On Aug 9, 2010, at 10:42, Kyle Sluder wrote: > On Mon, Aug 9, 2010 at 10:23 AM, Gideon King <gid...@novamind.com> wrote: >> I don't quite get why I should be passing in something for the context when >> setting up observing. I would have thought that that would be only for cases >> where either: >> a) you actually want to use some context data >> b) you have more than one key path the same and the observation object isn't >> enough information to give you the necessary context. >> ...neither of which is the case for my application. > > The context is the only unique identifier for your observation. If > both your implementation and your superclass's implementation register > for the same keypath on the same object, the context is the only way > to differentiate. > > The only sane thing to do is the technique espoused by Mike Ash > <http://www.mikeash.com/pyblog/key-value-observing-done-right.html>, > OFBinding > <http://github.com/omnigroup/OmniGroup/blob/master/Frameworks/OmniFoundation/OFBinding.h>, > and AppKit itself (all the various NS*Binder classes): create helper > objects for each individual binding, and have them use their self > pointers as the context arguments.
In many cases, it's sufficient to pass any ol' unique object pointer (or unique non-object value, of course, but getting uniqueness via object pointers is easier). Many people use globally unique strings. I generally use the observer's class object. That's assuming, of course, that there's no real context information that you need to pass in. To complete the sermon you started: Given the unique context identifier, the 'observeValueForKeyPath:ofObject:change:context:' *must* limit itself to handling invocations with the correct identifier and *must not* call the super implementation if it's correct. If the context is unrecognized, the method *must* call the super implementation instead. If Gideon didn't do that, then any number of application misbehaviors might result. _______________________________________________ Cocoa-dev mailing list (Cocoa-dev@lists.apple.com) Please do not post admin requests or moderator comments to the list. Contact the moderators at cocoa-dev-admins(at)lists.apple.com Help/Unsubscribe/Update your Subscription: http://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com This email sent to arch...@mail-archive.com
