Objective-C:KVC机制

KVC:key value coding    键值对的编码

功能：用来给对象属性设置值或者取出对象属性的值。虽然getter和setter方法也是该功能,但是如果类中没有设置属性特性或者重写这两个方法时,就无法存取属性值了。此时，采用KVC机制可以帮助完成这些要求。

 

先来个举例：给对象属性设值和取值

@interface Person

@property(strong,nonatomic)NSString name;

@end

一般模式：

Perosn *person = [[Person alloc]init];

[person setName:@“Tom”];

NSString *name = [person name];

 

KVC机制:

[perosn setValue:@“Tom” forKey:@“name”];

NSString *name = [person valueForKey:@“name”];

 

详细介绍：

@interface NSObject(NSKeyValueCoding)

//默认返回yes,如果没有setter方法,按_key,is_key,key,iskey的顺序搜索成员名

+ (BOOL)accessInstanceVariablesDirectly;

//通过属性的字符串键获取值

- (id)valueForKey:(NSString *)key;

//通过属性的字符串键设置值

-(void)setValue:(id)value forKey:(NSString*)key;

//通过属性路径字符串键获取值(格式：@“_xx._xx")

- (id)valueForKeyPath:(NSString *)keyPath;

//通过属性路径字符串键设置属性值(格式：@“_xx._xx")

- (void)setValue:(id)value forKeyPath:(NSString *)keyPath;

//通过属性字符串键检查值的正确性

- (BOOL)validateValue:(inout id *)ioValue forKey:(NSString *)inKey error:(out NSError **)outError;

//通过属性路径字符串键检查值的正确性

- (BOOL)validateValue:(inout id *)ioValue forKeyPath:(NSString *)inKeyPath error:(out NSError**)outError;

//通过属性数组键返回可变数组(有序一对多的关系：NSArray)

- (NSMutableArray *)mutableArrayValueForKey:(NSString *)key;

//通过属性数组路径键返回可变数组(有序一对多的关系：NSArray)

- (NSMutableArray *)mutableArrayValueForKeyPath:(NSString *)keyPath;

//通过属性集合键返回可变集合(无序一对多的关系：NSSet)

- (NSMutableSet *)mutableSetValueForKey:(NSString *)key;

//通过属性集合路径键返回可变集合(无序一对多的关系：NSSet)

- (NSMutableSet *)mutableSetValueForKeyPath:(NSString *)keyPath;

//通过数组(多个值)键获取字典值

- (NSDictionary *)dictionaryWithValuesForKeys:(NSArray *)keys;

//通过字典键设置多个值(数组)

- (void)setValuesForKeysWithDictionary:(NSDictionary *)keyedValues;

//通过属性字符串键返回可变有序集合

- (NSMutableOrderedSet *)mutableOrderedSetValueForKey:(NSString *)key；

//通过属性路径字符串键返回可变有序集合

- (NSMutableOrderedSet *)mutableOrderedSetValueForKeyPath:(NSString *)keyPath;

//当key不存在时,会处理异常,即会调用下面这两个方法(例如判断key==id时可以设取值)

- (id)valueForUndefinedKey:(NSString *)key;

- (void)setValue:(id)value forUndefinedKey:(NSString *)key;

//为nil设置一个合理的值,对于标量来说,多数情况下合理值为0

- (void)setNilValueForKey:(NSString *)key;

@end

@interface NSArray(NSKeyValueCoding)               //不可变数组

- (id)valueForKey:(NSString *)key;

- (void)setValue:(id)value forKey:(NSString *)key;

@end

@interface NSDictionary(NSKeyValueCoding)          //不可变字典

- (id)valueForKey:(NSString *)key;

@end

@interface NSMutableDictionary(NSKeyValueCoding)//可变字典

- (void)setValue:(id)value forKey:(NSString *)key;

@end

@interface NSOrderedSet(NSKeyValueCoding)       //有序set集合

- (id)valueForKey:(NSString *)key ;

- (void)setValue:(id)value forKey:(NSString *)key ;

@end

@interface NSSet(NSKeyValueCoding)                 //无序set集合

- (id)valueForKey:(NSString *)key;

- (void)setValue:(id)value forKey:(NSString *)key;

@end

具体的演示实例如下：

1、通过键访问属性：

首先创建一个Person类，perosn.h中声明person对象的属性姓名name,年龄age

#import <Foundation/Foundation.h>

@interface Person : NSObject
{
    NSString *_name;  //姓名
    NSInteger _age;   //年龄
}
@end

person.m中重写输出方法-(NSString*)description

#import "Person.h"

@implementation Person
-(NSString *)description
{
    return [NSString stringWithFormat:@"name:%@,age:%ld",_name,_age];
}
@end

然后再main方法中设置属值性和取出属性值

#import <Foundation/Foundation.h>
#import "Person.h"

int main(int argc, const char * argv[]) {
    @autoreleasepool
    {
        //使用KVC来存取对象中的数据成员
        Person *person = [[Person alloc]init];

        [person setValue:@"Tom" forKey:@"_name"];
        [person setValue:@ forKey:@"_age"];

        NSLog(@"%@",person);
    }
    return ;
}

打印结果如下：

-- ::36.736 Person[:] name:Tom,age:
Program ended with exit code: 

