KVC底层实现的是setter和getter方法。

KVC简介

代码准备
苹果官方文档查阅

//类LGPerson中的方法
#import "LGStudent.h"

NS_ASSUME_NONNULL_BEGIN

typedef struct {
    float x, y, z;
} ThreeFloats;

@interface LGPerson : NSObject{
   @public
   NSString *myName;
}

@property (nonatomic, copy)   NSString          *name;
@property (nonatomic, strong) NSArray           *array;
@property (nonatomic, strong) NSMutableArray    *mArray;
@property (nonatomic, assign) int age;
@property (nonatomic)         ThreeFloats       threeFloats;
@property (nonatomic, strong) LGStudent         *student;

@end
//类LGStudent中的方法
@interface LGStudent : NSObject
@property (nonatomic, copy)   NSString          *name;
@property (nonatomic, copy)   NSString          *subject;
@property (nonatomic, copy)   NSString          *nick;
@property (nonatomic, assign) int               age;
@property (nonatomic, assign) int               length;
@property (nonatomic, strong) NSMutableArray    *penArr;
@end

KVC设置的过程底层的代码是objc_setProperty_noatomic_copy(这个copy不一定，根据参数类型来定的)->reallySetProperty(self,_cmd,newValue,offSet,false,true,false)等方法。
（可以使用VSCode编译器，查看下源码）

 LGPerson *person = [[LGPerson alloc] init];
    // 一般setter 方法
    person.name      = @"LG_Cooci";
    person.age       = 18;
    person->myName   = @"cooci";
    NSLog(@"%@ - %d - %@",person.name,person.age,person->myName);
 // 1:Key-Value Coding (KVC) : 基本类型
    [person setValue:@"KC" forKey:@"name"];
    [person setValue:@19 forKey:@"age"];
    [person setValue:@"酷C" forKey:@"myName"];
    NSLog(@"%@ - %@ - %@",[person valueForKey:@"name"],[person valueForKey:@"age"],[person valueForKey:@"myName"]);

 // 2:KVC - 集合类型 -
    person.array = @[@"1",@"2",@"3"];
    // 由于不是可变数组 - 无法做到
    // person.array[0] = @"100";
    NSArray *array = [person valueForKey:@"array"];
    // 用 array 的值创建一个新的数组
    array = @[@"100",@"2",@"3"];
    [person setValue:array forKey:@"array"];
    NSLog(@"%@",[person valueForKey:@"array"]);
    // KVC 的方式
    NSMutableArray *ma = [person mutableArrayValueForKey:@"array"];
    ma[0] = @"100";
    NSLog(@"%@",[person valueForKey:@"array"]);
// 3:KVC - 集合操作符
    //[self dictionaryTest];
    //[self arrayMessagePass];
    //[self aggregationOperator];
    //[self arrayOperator];
    //[self arrayNesting];
    //[self setNesting];
    //[self arrayDemo];


    // 4:KVC - 访问非对象属性
    ThreeFloats floats = {1., 2., 3.};
    NSValue *value  = [NSValue valueWithBytes:&floats objCType:@encode(ThreeFloats)];
    [person setValue:value forKey:@"threeFloats"];
    NSValue *reslut = [person valueForKey:@"threeFloats"];
    NSLog(@"%@",reslut);
    
    ThreeFloats th;
    [reslut getValue:&th] ;
    NSLog(@"%f - %f - %f",th.x,th.y,th.z);
    
    // 5:KVC - 层层访问
    LGStudent *student = [[LGStudent alloc] init];
    student.subject    = @"iOS";
    person.student     = student;
    [person setValue:@"大师班" forKeyPath:@"student.subject"];
    NSLog(@"%@",[person valueForKeyPath:@"student.subject"]);

}


#pragma mark - array取值
- (void)arrayDemo{
    LGStudent *p = [LGStudent new];
    p.penArr = [NSMutableArray arrayWithObjects:@"pen0", @"pen1", @"pen2", @"pen3", nil];
    NSArray *arr = [p valueForKey:@"pens"]; // 动态成员变量
    NSLog(@"pens = %@", arr);
    //NSLog(@"%@",arr[0]);
    NSLog(@"%d",[arr containsObject:@"pen9"]);
    // 遍历
    NSEnumerator *enumerator = [arr objectEnumerator];
    NSString* str = nil;
    while (str = [enumerator nextObject]) {
        NSLog(@"%@", str);
    }
}

