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