iOS--KVO的概述與使用，iOS--KVO概述使用

iOS--KVO的概述與使用，iOS--KVO概述使用
一、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。

 二、使用方法

系統架構已經支援KVO，所以程式員在使用的時候非常簡單。

1. 註冊，指定被觀察者的屬性，

2. 實現回調方法

3. 移除觀察

 三、執行個體:　　假設一個情境,股票的價格顯示在當前螢幕上，當股票價格更改的時候，即時顯示更新其價格。程式目錄如下： 

工程程式如下：

StockData.h

 

#import <Foundation/Foundation.h>@interface StockData : NSObject{     NSString * stockName;     float price;}@end

 StockData.m

 

#import "StockData.h"@implementation StockData@end

 

這裡定義屬性是在ViewController.m檔案裡定義的，而ViewController.h裡沒有內容，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(@"舊資料--%@--，新資料--%@--",[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

 

程式解析如下：

1.定義DataModel,即自己定義的類

//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為Controller的屬性，執行個體化它，監聽它的屬性，並顯示在當前的View裡邊

- (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(@"舊資料--%@--，新資料--%@--",[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.增加觀察與取消觀察是成對出現的，所以需要在最後的時候，移除觀察者

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

 

四、小結 KVO這種編碼方式使用起來很簡單，很適用與datamodel修改後，引發的UIVIew的變化這種情況，就像上邊的例子那樣，當更改屬性的值後，監聽對象會立即得到通知。  五、程式