问题：KVC和KVO的keyPath一定是属性么？

• KVC 支持实例变量，
• KVO 只能手动支持 实例变量。即KVO需要自己在set方法里实现willChangeValueForKey didChangeValueForKey 还要自己实现 automaticallyNotifiesObserversForKey 手动进行监听。

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//
//  Student.h
//
#import <Foundation/Foundation.h>

@interface Student : NSObject {
    NSString  *_age;
}
- (void)setAge:(NSString *)age;
- (NSString *)age;

@property (nonatomic, strong) NSString  *name;
@end
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//
//  Student.m
//
#import "Student.h"

@implementation Student
@synthesize name = _name;
- (void)setName:(NSString *)name {
    _name = name;
}
- (NSString *)name {
    return _name;
}

// 手动设定KVO
- (void)setAge:(NSString *)age {
    [self willChangeValueForKey:@"age"];
    _age = age;
    [self didChangeValueForKey:@"age"];
}
- (NSString *)age {
    return _age;
}
+ (BOOL)automaticallyNotifiesObserversForKey:(NSString *)key {
    // 如果监测到键值为age,则指定为非自动监听对象
    if ([key isEqualToString:@"age"])   {
        return NO;
    }
    return [super automaticallyNotifiesObserversForKey:key];
}
@end
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