Direct Routed Event question

Question

A colleague and I were chatting about routed events, and we came up with some things that didn't make a lot of sense. Primarily, what is the purpose of a Direct event, and why are both LeftMouseButtonDown and PreviewLeftMouseButtonDown direct events as opposed to bubbling and tunneling respectively?

According to the documentation, the LeftMouseButtonDown (and preview) events appear to behave as bubbling and tunneling events, except that the event is raised and reraised as these events traverse the tree. Since direct events can only be handled by the source of the event, it follows that any UIElement could handle a direct event since each one is raising the event and is also therefore the source. (Unless reraising the event is different than raising?). Why are these then not tunneling and bubbling like so many other similar events?

Neither my colleague nor I could come up with a use case in which we would ever create a custom event using a direct strategy (although we came up with some for tunneling and bubbling), yet LeftMouseButtonDown, one of the most common events (if not THE most common) is using this strategy.

Anyone have any ideas about the rational behind this?

Thanks!

Answer 1

My guess would be performance and there are considerations for having multiple events for the same thing.

There is already an event for MouseDown, to route both that and LeftMouseDown wouldn't make much sense. Finally, which would fire first and would cancelling\handling the first prevent the other? It sounds like it would be too easy to handle the same event twice.