Flex Datagrid labelFunction query

Question

Main.mxl

<s:DataGrid dataProvider="{employeeData}"> // employeeData is an Arraycollection with predefined data


        <s:typicalItem>
            <s:DataItem firstName="Christopher" 
                        lastName="Winchester" 
                        hireDate="22/12/2013"/>
        </s:typicalItem>


        <s:columns>

            <s:ArrayList>

                <s:GridColumn labelFunction="employeeName"
                              headerText="Name"/>

                <s:GridColumn dataField="hireDate"
                              headerText="Hire Date"
                              labelFunction="dateFormat"/>
            </s:ArrayList>

        </s:columns>


    </s:DataGrid>   


<fx:Script>
        <![CDATA[
            import mx.collections.ArrayCollection;
            import mx.controls.dataGridClasses.DataGridColumn;
            import mx.rpc.events.ResultEvent;

            [Bindable]
            private var employeeData: ArrayCollection; 


            private function employeeName(item: Object, column: GridColumn): String
            {
                return item.firstName+" "+item.lastName;
            }


        ]]>
    </fx:Script>

A) Can anyone please explain me how does Datagrid internally works with employeeName function? I mean, Iam not even passing 2 parameters for labelFunction, BUT still how does it get called?

B) Why should I use s:ArrayList tag inside s:columns tag?

Answer 1

A) Can anyone please explain me how does Datagrid internally works with employeeName function? I mean, Iam not even passing 2 parameters for labelFunction, BUT still how does it get called?

The labelFunction is a property on the GridColumn class. Inside the Gridcolumn there is an itemToString() function which is used to determine what the label should be for that specific instance of the column. right out of the framework code:

/**
 *  @private
 *  Common logic for itemToLabel(), itemToDataTip().   Logically this code is
 *  similar to (not the same as) LabelUtil.itemToLabel().
 */
private function itemToString(item:Object, labelPath:Array, labelFunction:Function, formatter:IFormatter):String
{
    if (!item)
        return ERROR_TEXT;

    if (labelFunction != null)
        return labelFunction(item, this);

    var itemString:String = null;
    try 
    {
        var itemData:Object = item;
        for each (var pathElement:String in labelPath)
            itemData = itemData[pathElement];

        if ((itemData != null) && (labelPath.length > 0))
            itemString = (formatter) ? formatter.format(itemData) : itemData.toString();
    }
    catch(ignored:Error)
    {
    }        

    return (itemString != null) ? itemString : ERROR_TEXT;
}

B) Why should I use s:ArrayList tag inside s:columns tag?

Because the data type of the columns property on the DataGrid is an IList; and the ArrayList implements the IList interface. What you're looking at is the MXML way to create and define an ArrayList. You'd use a slightly different approach if you wanted to create the columns in ActionScript.

Answer 2

To answer your first question: the "labelFunction" property is a reference to the function that will be invoked by the DataGrid to format the text of a cell in a column. The function will be called dynamically and the DataGrid will pass in the required parameters. The DataGrid expects a label function to always have this signature. If you fail to do so, you will get a runtime error.

Technically, a function can be called dynamically with the following syntax:

var anObject:MyType;
var methodName:String = "myMethod";

anObject[methodName](param1, param2);

or if you have a Function object

var myFunction:Function;
myFunction(param1, param2);