KVC 補充及實例

怎麼獲取值 和 修改值—–>How

 - (void)setValue:(id)value forKey:(NSString *)key
 - (id)valueForKey:(NSString *)key
 - (id)valueForKey:(NSString *)key //以 key 作為標示符,獲取其對應的屬性值
 - (void)setValue:(id)value forKey:(NSString *)key //以 key 作為標示符設置其對應的屬性值
 - (id)valueForUndefinedKey:(NSString *)key
 - (void)setNilValueForKey:(NSString *)key
 ```

 Demo   
 創建一個QYPerson 類繼承於 NSObject   
 QYPerson.h

 ```
 #import 

@interface QYPerson : NSObject
@property (nonatomic, strong) NSString *name;
@property (nonatomic, assign) int age;

- (void)changeName;
@end

在 QYPerson.m 文件中實現上面的方法

#import QYPerson.h

@implementation QYPerson

- (void)changeName
{
    _name = @changeName;
}
@end

創建一個 QYPersonMonitor 的類用來監視 QYPerson 中的 name 屬性
在. m 文件中實現對 QYPerson 中 name 屬性的監視

 #import QYPersonMonitor.h

@implementation QYPersonMonitor
// 2. 回調的方法 當觀察的值改變的時候，該方法會被調用
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    if ([keyPath isEqual:@name]) {
        NSLog(@change name: old :%@,   new : %@,[change objectForKey:NSKeyValueChangeOldKey],[change objectForKey:NSKeyValueChangeNewKey]);
    }
}

@end

在控制器中初始化監視的對象以及被監視的對象,注冊觀察者

 #import ViewController.h
#import QYPerson.h
#import QYPersonMonitor.h

@interface ViewController ()
@property (nonatomic, strong) QYPerson *person;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    //初始化被監視的對象
    _person = [[QYPerson alloc] init];
    _person.name = @zhangsan;
    _person.age = 18;

    //監視對象
    QYPersonMonitor *personMonitor = [[QYPersonMonitor alloc] init];
    // 1. 注冊了一個觀察者
    [_person addObserver:personMonitor forKeyPath:@name options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:nil];

    //初始化的值
    NSLog(@person's name is %@,_person.name);

    //通過 setvalue 的方法,此時 QYPersonMonitor 的監視將被調用
    [_person setValue:@(lisi by KVC) forKey:@name];

    //打印通過 kvc 方式修改後的值
    NSLog(@person's name get by kvc is %@,[_person valueForKey:@name]);

    //通過.語法修改的效果和通過 setvalue 是一致的
    _person.name = @name change by .name= wangwu;
    //通過 person 自己的函數來更改 name 屬性
    [_person changeName];
}

@end

打印結果如下:
**2015-07-29 16:38:59.802 SetDemo[3764:1391043] person’s name is zhangsan
2015-07-29 16:38:59.802 SetDemo[3764:1391043] change name: old :zhangsan, new : (lisi by KVC)
2015-07-29 16:38:59.802 SetDemo[3764:1391043] person’s name get by kvc is (lisi by KVC)
2015-07-29 16:38:59.802 SetDemo[3764:1391043] change name: old :(lisi by KVC), new : name change by .name= wangwu**