键值观察者的使用及注意事项

来源：互联网发布：奥尼尔新秀体测数据编辑：程序博客网时间：2024/06/05 19:27

//
// Person.h
// Lesson_18_02_KVO(键值观察者)
//
// Created by lanou3g on 15/9/8.
// Copyright (c) 2015年 lanou3g.com. All rights reserved.
//

#import <Foundation/Foundation.h>

@interface Person : NSObject

@property (nonatomic,retain)NSString * name;

@end
//
// Person.m
// Lesson_18_02_KVO(键值观察者)
//
// Created by lanou3g on 15/9/8.
// Copyright (c) 2015年 lanou3g.com. All rights reserved.
//

#import "Person.h"

@implementation Person

- (void)dealloc
{
    [_name release];
    [super dealloc];
}

@end
//
// ViewController.h
// Lesson_18_02_KVO(键值观察者)
//
// Created by lanou3g on 15/9/8.
// Copyright (c) 2015年 lanou3g.com. All rights reserved.
//

#import <UIKit/UIKit.h>

@interface ViewController : UIViewController

@end
//
// ViewController.m
// Lesson_18_02_KVO(键值观察者)
//
// Created by lanou3g on 15/9/8.
// Copyright (c) 2015年 lanou3g.com. All rights reserved.
//

#import "ViewController.h"
#import "Person.h"

@interface ViewController ()
@property (retain, nonatomic) IBOutlet UITextField *texeField;
- (IBAction)didClickButton:(UIButton *)sender;
@property (nonatomic,retain)Person * person;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    self.person = [[Person alloc] init];
    // 1. 注册观察者(为被观察者添加一个观察者并设置观察的属性)

    [_person addObserver:self forKeyPath:@"name" options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld context:@"hello"];
    [_person release];
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

- (void)dealloc {
    // 5.移除观察者
    [_person removeObserver:self forKeyPath:@"name" context:@"hello"];
    [_texeField release];
    [super dealloc];
}
- (IBAction)didClickButton:(UIButton *)sender {
   // 2.改变属性name的值
    _person.name = _texeField.text;
}

// 3.只要是被观察者的属性值发生变化,就会执行下面的这个方法(这个方法会自动调用,不需要手动调用)
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context{
    NSLog(@"被检测的对象的属性所在的路径:%@",keyPath);
    NSLog(@"被观察者:%@",object);
    NSLog(@"被观察的属性所在状态下的值:%@",change);
    NSLog(@"在注册观察者的时候传过来的context的值:%@",context);
    // 4. 取出新值和旧值(nil:空对象 @"":空字符串)
    if (![change[@"new"] isEqualToString:change[@"old"]]) {
      self.view.backgroundColor = [UIColor colorWithRed:arc4random() %256/255.0 green:arc4random() %256/255.0 blue:arc4random() %256/255.0 alpha:1.0];
    }
    // 5.移除观察者(不写在这里,写在这里就不会有实现效果,应该写在dealloc里,即生命周期和person一致)
//    [_person removeObserver:self forKeyPath:keyPath context:context];
}
@end

使用观察者步骤和注意事项:
1:添加观察者
2.实现观察者方法
3.移除观察者(也是注意事项:使用完成后记得移除)

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