在dealloc之前观察并删除类别中的NSNotification

Question

I am currently observing a notification throughout the life of an object in a category . However, I am swizzling the dealloc method to have a spot to remove the observation. This feels bad and I am uncomfortable with it, additionally I am running into problems.

Does anyone have any ideas how I can stop observing the notification just before the object will be deallocated within a category ?

Answer 1

The best way to run code at the deallocation of an object for which you can't override dealloc is by using associated objects .

The object that you associate to will release its strongly-held associated objects when it deallocates. As long as that is the only owner, the associated object's dealloc will then be called. Using a class you control for the associated object, that's your entry point.

I've got a demonstration of using this trick to deregister for KVO in a GitHub repo https://github.com/woolsweater/UIViewController-WSSDataBindings

The controller associates a helper object to itself:

- (void)WSSBind:(NSString *)bindingName
   toObject:(id)target
withKeyPath:(NSString *)path
{
    WSSBinding * binding = [WSSBinding bindingWithBoundName:bindingName
                                                   onObject:self
                                                  toKeyPath:path
                                                   ofObject:target];

    // Attach the binding to both target and controller, but only make it
    // owned by the target. This provides automatic deregistration when the
    // target is destroyed, and allows the controller to unbind at will.
    // Disregard the target and bound path for the key to allow mirroring
    // Cocoa's unbind: method; this is simplest for the controller.
    NSUInteger key = [self WSSAssociateKeyForBinding:bindingName];
    objc_setAssociatedObject(target, (void *)key, binding,
                             OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    objc_setAssociatedObject(self, (void *)key, binding,
                             OBJC_ASSOCIATION_ASSIGN);
}

And the WSSBinding class implements dealloc to remove the observer that's been set up elsewhere. You can do the same for your NSNotification registration.