2、当对象中的数据成员如果是另一个对象,可以使用keyPath的键路径的形式访问这个对象中的数据成员

首先创建一个学生类Student，在Student.h中声明学生姓名、年龄、课程对象属性

#import <Foundation/Foundation.h>

@class Course;
@interface Student : NSObject
{
    NSString *_name;
    NSInteger _age;
    Course *_course;   //将课程对象定义为学生对象的属性
}
@end

然后再创建一个课程类Course，在Course.h中声明课程名属性

#import <Foundation/Foundation.h>

@interface Course : NSObject
{
    NSString *_CourseName;

CGFloat _score;  //分数

}
@end

最后再main函数中实现KVC机制

#import <Foundation/Foundation.h>
#import "Student.h"
#import "Course.h"

int main(int argc, const char * argv[]) {
    @autoreleasepool
    {

        //创建学生对象
        Student *stu = [[Student alloc]init];

        //设置stu属性值
        [stu setValue:@"Tom" forKey:@"_name"];
        [stu setValue:@ forKey:@"_age"];

        //通过键取出属性值
        NSString *name = [stu valueForKey:@"_name"];
        NSInteger age = [[stu valueForKey:@"_age"] integerValue];

        NSLog(@"name:%@,age:%ld",name,age);

        //创建科目对象
        Course *course = [[Course alloc]init];

        //通过键设置course属性值
        [course setValue:@"Chinese" forKey:@"_CourseName"];
        [course setValue:@99.3 forKey:@"_score"];
        [stu setValue:course forKey:@"_course"];

        //通过键路径取出course属性值
        NSString *courseName = [stu valueForKeyPath:@"_course._CourseName"];
        CGFloat score = [[stu valueForKeyPath:@"_course._score"]floatValue];

        NSLog(@"courseName:%@,score:%.1lf",courseName,score);

        //也可以使用键路径设置和取值
        [stu setValue:@"English" forKeyPath:@"_course._CourseName"];
        courseName = [stu valueForKeyPath:@"_course._CourseName"];

        //将基本数据类型封装成字符串
        [stu setValue:@"92.5" forKeyPath:@"_course._score"];
        NSString *score2 = [stu valueForKeyPath:@"_course._score"];

        NSLog(@"courseName:%@,score2:%@",courseName,score2);
    }
    return ;
}

打印结果如下：

-- ::38.412 KVC-键路径[:] name:Tom,age:
-- ::38.414 KVC-键路径[:] courseName:Chinese,score:99.3
-- ::38.414 KVC-键路径[:] courseName:English,score2:92.5
Program ended with exit code: 

3、使用KVC操作数组：当对象中有一个数组属性时,也可以使用KVC机制访问数组中的数据

首先创建一个动物类Animal

在anmial.h文件中声明属性

#import <Foundation/Foundation.h>

@interface Animal : NSObject
{
    NSString *_name;         //名字
    NSString *_food;         //食物
    NSArray *_otherAnimal;   //其他动物数组
}
@end

在animal.m中重写输出方法-(void)description

#import "Animal.h"

@implementation Animal
-(NSString*)description
{
    return [NSString stringWithFormat:@"name:%@,food:%@",_name,_food];
}
@end

在main主函数中进行KVC的操作

#import <Foundation/Foundation.h>
#import "Animal.h"

int main(int argc, const char * argv[]) {
    @autoreleasepool {

        //创建动物对象
        Animal *animal = [[Animal alloc]init];

        //设置属性
        [animal setValue:@"Cat" forKey:@"_name"];
        [animal setValue:@"fish" forKey:@"_food"];

        //取出属性
        NSString *animalName = [animal valueForKey:@"_name"];
        NSString *foodName = [animal valueForKey:@"_food"];

        NSLog(@"animalName:%@,foodName:%@",animalName,foodName);

        //其他的动物
        Animal *animal2 = [[Animal alloc]init];
        Animal *animal3 = [[Animal alloc]init];
        Animal *animal4 = [[Animal alloc]init];

        [animal2 setValue:@"dog" forKey:@"_name"];
        [animal3 setValue:@"rabbit" forKey:@"_name"];
        [animal4 setValue:@"pannada" forKey:@"_name"];

        [animal2 setValue:@"gone" forKey:@"_food"];
        [animal3 setValue:@"grass" forKey:@"_food"];
        [animal4 setValue:@"bamboo" forKey:@"_food"];

        NSArray *array = @[animal2,animal3,animal4];

        [animal setValue:array forKey:@"_otherAnimal"];

        //取出其他动物的属性
        NSLog(@"%@",[animal valueForKey:@"_otherAnimal"]); //打印所有动物的所有信息
        NSLog(@"%@",[animal valueForKeyPath:@"_otherAnimal.name"]);//输出所有动物的名字
        NSLog(@"%@",[animal valueForKeyPath:@"_otherAnimal._food"]);//输出所有动物的食物
    }
    return ;
}

打印结果如下：

-- ::30.375 KVC-操作数组[:] animalName:Cat,foodName:fish
-- ::30.377 KVC-操作数组[:] (
    "name:dog,food:gone",
    "name:rabbit,food:grass",
    "name:pannada,food:bamboo"
)
-- ::30.377 KVC-操作数组[:] (
    dog,
    rabbit,
    pannada
)
-- ::30.377 KVC-操作数组[:] (
    gone,
    grass,
    bamboo
)
Program ended with exit code: