KVC作为OC中重要的编程思想、在很多场景下我们都会使用，也是开发者必须掌握的基础知识。那么什么是KVC，他的实现原理或过程是什么？

什么是KVC

我的理解他就是一种数据访问的机制具体可参考如下的回答：

• KVC 提供一种机制来间接访问对象的属性，该机制通过遵循一个非正式的Protocol（ NSKeyValueCoding ）来实现相关的功能。
• KVC 是一种使用字符串标识符间接访问对象的属性和关系的机制。它的实现与Cocoa编程特有的几种机制和技术相关，其中包括core data、应用程序脚本、绑定技术和声明属性的语言特性。(在OS x上，脚本和绑定是特定于Cocoa的。)

实现原理

KVC的本质是在运行时动态的给对象发送setValue:forKey 消息,设置数值 -调用super.init 保证对象已经被创建完成 .当给对象发送setValue:forKey 消息时要判断对象是否存在key所对应的属性,有则直接赋值，如果没有就调用undefinedKey(默认崩溃,需要重写)
先去检查是否存在setter,getter方法，如果不存在，它将在内部查找名_key或key的实例变量。如果还是没有那就会报错误。通过KVC 可以获取不存在getter方法的对象值。当对象的实例变量为基本数据类型时(例如:char,int,float,BOOL)，我们需要对这些数据类型进行封装。
setValue:forKey的调用顺序首先会寻找set<key>方法,如果没有就去找_<key>、 _is<Key>、 <key> is<key> 顺序寻找,如何还没找到就调用undefinedKey(默认崩溃,需要重写).

KVC的价值或使用场景:

1.可以访问私有成员的值
2.可以间接修改私有成员变量的值(替换系统自带的导航栏,tabBar)
3.应用一些字符做简单运算(sum,min,max,avg,count)比如直接计算数组元素之间的关系。

       //属性相加
        NSString *sum= [persons valueForKeyPath:@"Person.@sum.age"];
        NSLog(@"sum = %@",sum);
        //数量
        NSString *count= [persons valueForKeyPath:@"Person.@count.age"];
        NSLog(@"count = %@",count);
        //最大值
        NSString *max= [persons valueForKeyPath:@"Person.@max.age"];
        NSLog(@"max = %@",max);
        //最小值
        NSString *min= [persons valueForKeyPath:@"Person.@min.age"];
        NSLog(@"min = %@",min);
        //平均值
        NSString *avg= [persons valueForKeyPath:@"Person.@avg.age"];
        NSLog(@"avg = %@",avg);

苹果对NSKeyValueCoding的接口定义如下

@interface NSObject(NSKeyValueCoding)
/* Return YES if -valueForKey:, -setValue:forKey:, -mutableArrayValueForKey:, -storedValueForKey:, -takeStoredValue:forKey:, and -takeValue:forKey: may directly manipulate instance variables when sent to instances of the receiving class, NO otherwise. The default implementation of this property returns YES.
*/
#if FOUNDATION_SWIFT_SDK_EPOCH_AT_LEAST(8)
@property (class, readonly) BOOL accessInstanceVariablesDirectly;
#endif
/* Given a key that identifies an attribute or to-one relationship, return the attribute value or the related object. Given a key that identifies a to-many relationship, return an immutable array or an immutable set that contains all of the related objects.
    