#pragma mark - 字典操作

- (void)dictionaryTest{
    
    NSDictionary* dict = @{
                           @"name":@"Cooci",
                           @"nick":@"KC",
                           @"subject":@"iOS",
                           @"age":@18,
                           @"length":@180
                           };
    LGStudent *p = [[LGStudent alloc] init];
    // 字典转模型
    [p setValuesForKeysWithDictionary:dict];
    NSLog(@"%@",p);
    // 键数组转模型到字典
    NSArray *array = @[@"name",@"age"];
    NSDictionary *dic = [p dictionaryWithValuesForKeys:array];
    NSLog(@"%@",dic);
}

#pragma mark - KVC消息传递
- (void)arrayMessagePass{
    NSArray *array = @[@"Hank",@"Cooci",@"Kody",@"CC"];
    NSArray *lenStr= [array valueForKeyPath:@"length"];
    NSLog(@"%@",lenStr);// 消息从array传递给了string
    NSArray *lowStr= [array valueForKeyPath:@"lowercaseString"];
    NSLog(@"%@",lowStr);
}

#pragma mark - 聚合操作符
// @avg、@count、@max、@min、@sum
- (void)aggregationOperator{
    NSMutableArray *personArray = [NSMutableArray array];
    for (int i = 0; i < 6; i++) {
        LGStudent *p = [LGStudent new];
        NSDictionary* dict = @{
                               @"name":@"Tom",
                               @"age":@(18+i),
                               @"nick":@"Cat",
                               @"length":@(175 + 2*arc4random_uniform(6)),
                               };
        [p setValuesForKeysWithDictionary:dict];
        [personArray addObject:p];
    }
    NSLog(@"%@", [personArray valueForKey:@"length"]);
    
    /// 平均身高
    float avg = [[personArray valueForKeyPath:@"@avg.length"] floatValue];
    NSLog(@"%f", avg);
    
    int count = [[personArray valueForKeyPath:@"@count.length"] intValue];
    NSLog(@"%d", count);
    
    int sum = [[personArray valueForKeyPath:@"@sum.length"] intValue];
    NSLog(@"%d", sum);
    
    int max = [[personArray valueForKeyPath:@"@max.length"] intValue];
    NSLog(@"%d", max);
    
    int min = [[personArray valueForKeyPath:@"@min.length"] intValue];
    NSLog(@"%d", min);
}

// 数组操作符 @distinctUnionOfObjects @unionOfObjects
- (void)arrayOperator{
    NSMutableArray *personArray = [NSMutableArray array];
    for (int i = 0; i < 6; i++) {
        LGStudent *p = [LGStudent new];
        NSDictionary* dict = @{
                               @"name":@"Tom",
                               @"age":@(18+i),
                               @"nick":@"Cat",
                               @"length":@(175 + 2*arc4random_uniform(6)),
                               };
        [p setValuesForKeysWithDictionary:dict];
        [personArray addObject:p];
    }
    NSLog(@"%@", [personArray valueForKey:@"length"]);
    // 返回操作对象指定属性的集合
    NSArray* arr1 = [personArray valueForKeyPath:@"@unionOfObjects.length"];
    NSLog(@"arr1 = %@", arr1);
    // 返回操作对象指定属性的集合 -- 去重
    NSArray* arr2 = [personArray valueForKeyPath:@"@distinctUnionOfObjects.length"];
    NSLog(@"arr2 = %@", arr2);
    
}

// 嵌套集合(array&set)操作 @distinctUnionOfArrays @unionOfArrays @distinctUnionOfSets
- (void)arrayNesting{
    
    NSMutableArray *personArray1 = [NSMutableArray array];
    for (int i = 0; i < 6; i++) {
        LGStudent *student = [LGStudent new];
        NSDictionary* dict = @{
                               @"name":@"Tom",
                               @"age":@(18+i),
                               @"nick":@"Cat",
                               @"length":@(175 + 2*arc4random_uniform(6)),
                               };
        [student setValuesForKeysWithDictionary:dict];
        [personArray1 addObject:student];
    }
    
    NSMutableArray *personArray2 = [NSMutableArray array];
    for (int i = 0; i < 6; i++) {
        LGPerson *person = [LGPerson new];
        NSDictionary* dict = @{
                               @"name":@"Tom",
                               @"age":@(18+i),
                               @"nick":@"Cat",
                               @"length":@(175 + 2*arc4random_uniform(6)),
                               };
        [person setValuesForKeysWithDictionary:dict];
        [personArray2 addObject:person];
    }
    
