KVC的基本使用

KVC的全称是Key-Value Coding，俗称“键值编码”，可以通过一个key来访问某个属性
常见的API有：

• (void)setValue:(id)value forKey:(NSString *)key;
• (void)setValue:(id)value forKeyPath:(NSString *)keyPath;
• (id)valueForKey:(NSString *)key;
• (id)valueForKeyPath:(NSString *)keyPath;
  基本用法如下：

Cat.h文件
#import <Foundation/Foundation.h>

@interface Cat : NSObject
@property (nonatomic,assign) int weight;
@end

Person.h文件
#import <Foundation/Foundation.h>
#import "Cat.h"
@interface Person : NSObject
@property (nonatomic,assign) int age;
@property (nonatomic,strong) Cat *cat;
@end

Person *person = [[Person alloc] init];
[person setValue:@10 forKey:@"age"];
person.cat = [[Cat alloc] init];
[person setValue:@20 forKeyPath:@"cat.weight"];
NSLog(@"age = %d",person.age);
NSLog(@"weight = %d",person.cat.weight);

Person *person = [[Person alloc] init];
person.age = 10;
person.cat = [[Cat alloc] init];
person.cat.weight = 20;
NSLog(@"age = %@",[person valueForKey:@"age"]);
NSLog(@"age = %@",[person valueForKeyPath:@"age"]);
NSLog(@"weight = %@",[person valueForKeyPath:@"cat.weight"]);

setValue:forKey:的原理

accessInstanceVariablesDirectly方法的默认返回值是YES

valueForKey：的原理