The default implementation of this method does the following:
    1. Searches the class of the receiver for an accessor method whose name matches the pattern -get<Key>, -<key>, or -is<Key>, in that order. If such a method is found it is invoked. If the type of the method's result is an object pointer type the result is simply returned. If the type of the result is one of the scalar types supported by NSNumber conversion is done and an NSNumber is returned. Otherwise, conversion is done and an NSValue is returned (new in Mac OS 10.5: results of arbitrary type are converted to NSValues, not just NSPoint, NRange, NSRect, and NSSize).
    2 (introduced in Mac OS 10.7). Otherwise (no simple accessor method is found), searches the class of the receiver for methods whose names match the patterns -countOf<Key> and -indexIn<Key>OfObject: and -objectIn<Key>AtIndex: (corresponding to the primitive methods defined by the NSOrderedSet class) and also -<key>AtIndexes: (corresponding to -[NSOrderedSet objectsAtIndexes:]). If a count method and an indexOf method and at least one of the other two possible methods are found, a collection proxy object that responds to all NSOrderedSet methods is returned. Each NSOrderedSet message sent to the collection proxy object will result in some combination of -countOf<Key>, -indexIn<Key>OfObject:, -objectIn<Key>AtIndex:, and -<key>AtIndexes: messages being sent to the original receiver of -valueForKey:. If the class of the receiver also implements an optional method whose name matches the pattern -get<Key>:range: that method will be used when appropriate for best performance.
    3. Otherwise (no simple accessor method or set of ordered set access methods is found), searches the class of the receiver for methods whose names match the patterns -countOf<Key> and -objectIn<Key>AtIndex: (corresponding to the primitive methods defined by the NSArray class) and (introduced in Mac OS 10.4) also -<key>AtIndexes: (corresponding to -[NSArray objectsAtIndexes:]). If a count method and at least one of the other two possible methods are found, a collection proxy object that responds to all NSArray methods is returned. Each NSArray message sent to the collection proxy object will result in some combination of -countOf<Key>, -objectIn<Key>AtIndex:, and -<key>AtIndexes: messages being sent to the original receiver of -valueForKey:. If the class of the receiver also implements an optional method whose name matches the pattern -get<Key>:range: that method will be used when appropriate for best performance.
    4 (introduced in Mac OS 10.4). Otherwise (no simple accessor method or set of ordered set or array access methods is found), searches the class of the receiver for a threesome of methods whose names match the patterns -countOf<Key>, -enumeratorOf<Key>, and -memberOf<Key>: (corresponding to the primitive methods defined by the NSSet class). If all three such methods are found a collection proxy object that responds to all NSSet methods is returned. Each NSSet message sent to the collection proxy object will result in some combination of -countOf<Key>, -enumeratorOf<Key>, and -memberOf<Key>: messages being sent to the original receiver of -valueForKey:.
    5. Otherwise (no simple accessor method or set of collection access methods is found), if the receiver's class' +accessInstanceVariablesDirectly property returns YES, searches the class of the receiver for an instance variable whose name matches the pattern _<key>, _is<Key>, <key>, or is<Key>, in that order. If such an instance variable is found, the value of the instance variable in the receiver is returned, with the same sort of conversion to NSNumber or NSValue as in step 1.
    6. Otherwise (no simple accessor method, set of collection access methods, or instance variable is found), invokes -valueForUndefinedKey: and returns the result. The default implementation of -valueForUndefinedKey: raises an NSUndefinedKeyException, but you can override it in your application.

Compatibility notes:
    - For backward binary compatibility, an accessor method whose name matches the pattern -_get<Key>, or -_<key> is searched for between steps 1 and 3. If such a method is found it is invoked, with the same sort of conversion to NSNumber or NSValue as in step 1. KVC accessor methods whose names start with underscores were deprecated as of Mac OS 10.3 though.
    - The behavior described in step 5 is a change from Mac OS 10.2, in which the instance variable search order was <key>, _<key>.
    - For backward binary compatibility, -handleQueryWithUnboundKey: will be invoked instead of -valueForUndefinedKey: in step 6, if the implementation of -handleQueryWithUnboundKey: in the receiver's class is not NSObject's.
*/
- (nullable id)valueForKey:(NSString *)key;

/* Given a value and a key that identifies an attribute, set the value of the attribute. Given an object and a key that identifies a to-one relationship, relate the object to the receiver, unrelating the previously related object if there was one. Given a collection object and a key that identifies a to-many relationship, relate the objects contained in the collection to the receiver, unrelating previously related objects if there were any.

The default implementation of this method does the following:
    1. Searches the class of the receiver for an accessor method whose name matches the pattern -set<Key>:. If such a method is found the type of its parameter is checked. If the parameter type is not an object pointer type but the value is nil -setNilValueForKey: is invoked. The default implementation of -setNilValueForKey: raises an NSInvalidArgumentException, but you can override it in your application. Otherwise, if the type of the method's parameter is an object pointer type the method is simply invoked with the value as the argument. If the type of the method's parameter is some other type the inverse of the NSNumber/NSValue conversion done by -valueForKey: is performed before the method is invoked.
    2. Otherwise (no accessor method is found), if the receiver's class' +accessInstanceVariablesDirectly property returns YES, searches the class of the receiver for an instance variable whose name matches the pattern _<key>, _is<Key>, <key>, or is<Key>, in that order. If such an instance variable is found and its type is an object pointer type the value is retained and the result is set in the instance variable, after the instance variable's old value is first released. If the instance variable's type is some other type its value is set after the same sort of conversion from NSNumber or NSValue as in step 1.
    3. Otherwise (no accessor method or instance variable is found), invokes -setValue:forUndefinedKey:. The default implementation of -setValue:forUndefinedKey: raises an NSUndefinedKeyException, but you can override it in your application.

