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

如下的例子：

使用KVC的好处

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，简化了很多代码。

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 处理未定义的键

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

4 字典转模型：会为我们把和dictionary的key名字相同的class proerty设置上dict中key对应的value

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

注意：要求字典中的key和对象属性一样，都是基本的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;

例子：

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中的关键字

如果将键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.

重写以下方法即可，处理未定义的键

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

解决方式：

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

字典里面还包含某些对应自定义类的字典或者数组

Person类增加了一个Car类型的属性

@property (nonatomic, strong) Car * car;

我们只需要重写以下方法

- (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)>)>"

)

如果不只是增加了Cars属性而是增加了Cars数组，也是类似的方式。