    // 嵌套数组
    NSArray* nestArr = @[personArray1, personArray2];
    
    NSArray* arr = [nestArr valueForKeyPath:@"@distinctUnionOfArrays.length"];
    NSLog(@"arr = %@", arr);
    
    NSArray* arr1 = [nestArr valueForKeyPath:@"@unionOfArrays.length"];
    NSLog(@"arr1 = %@", arr1);
}

- (void)setNesting{
    
    NSMutableSet *personSet1 = [NSMutableSet set];
    for (int i = 0; i < 6; i++) {
        LGStudent *person = [LGStudent new];
        NSDictionary* dict = @{
                               @"name":@"Tom",
                               @"age":@(18+i),
                               @"nick":@"Cat",
                               @"length":@(175 + 2*arc4random_uniform(6)),
                               };
        [person setValuesForKeysWithDictionary:dict];
        [personSet1 addObject:person];
    }
    NSLog(@"personSet1 = %@", [personSet1 valueForKey:@"length"]);
    
    NSMutableSet *personSet2 = [NSMutableSet set];
    for (int i = 0; i < 6; i++) {
        LGPerson *person = [LGPerson new];
        NSDictionary* dict = @{
                               @"name":@"Tom",
                               @"age":@(18+i),
                               @"nick":@"Cat",
                               @"length":@(175 + 2*arc4random_uniform(6)),
                               };
        [person setValuesForKeysWithDictionary:dict];
        [personSet2 addObject:person];
    }
    NSLog(@"personSet2 = %@", [personSet2 valueForKey:@"length"]);

    // 嵌套set
    NSSet* nestSet = [NSSet setWithObjects:personSet1, personSet2, nil];
    // 交集
    NSArray* arr1 = [nestSet valueForKeyPath:@"@distinctUnionOfSets.length"];
    NSLog(@"arr1 = %@", arr1);
}

KVC取值&赋值过程

pragma mark - 关闭或开启实例变量赋值

• (BOOL)accessInstanceVariablesDirectly{
  默认返回为YES，如果返回为NO，则报错（setValue:forUndefinedKey:）。
  return YES;
  }
  setvalue:forkey(是成员变量分析而不是属性分析)
  属性系统自动为我们实现了set和get方法了。

LGPerson *person = [[LGPerson alloc] init];

// 1: KVC - 设置值的过程 _<key>,_is<Key>,<key>,is<Key>的过程
// [person setValue:@"LG_Cooci" forKey:@"name"];

// NSLog(@"%@-%@-%@-%@",person->_name,person->_isName,person->name,person->isName);
// NSLog(@"%@-%@-%@",person->_isName,person->name,person->isName);
// NSLog(@"%@-%@",person->name,person->isName);
// NSLog(@"%@",person->isName);

// 2: KVC - 取值的过程
// person->_name = @"_name";
// person->_isName = @"_isName";
// person->name = @"name";
// person->isName = @"isName";

// NSLog(@"取值:%@",[person valueForKey:@"name"]);

//MARK: - valueForKey 流程分析 - get<Key>, <key>, is<Key>, or _<key>,

accessor method is an NSNumber scalar type or NSValue structure type but the value is nil,
只针对NSValue以及NSNumber的

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

自定义KVC

#import <objc/runtime.h>

@implementation NSObject (LGKVC)

- (void)lg_setValue:(nullable id)value forKey:(NSString *)key{
    
    // 1:非空判断一下
    if (key == nil  || key.length == 0) return;
    
    // 2:找到相关方法 set<Key> _set<Key> setIs<Key>
    // key 要大写
    NSString *Key = key.capitalizedString;
    // 拼接方法
    NSString *setKey = [NSString stringWithFormat:@"set%@:",Key];
    NSString *_setKey = [NSString stringWithFormat:@"_set%@:",Key];
    NSString *setIsKey = [NSString stringWithFormat:@"setIs%@:",Key];
    
    if ([self lg_performSelectorWithMethodName:setKey value:value]) {
        NSLog(@"*********%@**********",setKey);
        return;
    }else if ([self lg_performSelectorWithMethodName:_setKey value:value]) {
        NSLog(@"*********%@**********",_setKey);
        return;
    }else if ([self lg_performSelectorWithMethodName:setIsKey value:value]) {
        NSLog(@"*********%@**********",setIsKey);
        return;
    }
    
