The KVO mechanism is widely used in Foundation and AppKit (less so in UIKit). I frequently use it when working with NSOperations to observe isFinished. My biggest gripe with KVO is that all KVO changes are sent to one method, observeValueForKeyPath:ofObject:change:context:. The implementation of this method often becomes a ugly mass of if-else statements.
In an attempt to do away with observeValueForKeyPath:ofObject:change:context: I have written an NSObject category which augments KVO. It has three methods:
@interface NSObject (EMKKVOSelector) -(void)EMK_addObserver:(NSObject *)anObserver forKeyPath:(NSString *)keyPath selector:(SEL)observationSelector; -(void)EMK_addObserver:(NSObject *)anObserver forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(void *)context selector:(SEL)observationSelector; -(void)EMK_removeObserver:(NSObject *)anObserver withKeyPath:(NSString *)keyPath selector:(SEL)observationSelector; @end
The selectors used for observationSelector must match one of the following method signatures:
-(void)noArguements -(void)oneArguement:(NSString *)keyPath -(void)twoArguements:(NSString *) object:(id) -(void)threeArguements:(NSString *) object:(id) change:(NSDictionary) -(void)fourArguements:(NSString *) object:(id) change:(NSDictionary) context:(void *)
eg:
-(void)refreshView; -(void)refreshCellValue:(NSString *)keyPath representingPersonObject:(id)person;
All invocations to EMK_addObserver... methods must be balanced with a call to EMK_removeObserver.... These methods are not compatible with the standard KVO methods.
Download (from GitHub – the files are part of my open source library)
The code has only been briefly tested. Internally the code uses associate references and properties so it’s strictly Objective-C 2.0. Other people have implemented similar KVO fixes all of which offer slightly different features:
- Key-Value Observing Done Right (this includes a description of the design flaws of the KVO mechanism)
- Dispatching KVO Notifications
- Key-Value Observing Done Right. Again.
- KVO+Blocks: Block Callbacks for Cocoa Observers