Compatibility notes:
    - For backward binary compatibility with -takeValue:forKey:'s behavior, a method whose name matches the pattern -_set<Key>: is also recognized in step 1. KVC accessor methods whose names start with underscores were deprecated as of Mac OS 10.3 though.
    - For backward binary compatibility, -unableToSetNilForKey: will be invoked instead of -setNilValueForKey: in step 1, if the implementation of -unableToSetNilForKey: in the receiver's class is not NSObject's.
    - The behavior described in step 2 is different from -takeValue:forKey:'s, in which the instance variable search order is <key>, _<key>.
    - For backward binary compatibility with -takeValue:forKey:'s behavior, -handleTakeValue:forUnboundKey: will be invoked instead of -setValue:forUndefinedKey: in step 3, if the implementation of -handleTakeValue:forUnboundKey: in the receiver's class is not NSObject's.
*/
- (void)setValue:(nullable id)value forKey:(NSString *)key;

/* Given a pointer to a value pointer, a key that identifies an attribute or to-one relationship, and a pointer to an NSError pointer, return a value that is suitable for use in subsequent -setValue:forKey: messages sent to the same receiver. If no validation is necessary, return YES without altering *ioValue or *outError. If validation is necessary and possible, return YES after setting *ioValue to an object that is the validated version of the original value, but without altering *outError. If validation is necessary but not possible, return NO after setting *outError to an NSError that encapsulates the reason that validation was not possible, but without altering *ioValue. The sender of the message is never given responsibility for releasing ioValue or outError.

The default implementation of this method searches the class of the receiver for a validator method whose name matches the pattern -validate<Key>:error:. If such a method is found it is invoked and the result is returned. If no such method is found, YES is returned.
*/
- (BOOL)validateValue:(inout id _Nullable * _Nonnull)ioValue forKey:(NSString *)inKey error:(out NSError **)outError;

/* Given a key that identifies an _ordered_ to-many relationship, return a mutable array that provides read-write access to the related objects. Objects added to the mutable array will become related to the receiver, and objects removed from the mutable array will become unrelated.

The default implementation of this method recognizes the same simple accessor methods and array accessor methods as -valueForKey:'s, and follows the same direct instance variable access policies, but always returns a mutable collection proxy object instead of the immutable collection that -valueForKey: would return. It also:
    1. Searches the class of the receiver for methods whose names match the patterns -insertObject:in<Key>AtIndex: and -removeObjectFrom<Key>AtIndex: (corresponding to the two most primitive methods defined by the NSMutableArray class), and (introduced in Mac OS 10.4) also -insert<Key>:atIndexes: and -remove<Key>AtIndexes: (corresponding to -[NSMutableArray insertObjects:atIndexes:] and -[NSMutableArray removeObjectsAtIndexes:). If at least one insertion method and at least one removal method are found each NSMutableArray message sent to the collection proxy object will result in some combination of -insertObject:in<Key>AtIndex:, -removeObjectFrom<Key>AtIndex:, -insert<Key>:atIndexes:, and -remove<Key>AtIndexes: messages being sent to the original receiver of -mutableArrayValueForKey:. If the class of the receiver also implements an optional method whose name matches the pattern -replaceObjectIn<Key>AtIndex:withObject: or (introduced in Mac OS 10.4) -replace<Key>AtIndexes:with<Key>: that method will be used when appropriate for best performance.
    2. Otherwise (no set of array mutation methods is found), searches the class of the receiver for an accessor method whose name matches the pattern -set<Key>:. If such a method is found each NSMutableArray message sent to the collection proxy object will result in a -set<Key>: message being sent to the original receiver of -mutableArrayValueForKey:.
    3. Otherwise (no set of array mutation methods or simple accessor method is found), if the receiver's class' +accessInstanceVariablesDirectly property returns YES, searches the class of the receiver for an instance variable whose name matches the pattern _<key> or <key>, in that order. If such an instance variable is found, each NSMutableArray message sent to the collection proxy object will be forwarded to the instance variable's value, which therefore must typically be an instance of NSMutableArray or a subclass of NSMutableArray.
    4. Otherwise (no set of array mutation methods, simple accessor method, or instance variable is found), returns a mutable collection proxy object anyway. Each NSMutableArray message sent to the collection proxy object will result in a -setValue:forUndefinedKey: message being sent to the original receiver of -mutableArrayValueForKey:. The default implementation of -setValue:forUndefinedKey: raises an NSUndefinedKeyException, but you can override it in your application.

