一、Key Value Coding（KVC）提供了一种间接访问对象属性（用字符串表征，作为key值）的机制

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

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

例如对于Person类：

@property(nonatomic,copy)NSString* name;//属性为类类型
@property(nonatomic)CGFloat height;//属性为标量类型

Person *person = [[Person alloc] init];
        
        [person setValue:@"name" forKey:@"name"];
        [person setValue:@(20) forKey:@"height"];
        
        NSLog(@"name = %@",[person valueForKey:@"name"]);
        NSLog(@"height = %@",[person valueForKey:@"height"]);

注意到假如 Person 类有一个地址属性 address，地址 address 又有一个属性 city。

那么要访问Person类对象的 city 这个属性，就得通过Key Path，即使用下面的方法：

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

[person valueForKeyPath:@"address.city"]

二、Key-Value Coding Without @property

通过 key-value coding-compliant，可以取代 @property 和 @synthesize，通过实现 -<key>: 和 -set<key>: 方法即可。

例如对于属性 name，我们需要实现

- (NSString *)name;
- (void)setName:(NSString *)name;

- (CGFloat)height;
- (void)setHeight:(CGFloat)height;

[object setValue:nil forKey:@"height"]

*** Terminating app due to uncaught exception ‘NSInvalidArgumentException‘, reason: ‘[<Person 0x100108bf0> setNilValueForKey]: could not set nil as the value for the key height.‘

-(void)setNilValueForKey:(NSString *)key
{
    if ([key isEqualToString:@"height"]) {
        [self setValue:@0 forKey:key];
    }
    else
    {
        [super setNilValueForKey:key];
    }
}

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

这是个经常被忽视的KVC特性。

例如：

NSArray *a = @[@4, @84, @2];
        NSLog(@"max = %@", [a valueForKeyPath:@"@max.self"]);

又例如：某一类 Transaction，有一个 amount 属性，我们可以求出 Transaction 类对象中 amount 的最大值。

NSArray *a = @[transaction1, transaction2, transaction3];
NSLog(@"max = %@", [a valueForKeyPath:@"@max.amount"]);

对于 Collection Operator 官方文档中有详细的介绍：点击打开链接

四、KVC Through Collection Proxy Objects（比较少用到）

对于不可变集合类型：

When we call -valueForKey: on an object, that object can return collection proxy objects for an NSArray, an NSSet, or an NSOrderedSet. The class doesn’t implement the normal -<Key> method but instead implements a number of methods that the proxy uses.

例如：对于一个数组类型的属性 contracts 需要实现：

- (NSUInteger)countOfContacts;
- (id)objectInContactsAtIndex:(NSUInteger)idx;

Using these proxy objects only makes sense in special situations, but in those cases it can be very helpful. Imagine that we have a very large existing data structure and the caller doesn’t need to access all elements (at once).

对于可变集合类型：NSMutableArray,NSMutableSet,NSMutableOrderedSet.

获取这些可变集合类型，可以调用如下的方法：

- (NSMutableArray *)mutableArrayValueForKey:(NSString *)key;
- (NSMutableSet *)mutableSetValueForKey:(NSString *)key;
- (NSMutableOrderedSet *)mutableOrderedSetValueForKey:(NSString *)key;

- (NSMutableArray *)mutableContacts;
{
    return [self mutableArrayValueForKey:@"wrappedContacts"];
}

（1）We cannot observe an NSArray; we can only observe a property on an object – and that property may be an NSArray. 

As an example, if we have a ContactList object, we can observe itscontacts property, but we cannot pass an NSArray to -addObserver:forKeyPath:... as the object to be observed.

（2）Observing self doesn’t always work. It’s probably not a good design pattern, either.

六、KVO调试

(lldb) po [observedObject observationInfo]

七、Key-Value Validation（校验）

对于某些模型对象，需要校验其中的属性，那么通过在Key-Value Validation ，实现相关的校验方法，就可以在模型类中实现key-value校验。

例如：需要对 Contract 模型中的 name 属性进行校验：

- (BOOL)validateName:(NSString **)nameP error:(NSError * __autoreleasing *)error
{
    if (*nameP == nil) {
        *nameP = @"";
        return YES;
    } else {
        *nameP = [*nameP stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]];
        return YES;
    }
}

- (IBAction)nameFieldEditingDidEnd:(UITextField *)sender;
{
    NSString *name = [sender text];
    NSError *error = nil;
    if ([self.contact validateName:&name error:&error]) {
        self.contact.name = name;
    } else {
        // Present the error to the user
    }
    sender.text = self.contact.name;
}

更详细内容请见objc.io文章：点击打开链接

Objective-C KVC