How to get local path for payload in WiX/Burn Managed Bootstrapper Application?

I am currently working in a WiX/Burn Managed Bootstrapper Application and cannot figure out how to get the local path for a payload (MSI).

I let the user select which applications they want to install in my custom UI, and I want to not show applications for which the MSI is missing. I also need to see information in the MSI's database.

I know I can determine missing payloads by handling "ResolveSource" but that doesn't happen until right before the application in installed.

I deserialize the BootstrapperApplicationData.xml file first thing so I have information about which MSIs MIGHT be installed, but it still doesn't help me determine the source of the MSIs.

Does anyone know how to determine the local path to a payload?

EDIT: Here is an example for how I reference all the installers:

<MsiPackage Id="AppName"
      SourceFile="$(var.ProjectName.TargetDir)ProjectName.msi"
      Name="MSI\ProjectName.msi"
      Compressed="no"/>

Solution

In the GetLastUsedSourceFolder function in cache.cpp, you can see that the engine gets the source folder from the WixBundleLastUsedSource variable, and the parent directory of the WixBundleOriginalSource variable if WixBundleLastUsedSource isn't set.

You can use this along with the Name attribute of the WixPayloadProperties element in the BootstrapperApplicationData.xml file to predetermine where the engine will look for a payload. Note that the engine will actually look in the cache first.