    // 3:判断是否能够直接赋值实例变量
    if (![self.class accessInstanceVariablesDirectly] ) {
        @throw [NSException exceptionWithName:@"LGUnknownKeyException" reason:[NSString stringWithFormat:@"****[%@ valueForUndefinedKey:]: this class is not key value coding-compliant for the key name.****",self] userInfo:nil];
    }
    
    // 4.找相关实例变量进行赋值
    // 4.1 定义一个收集实例变量的可变数组
    NSMutableArray *mArray = [self getIvarListName];
    // _<key> _is<Key> <key> is<Key>
    NSString *_key = [NSString stringWithFormat:@"_%@",key];
    NSString *_isKey = [NSString stringWithFormat:@"_is%@",Key];
    NSString *isKey = [NSString stringWithFormat:@"is%@",Key];
    if ([mArray containsObject:_key]) {
        // 4.2 获取相应的 ivar
       Ivar ivar = class_getInstanceVariable([self class], _key.UTF8String);
        // 4.3 对相应的 ivar 设置值
       object_setIvar(self , ivar, value);
       return;
    }else if ([mArray containsObject:_isKey]) {
       Ivar ivar = class_getInstanceVariable([self class], _isKey.UTF8String);
       object_setIvar(self , ivar, value);
       return;
    }else if ([mArray containsObject:key]) {
       Ivar ivar = class_getInstanceVariable([self class], key.UTF8String);
       object_setIvar(self , ivar, value);
       return;
    }else if ([mArray containsObject:isKey]) {
       Ivar ivar = class_getInstanceVariable([self class], isKey.UTF8String);
       object_setIvar(self , ivar, value);
       return;
    }

    // 5:如果找不到相关实例
    @throw [NSException exceptionWithName:@"LGUnknownKeyException" reason:[NSString stringWithFormat:@"****[%@ %@]: this class is not key value coding-compliant for the key name.****",self,NSStringFromSelector(_cmd)] userInfo:nil];
}


- (nullable id)lg_valueForKey:(NSString *)key{
    
    // 1:刷选key 判断非空
    if (key == nil  || key.length == 0) {
        return nil;
    }

    // 2:找到相关方法 get<Key> <key> countOf<Key>  objectIn<Key>AtIndex
    // key 要大写
    NSString *Key = key.capitalizedString;
    // 拼接方法
    NSString *getKey = [NSString stringWithFormat:@"get%@",Key];
    NSString *countOfKey = [NSString stringWithFormat:@"countOf%@",Key];
    NSString *objectInKeyAtIndex = [NSString stringWithFormat:@"objectIn%@AtIndex:",Key];
        
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Warc-performSelector-leaks"
    if ([self respondsToSelector:NSSelectorFromString(getKey)]) {
        return [self performSelector:NSSelectorFromString(getKey)];
    }else if ([self respondsToSelector:NSSelectorFromString(key)]){
        return [self performSelector:NSSelectorFromString(key)];
    }else if ([self respondsToSelector:NSSelectorFromString(countOfKey)]){
        if ([self respondsToSelector:NSSelectorFromString(objectInKeyAtIndex)]) {
            int num = (int)[self performSelector:NSSelectorFromString(countOfKey)];
            NSMutableArray *mArray = [NSMutableArray arrayWithCapacity:1];
            for (int i = 0; i<num-1; i++) {
                num = (int)[self performSelector:NSSelectorFromString(countOfKey)];
            }
            for (int j = 0; j<num; j++) {
                id objc = [self performSelector:NSSelectorFromString(objectInKeyAtIndex) withObject:@(num)];
                [mArray addObject:objc];
            }
            return mArray;
        }
    }
#pragma clang diagnostic pop
    
    // 3:判断是否能够直接赋值实例变量
    if (![self.class accessInstanceVariablesDirectly] ) {
        @throw [NSException exceptionWithName:@"LGUnknownKeyException" reason:[NSString stringWithFormat:@"****[%@ valueForUndefinedKey:]: this class is not key value coding-compliant for the key name.****",self] userInfo:nil];
    }
    
