iOS语法之KVO的使用

//
//  Student.h
//  testOC
//
//  Created by Benjamin on 1/4/16.
//  Copyright © 2016 Benjamin. All rights reserved.
//

#import <Foundation/Foundation.h>

@interface Student : NSObject {
    
    NSString *_name;
    NSString *_courseName;
    int _age;
}

- (void)changeCourseName:(NSString *) newCourseName;
- (void)changeAge:(int) age;
@end

//
//  Student.m
//  testOC
//
//  Created by Benjamin on 1/4/16.
//  Copyright © 2016 Benjamin. All rights reserved.
//

#import "Student.h"

@implementation Student

-(void)changeCourseName:(NSString *)newCourseName {
    _courseName = newCourseName;
}

- (void)changeAge:(int)age {
    _age = age;
}


@end

//
//  PageView.m
//  testOC
//
//  Created by Benjamin on 1/4/16.
//  Copyright © 2016 Benjamin. All rights reserved.
//

#import "PageView.h"
#import "Student.h"

@implementation PageView

-(id)init:(Student *)initStudent {
    if (self == [super init]) {
        student = initStudent;
        // options确定返回的字典包含什么值：NSKeyValueObservingOptionNew包含新值
        // NSKeyValueObservingOptionNew包含旧值
        // keyPath就是要观察的属性值，options给你观察键值变化的选择，而context方便传输你需要的数据（注意这是一个void型）
        [student addObserver:self forKeyPath:@"courseName" options:NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew context:nil];
    }
    return self;
}

-(void)dealloc {
    [student removeObserver:self forKeyPath:@"courseName" context:nil];
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSString *,id> *)change context:(void *)context {
    if ([keyPath isEqual:@"courseName"]) {
        NSLog(@"PageView课程被改变了");
        NSLog(@"PageView新课程是: %@, 老课程是: %@", [change objectForKey: @"new"], [change objectForKey: @"old"]);
    }
}

@end

- (void)useKVO {
    Student *student = [[Student alloc] init];
    [student changeCourseName:@"语文课"];
    [student changeAge:19];
    NSLog(@"初始值: %@ %@", [student valueForKey: @"courseName"], [student valueForKey: @"age"]);
    
    PageView *page = [[PageView alloc]init:student];
    [student setValue:@"数学课" forKey:@"courseName"];
    [student changeCourseName:@"英语课"];  //这样直接改变不会触发observer,说明只有通过键值编码(KVC)改变的值，才会回调观察者注册的方法。
    
    [student willChangeValueForKey:@"age"];
    [student changeAge:20];
    [student didChangeValueForKey:@"age"];
    
    /**
     输出结果：
     2016-01-04 14:47:36.794 testOC[31517:1071934] 初始值: 语文课
     2016-01-04 14:47:36.795 testOC[31517:1071934] PageView课程被改变了
     2016-01-04 14:47:36.795 testOC[31517:1071934] PageView新课程是: 英语课, 老课程是: 语文课
     **/
}

2016-01-05 17:59:15.642 testOC[42635:1792662] 初始值: 语文课 19
2016-01-05 17:59:15.643 testOC[42635:1792662] PageView课程被改变了
2016-01-05 17:59:15.643 testOC[42635:1792662] PageView新课程是: 数学课, 老课程是: 语文课