Is not a custom tooltip... It a custom itemeditor in the datagrid. This itemeditor is a textinput with a validator.
When the text change the validator validate and show a red border with a tooltip... Something like this <mx:TextInput x="50" y="21" width="208" creationComplete="doInit()" > <![CDATA[ import mx.controls.dataGridClasses.DataGridListData; import mx.validators.EmailValidator; import mx.validators.DateValidator; import mx.core.UIComponent; import mx.managers.FocusManager; import mx.controls.Alert; import mx.events.ValidationResultEvent; import com.timeline.validators.HistoryDateValidator; import com.timeline.model.CustomDate public var validator:HistoryDateValidator= new HistoryDateValidator(); private function doInit():void { validator.source= this validator.property="text"; validator.addEventListener(ValidationResultEvent.VALID,handleResult) validator.addEventListener(ValidationResultEvent.INVALID,handleResult) validator.trigger=this; validator.triggerEvent ="change"; validator.validate(); } </mx:TextInput>