2008 04 22Key Value Bastard Observing
Making a class KVBO compliant
We'll make a class called SampleObject KVBO compliant.
setValue:forKey:to listen to all changes
selfand the key name (from
forKey:) with a class method called
setLastModifiedKey:forInstance:. They will be stored as static variables and will be read by KVBO observers
setValue:forKey:then sets a new value for a dummy key (keyChanged) on a shared instance (a singleton for the observed class, here SampleObject). Changing this dummy key's value dispatches KVO notifications of our shared instance
From here, an observer listening to the shared instance will receive notifications of all changes to all instances of SampleObject and be able to see which key of which instance changed.
Listening to KVBO
You can listen using 2 methods :
- standard KVO : use
addObserver:forKeyPath:options:context:to listen to your KVBO class' shared instance. All notifications will go through the KVO method
- bindings : use
bind:toObject:withKeyPath:options:to bind the shared instance to an observer. All notifications will go through the set method of the key you bind to
Which one to use ? That's a matter of taste. Using standard KVO, you have a fixed name method (
addObserver:forKeyPath:options:context:) to receive all notifications. Using bindings, you get to choose the name of your method, but it has to start with set.