Everything about announcements
The basic design strategy for OmniBrowser is simple: rather than modelling a browser with one large and complex object (like Browser does), break it up into a network of smaller, simpler objects. From there, the design is pretty straightforward, and it's much easier to build lots of kinds of browsers from the same code base.
This design does have a downside, though. It makes event handling more difficult, because the objects that need to communicate to respond to events are often in distant parts of the network, and can't rely on the the structure of the network to find each other. Early versions of OmniBrowser responded to events, such as a click, with a cascade of messages, with each object letting it's neighbors know about the the event. This had the advantage that each object only needed to know about it's immediate neighbors, but it was also fragile and prone to infinite loops as neighbors repeatedly notified each other of the same event.
My second attempt to address this problem involved the use of a Dispatcher. This was an central object that all notification messages would flow through. As the various parts of the browser were created, they would register with the dispatcher to receive messages. This was an improvement, because objects could send messages to "everybody" rather than to an explicit receiver. But it was still awkward, and the event handling code was still convoluted and difficult to understand.
I've just finished up the implementation of my third attempt, this time based on Vassili Bykov's notion of Announcements. I talked to the folks at Cincom about porting the code to Squeak, but that didn't work out. I ended up just doing a mini-implementation that meets my needs for OmniBrowser. (Actually this was probably what I should have done in the first place. It was probably less work for me to re-implement Announcements from scratch than it would have been for someone at Cincom to get corporate approval to release the code under an open source license.)
Despite all the positive things Vassili had to say about Announcements, I have to admit I was surprised what an improvement it made in OmniBrowser's event handling code. My first pass at the conversion was simple. I replaced messages sent to the dispatcher with announcements sent to the announcer. Then I installed an announcement spy and browsed around the image a bit. It turned out that every event resulted in 3 or 4 redundant announcements, and probably even more unnecessary updates to the UI.
So I made a second pass, explicitly aimed at removing all the redundant announcements. In many cases, this meant finding the ultimate source of a particular announcement. For example, OBSelectionChanged should only be announced from two places in the code. All the other places where it was being announced were redundant, and had to be removed. By spying on announcements, I was able to get a clearer idea of the code flow in response to different events, and find other ways to simplify.
I suspect there's even more simplification that can be made, but even without it, moving to Announcements was a big improvement.