iOS语法之KVO的使用

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////  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

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////  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

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////  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

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- (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     **/}

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2016-01-05 17:59:15.642 testOC[42635:1792662] 初始值: 语文课 192016-01-05 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