In the old days, one could listen on changes in the "activated nodes" of the active TopComponent with a listener on TopComponent.Registry paying attention to the PROP_ACTIVATED_NODES property.
Now Lookup is a much nicer way to show activate "objects". The way to listen globally to
changes in the Lookup of the active TopComponent is via Utilities.actionsGlobalContext() (which is a Lookup itself).
The trick is simple, you add a LookupListener on the Lookup.Result you get from Utilities.actionsGlobalContext().lookup(yourLookupTemplete) .
What is the garbage collection trick ?
Well, it's quite odd for me the existence of the Lookup.Result class. Because you don't do something like Lookup.addLookupListener(yourLookupTemplate, yourLookupListener). If you were to do something like this, you would expect that your listener lives as much as the Lookup. Meaning in the case of a "global" listener -- forever.
But what you do do is actually add a listener on Lookup.Result, which could be garbage collected and thus your listener also.
Generally, keep a hard reference on the Lookup.Result (or make a closure on it with some final keyword and a reference from the anonymous listener). Because if you don't -- the garbage collector might kick in quite soon and your listener won't be called.