观察者模式的具体应用——通知(notification)机制和KVO(Key-Value-Observing)机制

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    
    //将当前的controller注册为通知接收者
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(receiveNotification:) name:@"TestNotification" object:nil];
    
    //添加一个发送通知的按钮
    UIButton *button = [UIButton buttonWithType:UIButtonTypeSystem];
    [button setTitle:@"Send" forState:UIControlStateNormal];
    button.frame = CGRectMake(50, 300, 220, 25);
    [button addTarget:self action:@selector(sendNotification) forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:button];
    
    //监听系统通知
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(handleEnterBackground:) name:UIApplicationDidEnterBackgroundNotification object:nil];
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(handleEnterForeground:) name:UIApplicationWillEnterForegroundNotification object:nil];
}

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
    //解除通知
    [[NSNotificationCenter defaultCenter] removeObserver:self];
}

/**
 *  发送通知
 */
- (void)sendNotification {
    NSDictionary *dic = [NSDictionary dictionaryWithObject:@"Test" forKey:@"key"];
    [[NSNotificationCenter defaultCenter] postNotificationName:@"TestNotification" object:nil userInfo:dic];
}


/**
 *  接收自定义通知
 *
 *  @param notification 接收到的通知
 */
- (void)receiveNotification:(NSNotification *)notification {
    NSDictionary *dic = notification.userInfo;
    
    NSString *value = [dic objectForKey:@"key"];
    
    NSLog(@"Receive notification:%@", value);
}

/**
 *  接收应用进入后台的通知
 *
 *  @param notification 接收到的通知
 */
- (void)handleEnterBackground:(NSNotification *)notification {
    NSLog(@"应用进入到后台。");
}

/**
 *  接收应用进入前台的通知
 *
 *  @param notification 接收到的通知
 */
- (void)handleEnterForeground:(NSNotification *)notification {
    NSLog(@"应用进入前台。");
}

UIApplicationDidEnterBackgroundNotification

UIApplicationWillEnterForegroundNotification

//
//  WBViewController.h
//  KVODemo
//
//  Created by 韩学鹏 on 15/6/26.
//  Copyright (c) 2015年 韩学鹏. All rights reserved.
//

#import <UIKit/UIKit.h>

@interface WBViewController : UIViewController
@property (nonatomic, strong)NSString *textString;
@end

//
//  WBViewController.m
//  KVODemo
//
//  Created by 韩学鹏 on 15/6/26.
//  Copyright (c) 2015年 韩学鹏. All rights reserved.
//

#import "WBViewController.h"
#import "WBTextObserver.h"
@interface WBViewController () {
    UITextField *_textField;
    WBTextObserver *_observer;
}

@end

@implementation WBViewController

- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{
    self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
    if (self) {
        // Custom initialization
    }
    return self;
}

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    
    //输入框
    _textField = [[UITextField alloc] initWithFrame:CGRectMake(10, 200, 200, 25)];
    [_textField setBorderStyle:UITextBorderStyleRoundedRect];
    [self.view addSubview:_textField];
    
    //按钮，改变字符串
    UIButton *button = [UIButton buttonWithType:UIButtonTypeSystem];
    [button setTitle:@"Change" forState:UIControlStateNormal];
    button.frame = CGRectMake(200, 200, 80, 25);
    [button addTarget:self action:@selector(textChangeFunc) forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:button];
    
    //观察者
    _observer = [[WBTextObserver alloc] init];
    [self addObserver:_observer forKeyPath:@"textString" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:@"context"];
}

- (void)textChangeFunc {
    self.textString = _textField.text;
}

//
//  WBTextObserver.h
//  KVODemo
//
//  Created by 韩学鹏 on 15/6/26.
//  Copyright (c) 2015年 韩学鹏. All rights reserved.
//

#import <Foundation/Foundation.h>

@interface WBTextObserver : NSObject

@end

//
//  WBTextObserver.m
//  KVODemo
//
//  Created by 韩学鹏 on 15/6/26.
//  Copyright (c) 2015年 韩学鹏. All rights reserved.
//

#import "WBTextObserver.h"

@implementation WBTextObserver

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
    NSLog(@"WBTextObserver %@---%@", keyPath, [change objectForKey:NSKeyValueChangeNewKey]);
}

@end

[self addObserver:_observer forKeyPath:@"textString" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:@"context"];