WPF gives us a number of different mechanisms for handling events –
they are bubbling, tunneling, and direct. These are all known as
Routed events.Direct event
You are probably already used to the direct routed event. This is
where the item itself handles the event that occurred. A good example
would be handling heonClick-event of a mouse button in standard
WinForms. This is where the event is raised in the GUI item and gets
handled by said GUI element.Bubbling Event
Now we all like some bubbles in one form or another. Bubbling happens
when the event is not handled by the element ( say atextbox) and the
event “bubbles” its way up the UI containers which hold it. For
example, let’s say you have a window that contains a panel and inside
that panel you have a grid and inside the grid you have a textbox. If
the event is not handled by the textbox, then it moves, is passed or
“bubbles” up to the grid level (as the grid contains the textbox), if
it is not handled at that level then the event bubbles further up the
“tree” (known as a visual tree) to the panel where it may or may not
be handled. This process continues until it is handled or the event
“escapes” the top most element.Examples of a bubbling event would be something like a
MouseButtonDownevent. Or aKeydownevent.Tunneling
Tunneling is the opposite of Bubbling. So instead of an event going
“up” the visual tree, the event travels down the visual tree toward
the element that is considered the source. The standard WPF naming
definition of a tunneling event is that they all start with “preview”
for examplepreviewdownkeyandpreviewmousebuttondown. You can
catch them on their way to the “target” element and handle it. An
example for this might be perhaps you have some controls inside a grid
control and for some reason you have decided that no control within
that grid will be allowed to have the letter “t” reach it.
Source with the opinion of the author which I don’t support nor agree with.
And another StackOverflow question which is pretty much the same.
A nice demo project
And last but not least some explanation and another tutorial.