func value(forKey key: String) -> Any?                   通过字符串Key获取值
func setValue(_ value: Any?, forKey key: String)         通过字符串Key设置值
func value(forKeyPath keyPath: String) -> Any?           通过字符串keyPath获取值
func setValue(_ value: Any?, forKeyPath keyPath: String) 通过字符串keyPath设置值
func setNilValueForKey(_ key: String)                    如何设置值为nil会调用这个方法 
获取值:key不存在且KVC无法搜索到任何和Key有关的字段或者属性，会调用这个方法，默认抛出异常
func value(forUndefinedKey key: String) -> Any?
设置值:key不存在且KVC无法搜索到任何和Key有关的字段或者属性，会调用这个方法，默认抛出异常
func setValue(_ value: Any?, forUndefinedKey key: String)
直接访问实例变量，默认是true:表示如果没有找到setKey方法时，会按照_key，_iskey，key，iskey的顺序搜  
索成员,如何设置为false,则表示没有找到setKey方法时，就不会继续查找了
class var accessInstanceVariablesDirectly: Bool { get }
属性值的验证，用来检查设置的值是否正确,如何不正确会抛出异常
func validateValue(_ ioValue: AutoreleasingUnsafeMutablePointer<AnyObject?>,   
forKey inKey: String) throws
func validateValue(_ ioValue: AutoreleasingUnsafeMutablePointer<AnyObject?>,   
forKeyPath inKeyPath: String) throws
属性可以是NSMutableArray/NSMutableOrderedSet/NSMutableSet
可以调用下列方法通过Key来获取对应的类型
func mutableArrayValue(forKey key: String) -> NSMutableArray
func mutableArrayValue(forKeyPath keyPath: String) -> NSMutableArray
func mutableOrderedSetValue(forKey key: String) -> NSMutableOrderedSet
func mutableOrderedSetValue(forKeyPath keyPath: String) -> NSMutableOrderedSet
func mutableSetValue(forKey key: String) -> NSMutableSet
func mutableSetValue(forKeyPath keyPath: String) -> NSMutableSet
通过数组Key,返回这些Key对象的Value,通常用于Model转字典
func dictionaryWithValues(forKeys keys: [String]) -> [String : Any]
通过给定的字典为对象的属性设置值
func setValuesForKeys(_ keyedValues: [String : Any])

@objcMembers
public class WGAnimalModel : NSObject {
    private var name = ""  //KVC无法访问私有属性
    //var tuple = ("","")  无法对元组赋值
    var age = 0
    var isSex = false
    var dic = [String: Any]()
    var arr = [String]()
}

let entity = WGAnimalModel.init()   
//赋值
entity.setValue(18, forKey: "age")
entity.setValue(true, forKey: "isSex")
entity.setValue(["height": 130], forKey: "dic")
entity.setValue(["color","weight","room"], forKey: "arr")
//获取值
let age = entity.value(forKey: "age")!
let isSex = entity.value(forKey: "isSex")!
let dic = entity.value(forKey: "dic")!
let arr = entity.value(forKey: "arr")!
NSLog("age:\(age)\nisSex:\(isSex)\ndic:\(dic)\narr:\(arr)")

打印结果: age:18
isSex:1
dic:{
    height = 130;
}
arr:(
    color,
    weight,
    room
)

@objcMembers
public class WGAnimalModel : NSObject {
    private var name = ""  //KVC无法访问私有属性
    //var tuple = ("","")  无法对元组赋值
    var age = 0
    var isSex = false
    var dic = [String: Any]()
    var arr = [String]()
    
