iOS--KVO的概述與使用

1、概述

KVO,即：Key-Value Observing，它提供一種機制，當指定的對象的屬性被修改後，則對象就會接受到通知。簡單的說就是每次指定的被觀察的對象的屬性被修改後，KVO就會自動通知相應的觀察者了。

 

2、使用方法

系統框架已經支持KVO，因此程序員在使用的時候很是簡單。

1. 註冊，指定被觀察者的屬性，

2. 實現回調方法

3. 移除觀察

 

3、實例:

　　假設一個場景,股票的價格顯示在當前屏幕上，當股票價格更改的時候，實時顯示更新其價格。

程序目錄以下：

 

工程程序以下：

StockData.h

---

 

#import <Foundation/Foundation.h>

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

 StockData.m

---

 

#import "StockData.h"
@implementation StockData

@end

 

這裏定義屬性是在ViewController.m文件裏定義的，而ViewController.h裏沒有內容，故而沒有列舉出來。

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(100, 100, 100, 30 )];
    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(0, 0, 100, 30);
    b.backgroundColor=[UIColor redColor];
    [b addTarget:self action:@selector(buttonAction) forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:b];
}
-(void)buttonAction
{
    // 點擊按鈕 切換數值
    [self.stockforKVO setValue:[NSString stringWithFormat:@"%d",arc4random()%1000] 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(@"舊數據--%@--，新數據--%@--",[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"]];
    }
}
 */

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

@end

 

程序解析以下：

1.定義DataModel,即本身定義的類

---

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

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

---

//StockData.m

#import "StockData.h"

@implementation StockData

@end

 

2.定義此model爲Controller的屬性，實例化它，監聽它的屬性，並顯示在當前的View裏邊

- (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(100, 100, 100, 30 )];
    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(0, 0, 100, 30);
    b.backgroundColor=[UIColor redColor];
    [b addTarget:self action:@selector(buttonAction) forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:b];
}

 

3.當點擊button的時候，調用buttonAction方法，修改對象的屬性

-(void)buttonAction
{
    // 點擊按鈕 切換數值
    [self.stockforKVO setValue:[NSString stringWithFormat:@"%d",arc4random()%1000] forKey:@"price"];
}

 

4. 實現回調方法

-(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(@"舊數據--%@--，新數據--%@--",[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.增長觀察與取消觀察是成對出現的，因此須要在最後的時候，移除觀察者

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

 

4、小結

 

KVO這種編碼方式使用起來很簡單，很適用與datamodel修改後，引起的UIVIew的變化這種狀況，就像上邊的例子那樣，當更改屬性的值後，監聽對象會當即獲得通知。

 

5、程序效果圖