IOS学习笔记七之KVC和Key路径

1、KVC介绍

1）、KVC是由NSKeyValueCoding协议提供支持最基本的属性和两个方法如下

setValue:属性值  forkey:属性名：为指定属性设置值

valueForKey:属性名   （得到或者指定属性的值）


2）、当设置value为nil的时候，我们需要在类的实现里面重写setNilValueForKey方法，不然会抛出NSInvalidArgumentException

 
2、key路径介绍

KVC除了操作对象的属性之外，还可以操作对象的“复合属性”，比如类里面的成员变量是对象，然后给这个成员变量的对象进行赋值，就这样简单理解，赋值之前一定要记得把这个对象进行初始化操作

KVC协议中为操作Key路径的方法如下


setValue:forKeyPath:根据Key设置属性值

valueForKeyPath:根据key的路径获取属性值

 
2、测试简单Demo
 User.h

    #ifndef User_h
    #define User_h
    #import <Foundation/Foundation.h>
    @interface User : NSObject
    @property (nonatomic) NSString *name;
    @property (nonatomic, strong) NSString *city;
    @property (nonatomic, copy) NSString *add;
    @property NSString *pass;
    @property NSDate *birth;
    @property NSDate *birth1;
    @end
    #endif /* User_h */


User.m


    #import <Foundation/Foundation.h>
    #import "User.h"
     
    @implementation User
    @synthesize name = _name;
    @synthesize pass;
    @synthesize  birth;
    -(void) setName:(NSString *)name
    {
        self->_name = [NSString stringWithFormat:@"hello%@", name];
    }
    @end

 
KVCPerson.h

 
    #ifndef KVCPerson_h
    #define KVCPerson_h
    #import <Foundation/Foundation.h>
    #import "User.h"
     
    @interface KVCPerson : NSObject
    @property (nonatomic, copy) NSString *name;
    @property (nonatomic, copy) NSString *pass;
    @property (nonatomic, copy) NSDate *birth;
    @property (nonatomic) int price;
    @property (nonatomic) User *user;
    @end
    #endif /* KVCPerson_h */


KVCPerson.m

    #import <Foundation/Foundation.h>
    #import "KVCPerson.h"
    @implementation KVCPerson
     
    -(void)setNilValueForKey:(id)key
    {
        if ([key isEqualToString:@"price"])
        {
            _price = 0;
        }
        else
        {
            [super setNilValueForKey:key];
        }
    }
     
    @end


main.m

    #import <UIKit/UIKit.h>
    #import "AppDelegate.h"
    #import "Person.h"
    #import "Apple.h"
    #import "User.h"
    #import "Args.h"
    #import "KVCPerson.h"
    #import "FKItem.h"
    #import "FKOrder.h"
     
    int main(int argc, char * argv[]) {
        @autoreleasepool {
            KVCPerson *person = [KVCPerson new];
            [person setValue:@"chenyu" forKey:@"name"];
            [person setValue:@"1234" forKey:@"pass"];
            [person setValue:[NSDate date] forKey:@"birth"];
            NSLog(@"name is %@", [person valueForKey:@"name"]);
            NSLog(@"pass is %@", [person valueForKey:@"pass"]);
            NSLog(@"birth is %@", [person valueForKey:@"birth"]);
            NSLog(@"price is %@", [person valueForKey:@"price"]);
            [person setValue:nil forKey:@"name"];
            [person setValue:nil forKey:@"price"];
            NSLog(@"name is %@", [person valueForKey:@"name"]);
            NSLog(@"pass is %@", [person valueForKey:@"pass"]);
            NSLog(@"birth is %@", [person valueForKey:@"birth"]);
            NSLog(@"price is %@", [person valueForKey:@"price"]);
            
            [person setValue:[User new] forKeyPath:@"user"];
            //在使用valueForKeyPath之前一定要记得把成员对象进行初始化，不然直接设置无效
            [person setValue:@"hello" forKeyPath:@"user.city"];
            NSLog(@"user.city is%@", [person valueForKeyPath:@"user.city"]);
            
        }
    }

 
3、运行结果


    name is chenyu
    pass is 1234
    birth is Fri Jul  6 22:39:34 2018
    price is 0
    name is (null)
    pass is 1234
    birth is Fri Jul  6 22:39:34 2018
    price is 0
    user.city is hello


 
4、总结
在使用key的时候，一定要记得先把成员属性的对象进行初始化，不然设置无效

如下

            [person setValue:[User new] forKeyPath:@"user"];
            //在使用valueForKeyPath之前一定要记得把成员对象进行初始化，不然直接设置无效
            [person setValue:@"hello" forKeyPath:@"user.city"];
            NSLog(@"user.city is%@", [person valueForKeyPath:@"user.city"]);

而不是

            [person setValue:@"hello" forKeyPath:@"user.city"];
            NSLog(@"user.city is%@", [person valueForKeyPath:@"user.city"]);

这样结果会是null





 

作者：chen.yu
深信服三年半工作经验，目前就职游戏厂商，希望能和大家交流和学习,
微信公众号：编程入门到秃头 或扫描下面二维码
零基础入门进阶人工智能(链接)