Flex DataGrid with ComboBox itemRenderer

Question

I'm going spare trying to figure out the "correct" way to embed a ComboBox inside a Flex (3.4) DataGrid. By Rights (e.g. according to this page http://blog.flexmonkeypatches.com/2008/02/18/simple-datagrid-combobox-as-item-editor-example/) it should be easy, but I can't for the life of me make this work.

The difference I have to the example linked above is that my display value (what the user sees) is different to the id value I want to select on and store in my data provider.

So what I have is:

<mx:DataGridColumn headerText="Type" width="200" dataField="TransactionTypeID" editorDataField="value" textAlign="center" editable="true" rendererIsEditor="true">
    <mx:itemRenderer>
        <mx:Component>
            <mx:ComboBox dataProvider="{parentDocument.transactionTypesData}"/>
        </mx:Component>
    </mx:itemRenderer>
</mx:DataGridColumn>

Where transactionTypesData has both 'data' and 'label' fields (as per what the ComboBox - why on earth it doesn't provide both a labelField and idField I'll never know).

Anyway, the above MXML code doesn't work in two ways:

1. The combo box does not show up with any selected item.
2. After selecting an item, it does not store back that selected item to the datastore.

So, has anyone got a similar situation working?

Answer 1

While Jeff's answer is a partial answer for one approach for this (see http://flex.gunua.com/?p=119 for a complete example of this being used to good effect), it isn't as general as I wanted.

Thankfully, I finally found some great help on Experts Exchange (the answers by hobbit72) describes how to create a custom component that works in a grid as a ItemRenderer. I've extended that code to also support using the combo box as an ItemEditor as well. The full component is as follows:

<?xml version="1.0" encoding="utf-8"?>
<mx:ComboBox
    xmlns:mx="http://www.adobe.com/2006/mxml" 
    dataChange="setSelected()" 
    change="onSelectionChange(event)"
    focusEnabled="true">
    <mx:Script>
        <![CDATA[
            import mx.events.DataGridEvent;
            import mx.events.ListEvent;
            import mx.controls.dataGridClasses.DataGridListData;

            private var _ownerData:Object;
            private var _lookupField:String = "value";

            // When using this component as an itemEditor rather than an itemRenderer
            // then set ' editorDataField="selectedItemKey"' on the column to 
            // ensure that changes to the ComboBox are propogated.
            [Bindable] public var selectedItemKey:Object;

            public function set lookupField (value:String) : void {
                if(value) {
                    _lookupField = value;
                    setSelected();
                }
            }           
            override public function set data (value:Object) : void {
                if(value) {                    
                    _ownerData = value;
                    setSelected();
                }
            }
            override public function get data() : Object {
                return _ownerData;
            }            
            private function setSelected() : void {
                if (dataProvider && _ownerData) {
                    var col:DataGridListData = DataGridListData(listData);
                    for each (var dp:Object in dataProvider) {
                        if (dp[_lookupField] == _ownerData[col.dataField]) {
                            selectedItem = dp;
                            selectedItemKey = _ownerData[col.dataField];
                            return;     
                        }
                    }                    
                }
                selectedItem = null;
            }
            private function onSelectionChange (e:ListEvent) : void {
                if (selectedItem && _ownerData) {                    
                    var col:DataGridListData = DataGridListData(listData);
                    _ownerData[col.dataField] = selectedItem[_lookupField];
                    selectedItemKey = selectedItem[_lookupField];
                }
            }                   
        ]]>
    </mx:Script>    
</mx:ComboBox> 

Using this component is straight forward. As an ItemRenderer:

<mx:DataGridColumn headerText="Child" dataField="PersonID" editable="false" textAlign="center">
  <mx:itemRenderer>
    <mx:Component>
      <fx:GridComboBox dataProvider="{parentDocument.childrenData}" labelField="Name" lookupField="PersonID" change="dispatchEvent(new mx.events.DataGridEvent(mx.events.DataGridEvent.ITEM_FOCUS_OUT, true, true))"/>
    </mx:Component>
  </mx:itemRenderer>                      
</mx:DataGridColumn>

Using this component is straight forward. And as an ItemEditor:

<mx:DataGridColumn labelFunction="lookupChildName" headerText="Child" dataField="PersonID" editable="true" editorDataField="selectedItemKey">
    <mx:itemEditor>
        <mx:Component>
            <fx:GridComboBox dataProvider="{parentDocument.childrenData}" labelField="Name" lookupField="PersonID" change="dispatchEvent(new mx.events.DataGridEvent(mx.events.DataGridEvent.ITEM_FOCUS_OUT, true, true))"/>
        </mx:Component>
     </mx:itemEditor>      
</mx:DataGridColumn>

Note that when using it as an ItemEditor, a custom labelFunction (that looks up the Name from the PersonID in my case) must be used, otherwise you only see the key in the grid when the field isn't being edited (not a problem if your keys/values are the same).

Note that in my case, I wanted the item focus out event to propogate up to provide immediate feedback to the user (my DataGrid has itemFocusOut="handleChange()"), hence the change event creating an ITEM_FOCUS_OUT event.

Note that there are probably simpler ways to have a ComboBox as an ItemEditor when you don't mind the ComboBox only shown when the user clicks on the cell to edit. The approach I wanted was a generic way to show a combo box in a DataGrid for all rows, and being editable and with decent event propogation.