KVO：Key Value Observing，顾名思义，键值观察。
KVO主要用于视图交互方面，比如界面的某些数据变化了，界面的现实也跟着需要变化，那就要简历数据和页面的关联。
下面通过实例讲解KVO的简单使用。
KVO自动调用简单使用
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#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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#import "Student.h"
@implementation Student
-(void)changeCourseName:(NSString *)newCourseName {
_courseName = newCourseName;
}
- (void)changeAge:(int)age {
_age = age;
}
@end
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#import "PageView.h"
#import "Student.h"
@implementation PageView
-(id)init:(Student *)initStudent {
if (self == [super init]) {
student = initStudent;
[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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测试代码
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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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输出结果：
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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
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可以看到，只有通过KVC方式赋值才会触发Observer。
不懂KVC的童鞋，请看这篇文章： KVC
不懂观察者模式的童鞋可以看看这篇文章：Observer
参考：http://www.cnblogs.com/stoic/archive/2012/07/24/2606784.html
