Only allowing one Drilldown in SSRS - reporting-services

Question

It took me hours of searching and putting together piecemeal parts to find the solution to this, so I figured I'd post it on here in the hopes of helping someone else.
The Problem: We need to display a report, with proper grouping and drilldowns. However, we should only allow one group to be drilled down at one time.
SSRS doesn't exactly have robust scripting options - for instance, you can't close other groups "on click" or anything like that. So how do you do it?

Answer 1

In My example i'm using the AdventureworksDW database. I want to have a dataset that includes the total sales for each group and region. My Stored Procedure looks something like this:
SELECT dst.SalesTerritoryGroup,
dst.SalesTerritoryRegion,
SUM(fis.SalesAmount) AS SaleTotal,
DATEPART(YEAR,fis.OrderDate) AS OrderYear
FROM [dbo].FactInternetSales AS fis
INNER JOIN [dbo].DimSalesTerritory AS dst
ON fis.SalesTerritoryKey = dst.SalesTerritoryKey
WHERE fis.OrderDate < #QueryEndDate
GROUP BY
dst.SalesTerritoryGroup,
dst.SalesTerritoryRegion,
DATEPART(YEAR,fis.OrderDate)
UNION ALL /*The ResellerSales table. Same info.*/
From there I added a table with two groups: SalesTerritoryRegion and its parent, SalesTerritoryGroup. I also added a column to the left INSIDE the SalesTerritoryGroup, with an X (this can also be an image if you'd like). This is the "Drilldown" button that we'll use.
Create a string parameter, mine was #ExpandedGroup. Set the Default to an empty string (so that all the groups start out collapsed). Right click on the SalesTerritoryRegion group, or whatever your subgroup is, and go to the visibility tab. Click "Show or Hide based on Expression" and enter something like this:
=iif(Parameters!ExpandedGroup.Value="" or
Fields!SalesTerritoryGroup.Value<>Parameters!ExpandedGroup.Value,True,False)
This statement means: If we haven't opened a dropdown, or if the dropdown isn't the one selected, set hidden to true. Otherwise, false.
Next click on your "X" column to the left of SalesTerritoryGroup or your supergroup. Right click to go to Textbox Properties. Click the action tab. From there select "Go to Report". When you specify a report, make the target itself (For instance, mine is Main). Then, add parameters to the report.
The most important here is ExpandedGroup. The name should be ExpandedGroup, but the value is not just [ExpandedGroup]. Instead, it's an expression:
=IIF(Fields!SalesTerritoryGroup.Value=Parameters!ExpandedGroup.Value,
"",
Fields!SalesTerritoryGroup.Value)
This expression says: If the Group is the same as the Expanded group, make ExpandedGroup an empty string when you load the report. Otherwise, send the TerritoryGroup value. Essentially, this will let us toggle on and off the drilldown (same as you would in the report if you had traditional drilldowns).
Note: Also be sure to pass other parameters! For instance, my query requires a date to exclude some transaction data. If you don't pass this parameter in the "Go to Report" action, then you'll have to enter it again when you DrillDown. This also means you can give yourself even more flexibility when you click a drilldown (changing a chart that's displayed etc.) which is what I'm doing for this project.
Hope it helps someone out! Of course, if there is a more elegant or simpler solution I'd absolutely love to hear it.