Performance note: the repetitive -set<Key>: messages implied by step 2's description are a potential performance problem. For better performance implement insertion and removal methods that fulfill the requirements for step 1 in your KVC-compliant class. For best performance implement a replacement method too.
*/
- (NSMutableArray *)mutableArrayValueForKey:(NSString *)key;

/* Given a key that identifies an _ordered_ and uniquing to-many relationship, return a mutable ordered set that provides read-write access to the related objects. Objects added to the mutable ordered set will become related to the receiver, and objects removed from the mutable ordered set will become unrelated.

The default implementation of this method recognizes the same simple accessor methods and ordered set accessor methods as -valueForKey:'s, and follows the same direct instance variable access policies, but always returns a mutable collection proxy object instead of the immutable collection that -valueForKey: would return. It also:
    1. Searches the class of the receiver for methods whose names match the patterns -insertObject:in<Key>AtIndex: and -removeObjectFrom<Key>AtIndex: (corresponding to the two most primitive methods defined by the NSMutableOrderedSet class), and also -insert<Key>:atIndexes: and -remove<Key>AtIndexes: (corresponding to -[NSMutableOrderedSet insertObjects:atIndexes:] and -[NSMutableOrderedSet removeObjectsAtIndexes:). If at least one insertion method and at least one removal method are found each NSMutableOrderedSet message sent to the collection proxy object will result in some combination of -insertObject:in<Key>AtIndex:, -removeObjectFrom<Key>AtIndex:, -insert<Key>:atIndexes:, and -remove<Key>AtIndexes: messages being sent to the original receiver of -mutableOrderedSetValueForKey:. If the class of the receiver also implements an optional method whose name matches the pattern -replaceObjectIn<Key>AtIndex:withObject: or -replace<Key>AtIndexes:with<Key>: that method will be used when appropriate for best performance.
    2. Otherwise (no set of ordered set mutation methods is found), searches the class of the receiver for an accessor method whose name matches the pattern -set<Key>:. If such a method is found each NSMutableOrderedSet message sent to the collection proxy object will result in a -set<Key>: message being sent to the original receiver of -mutableOrderedSetValueForKey:.
    3. Otherwise (no set of ordered set mutation methods or simple accessor method is found), if the receiver's class' +accessInstanceVariablesDirectly property returns YES, searches the class of the receiver for an instance variable whose name matches the pattern _<key> or <key>, in that order. If such an instance variable is found, each NSMutableOrderedSet message sent to the collection proxy object will be forwarded to the instance variable's value, which therefore must typically be an instance of NSMutableOrderedSet or a subclass of NSMutableOrderedSet.
    4. Otherwise (no set of ordered set mutation methods, simple accessor method, or instance variable is found), returns a mutable collection proxy object anyway. Each NSMutableOrderedSet message sent to the collection proxy object will result in a -setValue:forUndefinedKey: message being sent to the original receiver of -mutableOrderedSetValueForKey:. The default implementation of -setValue:forUndefinedKey: raises an NSUndefinedKeyException, but you can override it in your application.

Performance note: the repetitive -set<Key>: messages implied by step 2's description are a potential performance problem. For better performance implement insertion and removal methods that fulfill the requirements for step 1 in your KVC-compliant class. For best performance implement a replacement method too.
*/
- (NSMutableOrderedSet *)mutableOrderedSetValueForKey:(NSString *)key NS_AVAILABLE(10_7, 5_0);

/* Given a key that identifies an _unordered_ and uniquing to-many relationship, return a mutable set that provides read-write access to the related objects. Objects added to the mutable set will become related to the receiver, and objects removed from the mutable set will become unrelated.

