On 19 Aug 2008, at 4:23 am, Ryan Brown wrote:
What is the best way to remove a KVO observer if you aren't sure if
the object already has an observer registered?
The right answer is to avoid this unsureness (is that a word?).
When I first started using KVO I too thought you could unregister
regardless - if it was registered it will unregister, if it wasn't it
does nothing. Unfortunately it doesn't work - Cocoa is very fussy
about this, and throws exceptions when you try to do things this way
with KVO. In addition, each observer has a separate connection so each
must be disconnected individually - there is no "disconnect all"
available. KVO is not like ordinary notifications.
The point is that observees should not know or care about their
observers. Once you stop trying to let observees control their
observers your problems will ease considerably. Instead, observers are
entirely responsible for their own actions - observing an object and
unobserving it later. The main problem usually is the need to
unobserve before the observee is deallocated "out from under it".
There's no one true way to do this, but just organise your code so
that the need doesn't arise. For example a controller that deletes an
object can send a "about to delete x" message before it does so that
any observers can disconnect from x, or the deletion of an object is
handled through a wrapper method that removes the observers. Typically
if you look at where and why you are observing an object, a simple
approach to managing its observers is usually straightforward. I must
admit though that getting it right can be tricky especially if you're
trying to fit KVO into an existing model that wasn't built with it in
mind.
hth,
Graham
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