iOS--KVO的概述与使用

一、概述

KVO,即：Key-Value Observing，它提供一种机制，当指定的对象的属性被修改后，则对象就会接受到通知。简单的说就是每次指定的被观察的对象的属性被修改后，KVO就会自动通知相应的观察者了。

二、使用方法

系统框架已经支持KVO，所以程序员在使用的时候非常简单。

1. 注册，指定被观察者的属性，

2. 实现回调方法

3. 移除观察

三、实例:

　　假设一个场景,股票的价格显示在当前屏幕上，当股票价格更改的时候，实时显示更新其价格。

程序目录如下：

工程程序如下：

StockData.h

---

#import <Foundation/Foundation.h>

@interface StockData : NSObject
{
     NSString * stockName;
     float price;
}
@end

StockData.m

---

#import "StockData.h"
@implementation StockData

@end

这里定义属性是在ViewController.m文件里定义的，而ViewController.h里没有内容，故而没有列举出来。

ViewController.m

---

#import "ViewController.h"
#import "StockData.h"
@interface ViewController ()

@property(strong,nonatomic) UILabel *myLable;
@property(strong,nonatomic) StockData *stockforKVO;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    self.stockforKVO=[[StockData alloc] init];
    [self.stockforKVO setValue:@"searph" forKey:@"stockName"];
    [self.stockforKVO setValue:@"10.0" forKey:@"price"];
    [self.stockforKVO addObserver:self forKeyPath:@"price" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:nil];
    self.myLable = [[UILabel alloc]initWithFrame:CGRectMake(, , ,  )];
    self.myLable.textColor = [UIColor redColor];
    self.myLable.text = [NSString stringWithFormat:@"%@",[self.stockforKVO valueForKey:@"price"]];
    [self.view addSubview:self.myLable];

    UIButton * b = [UIButton buttonWithType:UIButtonTypeRoundedRect];
    b.frame = CGRectMake(, , , );
    b.backgroundColor=[UIColor redColor];
    [b addTarget:self action:@selector(buttonAction) forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:b];
}
-(void)buttonAction
{
    // 点击按钮 切换数值
    [self.stockforKVO setValue:[NSString stringWithFormat:@"%d",arc4random()%] forKey:@"price"];
}
-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSString *,id> *)change context:(void *)context{

    if ([keyPath isEqualToString:@"price"]) {
        self.myLable.text= [NSString stringWithFormat:@"%@",[self.stockforKVO valueForKey:@"price"]];
        NSLog(@"旧数据--%@--，新数据--%@--",[change objectForKey:@"old"],[change objectForKey:@"new"]);
    }

}

/*
-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
     if([keyPath isEqualToString:@"price"])
    {
          self.myLable.text=[NSString stringWithFormat:@"%@",[self.stockforKVO valueForKey:@"price"]];
    }
}
 */

/**
 *  移除观察者
 */
-(void)dealloc
{
    [self.stockforKVO removeObserver:self forKeyPath:@"price"];
}
- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

@end

程序解析如下：

1.定义DataModel,即自己定义的类

---

//StockData.h
#import <Foundation/Foundation.h>

@interface StockData : NSObject
{
     NSString * stockName;
     float price;
}
@end

---

//StockData.m

#import "StockData.h"

@implementation StockData

@end

2.定义此model为Controller的属性，实例化它，监听它的属性，并显示在当前的View里边

- (void)viewDidLoad {
    [super viewDidLoad];
    self.stockforKVO=[[StockData alloc] init];
    [self.stockforKVO setValue:@"searph" forKey:@"stockName"];
    [self.stockforKVO setValue:@"10.0" forKey:@"price"];
    [self.stockforKVO addObserver:self forKeyPath:@"price" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:nil];
    self.myLable = [[UILabel alloc]initWithFrame:CGRectMake(, , ,  )];
    self.myLable.textColor = [UIColor redColor];
    self.myLable.text = [NSString stringWithFormat:@"%@",[self.stockforKVO valueForKey:@"price"]];
    [self.view addSubview:self.myLable];

    UIButton * b = [UIButton buttonWithType:UIButtonTypeRoundedRect];
    b.frame = CGRectMake(, , , );
    b.backgroundColor=[UIColor redColor];
    [b addTarget:self action:@selector(buttonAction) forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:b];
}

3.当点击button的时候，调用buttonAction方法，修改对象的属性

-(void)buttonAction
{
    // 点击按钮 切换数值
    [self.stockforKVO setValue:[NSString stringWithFormat:@"%d",arc4random()%] forKey:@"price"];
}

4. 实现回调方法

-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSString *,id> *)change context:(void *)context{

    if ([keyPath isEqualToString:@"price"]) {
        self.myLable.text= [NSString stringWithFormat:@"%@",[self.stockforKVO valueForKey:@"price"]];
        NSLog(@"旧数据--%@--，新数据--%@--",[change objectForKey:@"old"],[change objectForKey:@"new"]);
    }

}

/*
-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
     if([keyPath isEqualToString:@"price"])
    {
          self.myLable.text=[NSString stringWithFormat:@"%@",[self.stockforKVO valueForKey:@"price"]];
    }
}
 */

5.增加观察与取消观察是成对出现的，所以需要在最后的时候，移除观察者

/**
 *  移除观察者
 */
-(void)dealloc
{
    [self.stockforKVO removeObserver:self forKeyPath:@"price"];
}

四、小结

KVO这种编码方式使用起来很简单，很适用与datamodel修改后，引发的UIVIew的变化这种情况，就像上边的例子那样，当更改属性的值后，监听对象会立即得到通知。

五、程序效果图