                        dg1.columns[v.columnIndex].editable) {
                        
                        if (v.columnIndex != 2) {
                            dataEditable = true;
                        } else {
                            dataEditable = false;
                        }
                    }
                }
                return v;
            }

            private function keyFocusChangeHandler(event:FocusEvent):void
            {
                if (event.keyCode == Keyboard.TAB)
                {
                    event.preventDefault();
                }
            }
        ]]>
    </mx:Script>

    <mx:Canvas verticalScrollBarStyleName="hh"
               height="614"
               width="831.5"
               horizontalCenter="-1"
               verticalCenter="0"
               verticalScrollPolicy="auto">

        <mx:AdvancedDataGrid id="dg1"
                             designViewDataType="flat"
                             sortExpertMode="true"
                             sortableColumns="false"
                             rowCount="5"
                             dataProvider="{data1}"
                             height="441"
                             width="811.5"
                             fontSize="14"
                             fontFamily="Times New Roman"
                             editable="true"
                             enabled="true"
                             x="10"
                             y="55"
                             itemEditBegin="itemEditBeginHandler(event)"
                             keyDown="keydownDataGrid(event)">
                             

            <mx:groupedColumns>
                <mx:AdvancedDataGridColumn headerText="序号"
                                           dataField="xh"
                                           width="150"
                                           editable="false"/>
                <mx:AdvancedDataGridColumn headerText="COLUMN1"
                                           dataField="col1"
                                           width="200"/>
                <mx:AdvancedDataGridColumn headerText="COLUMN2"
                                           dataField="col2"
                                           width="200"/>
                <mx:AdvancedDataGridColumn headerText="COLUMN3"
                                           dataField="col3"
                                           width="200"/>
                <mx:AdvancedDataGridColumn headerText="COLUMN4"
                                           dataField="col4"
                                           width="200"/>
                <mx:AdvancedDataGridColumn headerText="COLUMN5"
                                           dataField="col5"
                                           width="200"/>

            </mx:groupedColumns>
        </mx:AdvancedDataGrid>

    </mx:Canvas>

</mx:Application>