How to set a Property in Wix - wix

Question

I'm confused about the correct procedure for setting a Property in Wix. Is it necessary to use an Event? Are there existing, built-in, events and can they be tailored to custom Wix Properties? What I'm trying to do (with Wix 3.0) is install a IIS Virtual Directory using a custom UI. The problem here is that the property I'm attempting to set the virtual directory name with is not being over-written from the value provided in the interface. The other property here used for the installation path is being over-written and it looks like the Event="SetTargetPath" is what is taking care of that, am I wrong or missing something with this installation UI? Thanks much for your help.
<Directory Id="TARGETDIR" Name="SourceDir">
<Directory Id="ProgramFilesFolder" Name="PFiles">
<Directory Id="MyCo" Name="MyCo">
<Directory Id="FILEINSTALLDIR" Name="MyCo Admin">
<Component Id="MyCo_AdminComponent" Guid="########-####-####-####-############">
<File Source="Default.aspx" Vital="yes" KeyPath="yes" DiskId="1"/>
</Component>
</Directory>
</Directory>
</Directory>
<Component Id='ADMINVIRTUALDIRCOMP' Guid='########-####-####-####-############' Permanent='no'>
<iis:WebVirtualDir Id='VIRTUALDIRECTORY' Alias='[WIXUI_VIRTUALDIR]' Directory='FILEINSTALLDIR' WebSite='DefaultWebSite'>
<iis:WebApplication Id='ADMINWEBAPPLICATION' Name='[WIXUI_VIRTUALDIR]' />
</iis:WebVirtualDir>
</Component>
</Directory>
<iis:WebSite Id='DefaultWebSite' Description='Default Web Site'>
<iis:WebAddress Id='AllUnassigned' Port='80' />
</iis:WebSite>
<Feature Id="ProductFeature" Title="MyCo WebApp" Level="1">
<ComponentRef Id="MyCo_AdminComponent" />
<ComponentRef Id="ADMINVIRTUALDIRCOMP" />
</Feature>
<UIRef Id="WixUI_Common" />
<UIRef Id="WixUI_InstallWeb" />
<Property Id="WIXUI_INSTALLDIR" Value="FILEINSTALLDIR" />
<Property Id="WIXUI_VIRTUALDIR" Value ="VIRTUALDIR" ></Property>
<Property Id="VIRTUALDIR" ><![CDATA[AdminCompName]]></Property>
<UI Id="WixUI_InstallWeb">
<TextStyle Id="WixUI_Font_Normal" FaceName="Tahoma" Size="8" />
<TextStyle Id="WixUI_Font_Bigger" FaceName="Tahoma" Size="12" />
<TextStyle Id="WixUI_Font_Title" FaceName="Tahoma" Size="9" Bold="yes" />
<Property Id="DefaultUIFont" Value="WixUI_Font_Normal" />
<Property Id="WixUI_Mode" Value="InstallDir" />
<DialogRef Id="BrowseDlg" />
<DialogRef Id="DiskCostDlg" />
<DialogRef Id="ErrorDlg" />
<DialogRef Id="FatalError" />
<DialogRef Id="FilesInUse" />
<DialogRef Id="MsiRMFilesInUse" />
<DialogRef Id="PrepareDlg" />
<DialogRef Id="ProgressDlg" />
<DialogRef Id="ResumeDlg" />
<DialogRef Id="UserExit" />
<Publish Dialog="BrowseDlg" Control="OK" Event="DoAction" Value="WixUIValidatePath" Order="3">1</Publish>
<Publish Dialog="BrowseDlg" Control="OK" Event="SpawnDialog" Value="InvalidDirDlg" Order="4">
<![CDATA[WIXUI_INSTALLDIR_VALID<>"1"]]>
</Publish>
<Publish Dialog="ExitDialog" Control="Finish" Event="EndDialog" Value="Return" Order="999">1</Publish>
<Publish Dialog="WelcomeDlg" Control="Next" Event="NewDialog" Value="InstallWebDlg">1</Publish>
<Publish Dialog="InstallWebDlg" Control="Back" Event="NewDialog" Value="WelcomeDlg">1</Publish>
<Publish Dialog="InstallWebDlg" Control="Next" Event="SetTargetPath" Value="[WIXUI_INSTALLDIR]" Order="1">1</Publish>
<Publish Dialog="InstallWebDlg" Control="Next" Event="DoAction" Value="WixUIValidatePath" Order="2">
NOT WIXUI_DONTVALIDATEPATH</Publish>
<Publish Dialog="InstallWebDlg" Control="Next" Event="SpawnDialog" Value="InvalidDirDlg" Order="3">
<![CDATA[NOT WIXUI_DONTVALIDATEPATH AND WIXUI_INSTALLDIR_VALID<>"1"]]></Publish>
