KVO

涉及3个方法：

1.为对象的某个属性添加观察者

例如为lable的text属性添加观察者，self表示控制器是lable的text属性的观察者

    [lable addObserver:selfforKeyPath:@"text"options:0context:nil];

2.重写回调方法

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context

{ 

//当lable的text属性被修改了之后，你想干什么，就写在这里

}

3.当对象被销毁时，移除观察者（如果不移除，可能因为对象已经不在了，但通知还在，导致崩溃）

- (void)dealloc

{

    [self.myLableremoveObserver:selfforKeyPath:@"text"];

}

源代码如下，拷贝后即可运行：

//

//  ViewController.m

//  KVOLearn

//

//  Created by wusiping on 15/11/30.

//  Copyright (c) 2015年 wusiping. All rights reserved.

//

#import "ViewController.h"

@interface ViewController ()

@property(nonatomic,weak)UIButton *btn;

@property(nonatomic,weak)UILabel *myLable;

@property(nonatomic,weak)UILabel *myLable2;

@end

@implementation ViewController

- (void)viewDidLoad {

    [superviewDidLoad];

    // Do any additional setup after loading the view, typically from a nib.

    [self addBtn];

    [self addLable];

    [selfaddLable2];

}

//添加一个按钮，来改变lable的属性

- (void)addBtn

{

    CGFloat btnX = 200;

    CGFloat btnY = 200;

    CGFloat btnW = 100;

    CGFloat btnH = 50;

    CGRect btnFrame = CGRectMake(btnX, btnY, btnW, btnH);

    UIButton *btn = [[UIButtonalloc]initWithFrame:btnFrame];

    btn.backgroundColor = [UIColorredColor];

    [btn setTitle:@"数字加一"forState:UIControlStateNormal];

    _btn = btn;

    [_btnaddTarget:selfaction:@selector(touchBtn:)forControlEvents:UIControlEventTouchUpInside];

    [self.viewaddSubview:_btn];

    

}

//点击按钮时，触发的操作

- (void)touchBtn:(UIButton *)btn

{

    static int i =0 ;

    _myLable.text = [NSStringstringWithFormat:@"哈哈%d",i ];

    i ++;

}

//用于展示lable1

- (void)addLable

{

    CGFloat lableX = 50;

    CGFloat lableY = 50;

    CGFloat lableW = 100;

    CGFloat lableH = 50;

    CGRect lableFrame = CGRectMake(lableX, lableY, lableW, lableH);

    UILabel *lable = [[UILabelalloc]initWithFrame:lableFrame];

    lable.text = @"hello";

    //添加观察者

    [lable addObserver:selfforKeyPath:@"text"options:0context:nil];

    _myLable = lable;

    [self.viewaddSubview:_myLable];

}

//取消观察者

- (void)dealloc

{

    [self.myLableremoveObserver:selfforKeyPath:@"text"];

}

//lable显示labe1的变化

- (void)addLable2

{

    CGFloat lable2X = 50;

    CGFloat lable2Y = 150;

    CGFloat lable2W = 300;

    CGFloat lable2H = 50;

    CGRect lable2Frame = CGRectMake(lable2X, lable2Y, lable2W, lable2H);

    UILabel *lable2 = [[UILabelalloc]initWithFrame:lable2Frame];

    _myLable2 = lable2;

    [self.viewaddSubview:_myLable2];

    

}

//当labe1的text属性被修改时，就会回调下面这个方法

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context

{

    NSLog(@"mylable属性被修改为%@",_myLable.text);

    _myLable2.text = [NSStringstringWithFormat:@"lable1的属性被修改为%@",_myLable.text];

}

@end