一、KVC设值的原理

二、KVC取值原理

• 验证设置和取值代码：
• 1、MJPerson类

----------------------------MJPerson.h--------------------------
#import <Foundation/Foundation.h>

@interface MJPerson : NSObject
{
    @public
//    int age;
//    int isAge;
//    int _isAge;
//    int _age;
}

@end

-------------------------------MJPerson.m-------------------------------
#import "MJPerson.h"

@implementation MJPerson

//- (int)getAge
//{
//    return 11;
//}

//- (int)age
//{
//    return 12;
//}

//- (int)isAge
//{
//    return 13;
//}

//- (int)_age
//{
//    return 14;
//}

//- (void)setAge:(int)age
//{
//    NSLog(@"setAge: - %d", age);
//}

//- (void)_setAge:(int)age
//{
//    NSLog(@"_setAge: - %d", age);
//}

//- (void)willChangeValueForKey:(NSString *)key
//{
//    [super willChangeValueForKey:key];
//    NSLog(@"willChangeValueForKey - %@", key);
//}
//
//- (void)didChangeValueForKey:(NSString *)key
//{
//    NSLog(@"didChangeValueForKey - begin - %@", key);
//    [super didChangeValueForKey:key];
//    NSLog(@"didChangeValueForKey - end - %@", key);
//}

// 默认的返回值就是YES
//+ (BOOL)accessInstanceVariablesDirectly
//{
//    return YES;
//}

@end

• 2、自定义监听MJObserver类

------------------------------MJObserver.h-----------------------------
#import <Foundation/Foundation.h>

@interface MJObserver : NSObject

@end

------------------------------MJObserver.m------------------------------
#import "MJObserver.h"

@implementation MJObserver

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context
{
    NSLog(@"observeValueForKeyPath - %@", change);
}

@end

• 3、main.m

#import <Foundation/Foundation.h>
#import "MJPerson.h"
#import "MJObserver.h"

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        MJPerson *person = [[MJPerson alloc] init];
//        person->_age = 10;
//        person->_isAge = 11;
//        person->age = 12;
//        person->isAge = 13;
        
        
        NSLog(@"%@", [person valueForKey:@"age"]);
    }
    return 0;
}


void test()
{
    MJObserver *observer = [[MJObserver alloc] init];
    MJPerson *person = [[MJPerson alloc] init];
    
    // 添加KVO监听
    [person addObserver:observer forKeyPath:@"age" options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld context:NULL];
    
    // 通过KVC修改age属性
    [person setValue:@10 forKey:@"age"];
    // [person willChangeValueForKey:@"age"];
    // person->_age = 10;
    // [person didChangeValueForKey:@"age"];
    
    // 移除KVO监听
    [person removeObserver:observer forKeyPath:@"age"];
}