<Publish Dialog="InstallWebDlg" Control="Next" Event="NewDialog" Value="VerifyReadyDlg" Order="4">
WIXUI_DONTVALIDATEPATH OR WIXUI_INSTALLDIR_VALID="1"</Publish>
<Publish Dialog="InstallWebDlg" Control="ChangeFolder" Property="_BrowseProperty" Value="[WIXUI_INSTALLDIR]" Order="1">1</Publish>
<Publish Dialog="InstallWebDlg" Control="ChangeFolder" Event="SpawnDialog" Value="BrowseDlg" Order="2">1</Publish>
<Publish Dialog="VerifyReadyDlg" Control="Back" Event="NewDialog" Value="InstallWebDlg" Order="1">NOT Installed</Publish>
<Publish Dialog="VerifyReadyDlg" Control="Back" Event="NewDialog" Value="MaintenanceTypeDlg" Order="2">Installed</Publish>
<Publish Dialog="MaintenanceWelcomeDlg" Control="Next" Event="NewDialog" Value="MaintenanceTypeDlg">1</Publish>
<Publish Dialog="MaintenanceTypeDlg" Control="RepairButton" Event="NewDialog" Value="VerifyReadyDlg">1</Publish>
<Publish Dialog="MaintenanceTypeDlg" Control="RemoveButton" Event="NewDialog" Value="VerifyReadyDlg">1</Publish>
<Publish Dialog="MaintenanceTypeDlg" Control="Back" Event="NewDialog" Value="MaintenanceWelcomeDlg">1</Publish>
<Property Id="ARPNOMODIFY" Value="1" />
<Dialog Id="InstallWebDlg" Width="370" Height="270" Title="!(loc.InstallDirDlg_Title)">
<Control Id="Next" Type="PushButton" X="236" Y="243" Width="56" Height="17" Default="yes" Text="!(loc.WixUINext)" >
</Control>
<Control Id="Back" Type="PushButton" X="180" Y="243" Width="56" Height="17" Text="!(loc.WixUIBack)" />
<Control Id="Cancel" Type="PushButton" X="304" Y="243" Width="56" Height="17" Cancel="yes" Text="!(loc.WixUICancel)">
<Publish Event="SpawnDialog" Value="CancelDlg">1</Publish>
</Control>
<Control Id="Description" Type="Text" X="25" Y="23" Width="280" Height="15" Transparent="yes" NoPrefix="yes" Text="!(loc.InstallDirDlgDescription)" />
<Control Id="Title" Type="Text" X="15" Y="6" Width="200" Height="15" Transparent="yes" NoPrefix="yes" Text="!(loc.InstallDirDlgTitle)" />
<Control Id="BannerBitmap" Type="Bitmap" X="0" Y="0" Width="370" Height="44" TabSkip="no" Text="!(loc.InstallDirDlgBannerBitmap)" />
<Control Id="BannerLine" Type="Line" X="0" Y="44" Width="370" Height="0" />
<Control Id="BottomLine" Type="Line" X="0" Y="234" Width="370" Height="0" />
<Control Id="FolderLabel" Type="Text" X="20" Y="60" Width="290" Height="30" NoPrefix="yes" Text="!(loc.InstallDirDlgFolderLabel)" />
<Control Id="Folder" Type="PathEdit" X="20" Y="100" Width="320" Height="18" Property="WIXUI_INSTALLDIR" Indirect="yes" />
<Control Id="ChangeFolder" Type="PushButton" X="20" Y="120" Width="56" Height="17" Text="!(loc.InstallDirDlgChange)" />
<Control Id="VirtualDirLabel" Type="Text" X="20" Y="160" Width="290" Height="10" NoPrefix="yes" Text="Virtual Directory:" />
<Control Id="VirtualDir" Type="Edit" X="20" Y="172" Width="320" Height="18" Property="WIXUI_VIRTUALDIR" Indirect="yes" />
</Dialog>
</UI>

Answer 1

Is it necessary to use an Event? Are there existing, built-in, events and can they be tailored to custom Wix Properties?
No, it is not necessary to use an Event to set property values; they can be set explicitly, via custom actions, or from conditions. To answer the second question, no, there isn't an event model which you can tailor to custom properties within Wix.
I can't help but wonder if your problem of the virtual directory name not being set has to do with your VIRTUALDIR property not having the Secure attribute. Take a look at this model. I'm not certain, but I think only secure properties can move from the client (pink) area to the server (green) area.
Perhaps try adding Secure="yes" to the property and see if that does the trick?