kvo

//  BOY.h

#import <Foundation/Foundation.h>

@interface BOY : NSObject

@property (nonatomic,retain) NSString * boyName;
@end

//  GIRL.h

#import <Foundation/Foundation.h>

@interface GIRL : NSObject
@property (nonatomic,retain) NSString * girlName;
@end

//  RootViewController.m

#import "RootViewController.h"
#import "GIRL.h"
#import "BOY.h"
@interface RootViewController ()
{
    GIRL * xiaoGirl;
    BOY * xiaoBoy;
}
@end

@implementation RootViewController

-(void)createUI
{
    NSArray * arr = @[@"BOY",@"GIRL"];
    for(int i = 0;i<2;i++)
    {
        UIButton * btn = [UIButton buttonWithType:UIButtonTypeCustom];
        btn.frame = CGRectMake(50 + 130 * i, 300, 100, 100);
        [btn setTitle:arr[i] forState:UIControlStateNormal];
        [btn setTitleColor:[UIColor redColor] forState:UIControlStateNormal];
        btn.tag = i + 1;
        [btn addTarget:self action:@selector(pressBtn:) forControlEvents:UIControlEventTouchUpInside];
        [self.view addSubview:btn];
        
        UILabel * label = [[UILabel alloc]initWithFrame:CGRectMake(10, 100 + 50 * i, 300, 40)];
        label.backgroundColor = [UIColor greenColor];
        label.tag = 100 + i;
        [self.view addSubview:label];
        label.textAlignment = NSTextAlignmentCenter;
        [label release];
    }
}
-(void)pressBtn:(id)sender
{
    UIButton * btn = (UIButton *)sender;
    if(btn.tag == 1)
    {
        [xiaoBoy setValue:@"赵又廷" forKey:@"boyName"];
    }
    else
    {
        [xiaoGirl setValue:@"高圆圆" forKey:@"girlName"];
    }
}
- (void)viewDidLoad
{
    [super viewDidLoad];
    [self createUI];
    
    xiaoGirl = [[GIRL alloc]init];
    xiaoBoy = [[BOY alloc]init];
    
    //通过KVC对成员属性进行赋值
    [xiaoBoy setValue:@"黄晓明" forKey:@"boyName"];
    [xiaoGirl setValue:@"Boby" forKey:@"girlName"];
    
    //初始情况下label上的内容显示
    UILabel * label1 = (UILabel *)[self.view viewWithTag:100];
    UILabel * label2 = (UILabel *)[self.view viewWithTag:101];
    
    label1.text = [xiaoBoy valueForKey:@"boyName"];
    label2.text = [xiaoGirl valueForKey:@"girlName"];
    
    //KVO
    //KVO 是 key-value-observer 键值观察者
    //观察某个属性发生改变触发相应方法
    
    //为BOY和GIRL的对象所包含的成员属性添加观察者
    
    //1、添加观察者对象指针 谁观察成员变量的变化 就添加哪个对象指针 一般都是当前类的对象作为观察者
    //2、添加的是被观察的成员属性名称
    //3、被观察的成员属性变化的前后值
    //4、直接设置为nil
    [xiaoBoy addObserver:self forKeyPath:@"boyName" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context: nil];
    [xiaoGirl addObserver:self forKeyPath:@"girlName" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:nil];
    //如果被观察的对象的成员属性发生改变 就会调用下面的方法
}
-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    /*
     1、被观察的成员属性的名称
     2、被观察的成员属性所属的对象指针
     3、存放的是被观察的成员属性变化的前后值(旧值的key是old   新值的key是new)
     */
    UILabel * label1 = (UILabel *)[self.view viewWithTag:100];
    UILabel * label2 = (UILabel *)[self.view viewWithTag:101];
    
    //isMemberOfClass:判断对象指针是否属于某个类
    if([object isMemberOfClass:[BOY class]])
    {
        NSString * older = [change objectForKey:@"old"];
        NSString * newer = [change objectForKey:@"new"];
        NSString * string = [NSString stringWithFormat:@"old:%@ -- new:%@",older,newer];
        label1.text = string;
    }
    else
    {
        NSString * older = [change objectForKey:@"old"];
        NSString * newer = [change objectForKey:@"new"];
        NSString * string = [NSString stringWithFormat:@"old:%@ -- new:%@",older,newer];
        label2.text = string;
    }
    
    
}