    // 4.找相关实例变量进行赋值
    // 4.1 定义一个收集实例变量的可变数组
    NSMutableArray *mArray = [self getIvarListName];
    // _<key> _is<Key> <key> is<Key>
    // _name -> _isName -> name -> isName
    NSString *_key = [NSString stringWithFormat:@"_%@",key];
    NSString *_isKey = [NSString stringWithFormat:@"_is%@",Key];
    NSString *isKey = [NSString stringWithFormat:@"is%@",Key];
    if ([mArray containsObject:_key]) {
        Ivar ivar = class_getInstanceVariable([self class], _key.UTF8String);
        return object_getIvar(self, ivar);;
    }else if ([mArray containsObject:_isKey]) {
        Ivar ivar = class_getInstanceVariable([self class], _isKey.UTF8String);
        return object_getIvar(self, ivar);;
    }else if ([mArray containsObject:key]) {
        Ivar ivar = class_getInstanceVariable([self class], key.UTF8String);
        return object_getIvar(self, ivar);;
    }else if ([mArray containsObject:isKey]) {
        Ivar ivar = class_getInstanceVariable([self class], isKey.UTF8String);
        return object_getIvar(self, ivar);;
    }

    return @"";
}

#pragma mark - 相关方法
- (BOOL)lg_performSelectorWithMethodName:(NSString *)methodName value:(id)value{
 
    if ([self respondsToSelector:NSSelectorFromString(methodName)]) {
        
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Warc-performSelector-leaks"
        [self performSelector:NSSelectorFromString(methodName) withObject:value];
#pragma clang diagnostic pop
        return YES;
    }
    return NO;
}

- (id)performSelectorWithMethodName:(NSString *)methodName{
    if ([self respondsToSelector:NSSelectorFromString(methodName)]) {
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Warc-performSelector-leaks"
        return [self performSelector:NSSelectorFromString(methodName) ];
#pragma clang diagnostic pop
    }
    return nil;
}

- (NSMutableArray *)getIvarListName{
    
    NSMutableArray *mArray = [NSMutableArray arrayWithCapacity:1];
    unsigned int count = 0;
    Ivar *ivars = class_copyIvarList([self class], &count);
    for (int i = 0; i<count; i++) {
        Ivar ivar = ivars[i];
        const char *ivarNameChar = ivar_getName(ivar);
        NSString *ivarName = [NSString stringWithUTF8String:ivarNameChar];
        NSLog(@"ivarName == %@",ivarName);
        [mArray addObject:ivarName];
    }
    free(ivars);
    return mArray;
}

KVC异常处理技巧

LGPerson *person = [[LGPerson alloc] init];
    
    // 1: KVC 自动转换类型
    NSLog(@"******1: KVC - int -> NSNumber - 结构体******");

    [person setValue:@18 forKey:@"age"];
    // 上面那个表达 大家应该都会! 但是下面这样操作可以?
    [person setValue:@"20" forKey:@"age"]; // int - string
    NSLog(@"%@-%@",[person valueForKey:@"age"],[[person valueForKey:@"age"] class]);//__NSCFNumber
    
    [person setValue:@"20" forKey:@"sex"];
    NSLog(@"%@-%@",[person valueForKey:@"sex"],[[person valueForKey:@"sex"] class]);//__NSCFNumber

    ThreeFloats floats = {1., 2., 3.};
    NSValue *value  = [NSValue valueWithBytes:&floats objCType:@encode(ThreeFloats)];
    [person setValue:value forKey:@"threeFloats"];
    NSLog(@"%@-%@",[person valueForKey:@"threeFloats"],[[person valueForKey:@"threeFloats"] class]);//NSConcreteValue
    
    // 2: 设置空值
    NSLog(@"******2: 设置空值******");
    [person setValue:nil forKey:@"age"]; // subject不会走 - 官方注释里面说只对 NSNumber - NSValue
    [person setValue:nil forKey:@"subject"];
    
    // 3: 找不到的 key
    NSLog(@"******3: 找不到的 key******");
    [person setValue:nil forKey:@"KC"]; 

    // 4: 取值时 - 找不到 key
    NSLog(@"******4: 取值时 - 找不到 key******");
    NSLog(@"%@",[person valueForKey:@"KC"]);
    
    // 5: 键值验证
    NSLog(@"******5: 键值验证******");
    NSError *error;
    NSString *name = @"LG_Cooci";
    if (![person validateValue:&name forKey:@"names" error:&error]) {
        NSLog(@"%@",error);
    }else{
        NSLog(@"%@",[person valueForKey:@"name"]);
    }

kvc.png