Invokes a block when an observed KVO property changes

PropertyObserver.h

// Copyright (C) 2012 Kristopher Johnson
// 
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.

#import <Foundation/Foundation.h>

typedef void (^KJPropertyChangeBlock)(id newValue);

// Object that invokes a block when a specified property is changed for an observed object

@interface KJPropertyObserver : NSObject

@property (nonatomic, readonly) NSObject *subject;
@property (nonatomic, readonly, copy) NSString *keyPath;
@property (nonatomic, readonly, copy) KJPropertyChangeBlock changeBlock;

// Return an observer for the specified key path on the specified object
+ (KJPropertyObserver *)observerForKeyPath:(NSString *)keyPath ofSubject:(NSObject *)subject onChange:(KJPropertyChangeBlock)changeBlock;

// Initialize observation for the specified key path on the specified object
- (id)initObservingKeyPath:(NSString *)keyPath ofSubject:(NSObject *)subject onChange:(KJPropertyChangeBlock)changeBlock;

@end

PropertyObserver.m

// Copyright (C) 2012 Kristopher Johnson
// 
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.

#import "PropertyObserver.h"

@interface KJPropertyObserver ()

@property (nonatomic, readwrite) NSObject *subject;
@property (nonatomic, copy, readwrite) NSString *keyPath;
@property (nonatomic, copy, readwrite) KJPropertyChangeBlock changeBlock;

@end

@implementation KJPropertyObserver

@synthesize subject = _subject;
@synthesize keyPath = _keyPath;
@synthesize changeBlock = _changeBlock;

+ (KJPropertyObserver *)observerForKeyPath:(NSString *)keyPath ofSubject:(NSObject *)subject onChange:(KJPropertyChangeBlock)changeBlock {
    return [[KJPropertyObserver alloc] initObservingKeyPath:keyPath ofSubject:subject onChange:changeBlock];
}

- (id)initObservingKeyPath:(NSString *)keyPath ofSubject:(NSObject *)subject onChange:(KJPropertyChangeBlock)changeBlock {
    self = [super init];
    if (!self)
        return nil;
    
    self.subject = subject;
    self.keyPath = keyPath;
    self.changeBlock = changeBlock;
    [subject addObserver:self
              forKeyPath:_keyPath
                 options:(NSKeyValueObservingOptionNew | NSKeyValueObservingOptionInitial)
                 context:nil];
    return self;
}

- (void)dealloc {
    [_subject removeObserver:self forKeyPath:_keyPath];
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
    if ([keyPath isEqualToString:_keyPath]) {
        id newValue = [change valueForKey:NSKeyValueChangeNewKey];
        _changeBlock(newValue);
    }
}

@end