switches to a new event whenever it receives one. At the moment of
switching, the old event will be ignored if it fires, and the new one
will be used if it fires; this is the opposite of
switch, which
will use only the old value.
switchHold, by always forwarding the old event the moment it is
switched out, avoids many potential cyclic dependency problems /
metastability problems. It's also more performant. Use it instead
unless you are sure you cannot.