Key Value Coding是cocoa的一个标准组成部分，它能让我们能够通过name(key)的方式訪问属性，某些情况下极大地简化了代码。可称之为cocoa的大招。

例如以下的样例：

使用KVC的优点

不使用KVC

- (id)tableView:(NSTableView *)tableview
objectValueForTableColumn:(id)column row:(NSInteger)row {
    ChildObject *child = [childrenArray objectAtIndex:row];
    if ([[column identifier] isEqualToString:@"name"]) {
       return [child name];
    }
   if ([[column identifier] isEqualToString:@"age"]) {
       return [child age];
    }
   if ([[column identifier] isEqualToString:@"favoriteColor"]) {
       return [child favoriteColor];
    }
    // And so on.
}

使用KVC

- (id)tableView:(NSTableView *)tableview
objectValueForTableColumn:(id)column row:(NSInteger)row {
    ChildObject *child = [childrenArray objectAtIndex:row];
    return [child valueForKey:[column identifier]];
}

显而易见，简化了非常多代码。

KVC操作

KVC赋值

1 给当前对象属性赋值

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

2给对象的属性的属性赋值

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

3 处理没有定义的键

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

4 字典转模型：会为我们把和dictionary的key名字同样的class proerty设置上dict中key相应的value

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

注意：要求字典中的key和对象属性一样。都是主要的OC数据类型：Array/Dictionary/Boolean/Data/Number/String

KVC取值

1 获取对象属性的值

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

2 获取对象属性的属性的值

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

样例：

Person * p = [[Person alloc]init];
Car *car = [[Car alloc]init];
p.car = car;
[p setValue:@"qhyuan" forKeyPath:@"name"];
[p setValue:@(20) forKey:@"id"];
[p setValue:@"baoshijie" forKeyPath:@"car.brand"];
[p setValue:@"180000" forKeyPath:@"car.price"];
NSLog(@"kvc賦值的person对象----%@",p);
NSString * name = [p valueForKey:@"name"];
NSString * brand = [p valueForKeyPath:@"car.brand"];
NSLog(@"%@ %@",name, brand);

字典转模型

常规情况

模型

Person.h

@interface Person : NSObject
@property (nonatomic, copy) NSString * name;
@property (nonatomic, assign) int age;
- (instancetype) initWithDict:(NSDictionary *) dict;
+ (instancetype) personWithDict:(NSDictionary *) dict;
+ (NSArray *) person;
@end

Person.m

@implementation Person
- (instancetype) initWithDict:(NSDictionary *) dict
{
    if(self = [self init])
{
// 使用KVC 字典转模型 如此方便。省去了大量的赋值代码
[self setValuesForKeysWithDictionary:dict];
    //self.name = dict[@"name"];
    //self.age = [dict[@"age"] integerValue];
    }
    return self;
}
+ (instancetype) personWithDict:(NSDictionary *) dict
{
    return [[self alloc]initWithDict:dict];
}
+ (NSArray *) person
{
    NSMutableArray * mutablePersons = [NSMutableArray array];
    NSString * path = [[NSBundle mainBundle] pathForResource:@"persons.plist" ofType:nil];
    NSArray *persons = [[NSArray alloc] initWithContentsOfFile:path];
    for (NSDictionary * person in persons) {
        [mutablePersons addObject:[self personWithDict:person]];
    }
    return mutablePersons;
}
- (NSString *) description
{
    NSString * desc = [NSString stringWithFormat:@"<%p:(%@,%d)>",self,self.name,self.age];
    return desc;
}
@end

字典中多个某些key是OC中的keyword

假设将键age换成了id

会抛出异常：

*** Terminating app due to uncaught exception ‘NSUnknownKeyException‘,reason: ‘[<Person 0x8c419a0> setValue:forUndefinedKey:]: this class isnot key value coding-compliant for the key id.

重写下面方法就可以，处理没有定义的键

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

解决方案：

- (void) setValue:(id)value forUndefinedKey:(NSString *)key
{
    if([key isEqualToString:@"id"])
        key = @"age";
    [super setValue:value forKey:key];
}

字典里面还包括某些相应自己定义类的字典或者数组

Person类添加了一个Car类型的属性

@property (nonatomic, strong) Car * car;

我们仅仅须要重写下面方法

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

解决方法：

- (void)setValue:(id)value forKey:(NSString *)key
{
    if([key isEqualToString:@"cars"])
    {
        Car *car = [Car carWithDict:(NSDictionary *)value];
        self.car = car;
    }
    else
        [super setValue:value forKey:key];
}

打印结果

字典转模型[5525:60b] (

"<Person:(zhangsan,20,<Car:(benchi,180000)>)>",

"<Person:(lisi,22,<Car:(baoma,180000)>)>",

"<Person:(wangwu,24,<Car:(aodi,180000)>)>"

)

假设不仅仅是添加了Cars属性而是添加了Cars数组，也是类似的方式。