KVO初探

KVO全称为key-value oberserving, 源于观察者模式，即key对应的value改变，observer要做出反应。类似Model对Controller发出通知Notification. Notification通常用于响应系统的一些变化，例如键盘出现、消失等。

在数据模型中，将myController设置为观察者，keyPath为myModel的属性，options通常为NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld， context为任意data, 用于传入observeValueForKeyPath:ofObject:change:context:

[myModel addObserver:(NSObject *)myController forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(void *)context]

在myController中，实现observeValueForKeyPath:ofObject:change:context:方法来相应value的变化。

//
//  ViewController.m
//  KvoTest
//
//  Created by luzheng1208 on 14/12/7.
//  Copyright (c) 2014年 luzheng. All rights reserved.
//

#import "ViewController.h"

@interface ViewController () {

}

@property (strong, nonatomic) NSString *name;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    self.name = @"Xiao Ming";
    [self registerForKVO];

    //[self willChangeValueForKey:@"name"];
    self.name = @"Xiao Hong";
    self.name = @"Xiao Li";
    //[self didChangeValueForKey:@"name"];
}

- (void)dealloc {
    [self unregisterForKVO];
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

#pragma mark - KVO

- (void)registerForKVO {
    [self addObserver:self forKeyPath:@"name" options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld context:NULL];
}

- (void)unregisterForKVO {
    [self removeObserver:self forKeyPath:@"name"];
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
    if ([keyPath isEqualToString:@"name"]) {
        NSLog(@"The old value is %@", [change objectForKey:NSKeyValueChangeOldKey]);
        NSLog(@"The new value is %@", [change objectForKey:NSKeyValueChangeNewKey]);
    }
    else
    {
        [super observeValueForKeyPath:keyPath
                             ofObject:object
                               change:change
                              context:context];
    }
}

@end

其输出为：

2014-12-07 23:02:38.951 KvoTest[19553:2584967] The old value is Xiao Ming
2014-12-07 23:02:38.954 KvoTest[19553:2584967] The new value is Xiao Hong
2014-12-07 23:02:38.954 KvoTest[19553:2584967] The old value is Xiao Hong
2014-12-07 23:02:38.954 KvoTest[19553:2584967] The new value is Xiao Li

上面的例子中未引入Model。

KVO还有手动实现的方法，具体参见blog: http://www.cppblog.com/kesalin/archive/2012/11/17/kvo.html