Visual Studio 2017: Event-call when user changes tab

Question

I'm trying to write an outliner for XML code in VS (an extension using their SDK from a VSIX template) and I would like to get an event-call whenever the user changes to a different codeview/document.

Then, I plan to check the document type, create and render an interactive outline if it is indeed an xml document.

How would I go about creating such a hooking and is it even necessary?

EDIT

I've attempted the following implementation, but I'm being told that object does not contain a definition for "GetGlobalService"

using System;
using System.Runtime.InteropServices;
using EnvDTE;
using Microsoft.VisualStudio.Shell;

[Guid("bc4c5e8f-a492-4a44-9e57-ec9ad945140e")]
public class OutlineWindow : ToolWindowPane
{

    private DTE dte;

    public OutlineWindow() : base(null)
    {
        this.Caption = "OutlineWindow";

        this.Content = new OutlineWindowControl();

        dte = Package.GetGlobalService(typeof(EnvDTE.DTE)) as EnvDTE.DTE;
        dte.Events.WindowEvents.WindowActivated += OnWindowActivated;
    }

    private void OnWindowActivated(Window gotFocus, Window lostFocus)
    {
        throw new NotImplementedException();
    }
}

Answer 1

Thanks to @stuartd I managed to get this working! My problem was actually that I was putting it in the wrong class, the inheritance was messing it up.

public class OutlineManager
{
    private DTE dte;

    public OutlineManager()
    {
        dte = Package.GetGlobalService(typeof(DTE)) as DTE;
        dte.Events.WindowEvents.WindowActivated += OnWindowActivated;
    }

    private void OnWindowActivated(Window gotFocus, Window lostFocus)
    {
        //This is run when a new "window"(panel) gains focus (not only the code window though)
    }
}