【MJ-iOS底层原理总结】KVC的原理

1. KVC的使用

1. 相关API

   /// 赋值
   -(void)setValue:(id)value forKeyPath:(NSString *)keyPath;
   -(void)setValue:(id)value forKey:(NSString *)key;
   /// 取值
   -(id)valueForKeyPath:(NSString *)keyPath;
   -(id)valueForKey:(NSString *)key;
   

2. 假设当前有Person和Cat类

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

3. keyPath和key的区别？

   • keyPath的功能包含key，如下所示有相同效果：

     Person *person = [Person new];
     /// 赋值
     [person setValue:@(10) forKey:@"age"];
     [person setValue:@(10) forKeyPath:@"age"];
     /// 取值
     [person valueForKey:@"age"];
     [person valueForKeyPath:@"age"];
     

   • keyPath还可以指定路径进行取值和赋值：

     /// 赋值
     [person setValue:@(10) forKeyPath:@"cat.weight"];
     /// 取值
     [person valueForKeyPath:@"cat.weight"];  
     

2. KVC赋值的原理

1. 流程：
2. 问题：KVC是否会触发KVO？
   • 进行验证：

   self.person = [Person new];
   [self.person addObserver:self forKeyPath:@"age" options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld context:nil];
   [self.person setValue:@(10) forKey:@"age"];
   

   • 验证结果为：会进入observeValueForKeyPath监听方法
3. + (BOOL)accessInstanceVariablesDirectly;
   • 是否允许访问成员变量

3. KVC取值的原理