One view does not rerender when @State var changes based on Picker selection

Trying to build a simple MacOS app using SwiftUI. I have a View that contains a Picker bound to a State var. As a sanity check I have added a Text Views (the dwarves one and the volumes...itemName) that should also change with the Picker changes. And they do, but the View I want to rerender (the FileList) does not.

I suspect it has something to do with how I am trying to pass the new FileSystemItem (internal class) to the FileList. Like when the FilePanel rerenders the volumeSelection is back to 0 and the state is applied afterwards. So my problem is that I seem to be missing a fundamental concept on how this data is supposed to flow. I have gone through the WWDC info again and been reading other articles but I am not seeing the answer.

The desired behavior is changing the selection on the picker should cause a new FileSystemItem to be displayed in the FileList view. What is the right way to get this to happen? To state it more generically, how to you get a child view to display new data when a Picker selection changes?

struct FilePanel: View 
{
    @State var volumeSelection = 0
    @State var pathSelection = 0

    var volumes = VolumesModel() //should be passed in?
    var dwarves = ["Schlumpy","Wheezy","Poxxy","Slimy","Pervy","Drooly"]

    var body: some View {
        VStack {
            Picker(selection: $volumeSelection, label:
                Text("Volume")
                , content: {
                    ForEach (0 ..< volumes.count()) {
                        Text(self.volumes.volumeAtIndex(index: $0).itemName).tag($0)
                    }
            })

            FileList(item:volumes.volumeAtIndex(index: volumeSelection)).frame(minWidth: CGFloat(100.0), maxHeight: .infinity)
            Text(dwarves[volumeSelection])
            Text(volumes.volumeAtIndex(index: volumeSelection).itemName)

        }
    }
 }

struct FileList: View {
    @State var item : FileSystemItem

    var body: some View {
        VStack {
        List(item.childItems){fsi in
            FileCell(expanded:false, item: fsi)
        }
        Text(item.itemName)
        }
    }
}