The default implementation of this method recognizes the same simple accessor methods and set accessor methods as -valueForKey:'s, and follows the same direct instance variable access policies, but always returns a mutable collection proxy object instead of the immutable collection that -valueForKey: would return. It also:
    1. Searches the class of the receiver for methods whose names match the patterns -add<Key>Object: and -remove<Key>Object: (corresponding to the two primitive methods defined by the NSMutableSet class) and also -add<Key>: and -remove<Key>: (corresponding to -[NSMutableSet unionSet:] and -[NSMutableSet minusSet:]). If at least one addition method and at least one removal method are found each NSMutableSet message sent to the collection proxy object will result in some combination of -add<Key>Object:, -remove<Key>Object:, -add<Key>:, and -remove<Key>: messages being sent to the original receiver of -mutableSetValueForKey:. If the class of the receiver also implements an optional method whose name matches the pattern -intersect<Key>: or -set<Key>: that method will be used when appropriate for best performance.
    2. Otherwise (no set of set mutation methods is found), searches the class of the receiver for an accessor method whose name matches the pattern -set<Key>:. If such a method is found each NSMutableSet message sent to the collection proxy object will result in a -set<Key>: message being sent to the original receiver of -mutableSetValueForKey:.
    3. Otherwise (no set of set mutation methods or simple accessor method is found), if the receiver's class' +accessInstanceVariablesDirectly property returns YES, searches the class of the receiver for an instance variable whose name matches the pattern _<key> or <key>, in that order. If such an instance variable is found, each NSMutableSet message sent to the collection proxy object will be forwarded to the instance variable's value, which therefore must typically be an instance of NSMutableSet or a subclass of NSMutableSet.
    4. Otherwise (no set of set mutation methods, simple accessor method, or instance variable is found), returns a mutable collection proxy object anyway. Each NSMutableSet message sent to the collection proxy object will result in a -setValue:forUndefinedKey: message being sent to the original receiver of -mutableSetValueForKey:. The default implementation of -setValue:forUndefinedKey: raises an NSUndefinedKeyException, but you can override it in your application.

Performance note: the repetitive -set<Key>: messages implied by step 2's description are a potential performance problem. For better performance implement methods that fulfill the requirements for step 1 in your KVC-compliant class.
*/
- (NSMutableSet *)mutableSetValueForKey:(NSString *)key;

/* Key-path-taking variants of like-named methods. The default implementation of each parses the key path enough to determine whether or not it has more than one component (key path components are separated by periods). If so, -valueForKey: is invoked with the first key path component as the argument, and the method being invoked is invoked recursively on the result, with the remainder of the key path passed as an argument. If not, the like-named non-key-path-taking method is invoked.
*/
- (nullable id)valueForKeyPath:(NSString *)keyPath;
- (void)setValue:(nullable id)value forKeyPath:(NSString *)keyPath;
- (BOOL)validateValue:(inout id _Nullable * _Nonnull)ioValue forKeyPath:(NSString *)inKeyPath error:(out NSError **)outError;
- (NSMutableArray *)mutableArrayValueForKeyPath:(NSString *)keyPath;
- (NSMutableOrderedSet *)mutableOrderedSetValueForKeyPath:(NSString *)keyPath NS_AVAILABLE(10_7, 5_0);
- (NSMutableSet *)mutableSetValueForKeyPath:(NSString *)keyPath;

/* Given that an invocation of -valueForKey: would be unable to get a keyed value using its default access mechanism, return the keyed value using some other mechanism. The default implementation of this method raises an NSUndefinedKeyException. You can override it to handle properties that are dynamically defined at run-time.
*/
- (nullable id)valueForUndefinedKey:(NSString *)key;

/* Given that an invocation of -setValue:forKey: would be unable to set the keyed value using its default mechanism, set the keyed value using some other mechanism. The default implementation of this method raises an NSUndefinedKeyException. You can override it to handle properties that are dynamically defined at run-time.
*/
- (void)setValue:(nullable id)value forUndefinedKey:(NSString *)key;

/* Given that an invocation of -setValue:forKey: would be unable to set the keyed value because the type of the parameter of the corresponding accessor method is an NSNumber scalar type or NSValue structure type but the value is nil, set the keyed value using some other mechanism. The default implementation of this method raises an NSInvalidArgumentException. You can override it to map nil values to something meaningful in the context of your application.
*/
- (void)setNilValueForKey:(NSString *)key;

/* Given an array of keys, return a dictionary containing the keyed attribute values, to-one-related objects, and/or collections of to-many-related objects. Entries for which -valueForKey: returns nil have NSNull as their value in the returned dictionary.
*/
- (NSDictionary<NSString *, id> *)dictionaryWithValuesForKeys:(NSArray<NSString *> *)keys;

/* Given a dictionary containing keyed attribute values, to-one-related objects, and/or collections of to-many-related objects, set the keyed values. Dictionary entries whose values are NSNull result in -setValue:nil forKey:key messages being sent to the receiver.
*/
- (void)setValuesForKeysWithDictionary:(NSDictionary<NSString *, id> *)keyedValues;
@end

ValueForKey的调用顺序 按照get<key>, <key> is<key> 顺序寻找,如果没有找到按照_<key> _is<key> <key> is<key>顺序寻找,如何还没找到就调用undefinedKey(默认崩溃,需要重写).