    //如果将Key对应的值设置为nil，会导致程序crash,重写这个方法可避免crash
    public override func setNilValueForKey(_ key: String) {
        NSLog("\(key)的值被设置成了nil")
    }
    //如果对不存在的Key赋值，会导致程序crash,重写该方法可避免crash
    public override func setValue(_ value: Any?, forUndefinedKey key: String) {
        NSLog("\(key)不存在")
    }
    //如果去获取一个不存在的Key对应的值，会导致程序crash,重写该方法可避免crash
    public override func value(forUndefinedKey key: String) -> Any? {
        NSLog("\(key)不存在，所以无法获取值")
        return nil
    }
}

let entity = WGAnimalModel.init()
entity.setValue(nil, forKey: "age")
entity.setValue(18, forKey: "ages")
entity.value(forKey: "ages")

打印结果: age的值被设置成了nil
        ages不存在
        ages不存在，所以无法获取值

let entity = WGAnimalModel.init()
entity.setValue(18, forKeyPath: #keyPath(WGAnimalModel.age))
let age = entity.value(forKeyPath: #keyPath(WGAnimalModel.age))!
NSLog("age:\(age)")

打印结果: age:18

文档: A key path that supports reading from and writing to the resulting value   
with reference semantics
public class ReferenceWritableKeyPath<Root, Value> : WritableKeyPath<Root, Value>(支持语义)

文档: A key path that supports reading from and writing to the resulting value(支持读写)
public class WritableKeyPath<Root, Value> : KeyPath<Root, Value>

文档: A key path from a specific root type to a specific resulting value type(仅可读)
public class KeyPath<Root, Value> : PartialKeyPath<Root>

文档: A partially type-erased key path, from a concrete root type to any resulting value type.
public class PartialKeyPath<Root> : AnyKeyPath

public class AnyKeyPath : Hashable, _AppendKeyPath
public protocol _AppendKeyPath

public class WGAnimalModel {
    var age = 0
    var isSex = false
    var dic = [String: Any]()
    var arr = [String]()
    var name: String?=nil
    //只读计算属性
    var cardId: String {
        return "sdfdsf"
    }
    var info: WGInfoModel?
}
public class WGInfoModel {
    var weight = 0
    var height = 0
}

let agePath: WritableKeyPath<WGAnimalModel,Int> = \WGAnimalModel.age
let sexPath: WritableKeyPath<WGAnimalModel,Bool> = \WGAnimalModel.isSex
let arrPath: WritableKeyPath<WGAnimalModel,[String]> = \WGAnimalModel.arr
let dicPath: WritableKeyPath<WGAnimalModel,[String: Any]> = \WGAnimalModel.dic
let namePath: WritableKeyPath<WGAnimalModel,String?> = \WGAnimalModel.name
let cardIdPath: KeyPath<WGAnimalModel,String> = \WGAnimalModel.cardId
let infoPath: ReferenceWritableKeyPath<WGAnimalModel, WGInfoModel?> = \WGAnimalModel.info
let infoWeight: KeyPath<WGAnimalModel, Int?> = \WGAnimalModel.info?.weight

类属性
@property (class, readonly) BOOL accessInstanceVariablesDirectly;
设置值
- (void)setValue:(nullable id)value forKey:(NSString *)key;
- (void)setValue:(nullable id)value forKeyPath:(NSString *)keyPath;
通过Key获取值
- (nullable id)valueForKey:(NSString *)key;
- (NSMutableArray *)mutableArrayValueForKey:(NSString *)key;
- (NSMutableSet *)mutableSetValueForKey:(NSString *)key;
- (NSMutableOrderedSet *)mutableOrderedSetValueForKey:(NSString *)key;
通过keyPath获取值
- (nullable id)valueForKeyPath:(NSString *)keyPath;
- (NSMutableArray *)mutableArrayValueForKeyPath:(NSString *)keyPath;
- (NSMutableSet *)mutableSetValueForKeyPath:(NSString *)keyPath;
- (NSMutableOrderedSet *)mutableOrderedSetValueForKeyPath:(NSString *)keyPath;
验证值
- (BOOL)validateValue:(inout id _Nullable * _Nonnull)ioValue forKey:(NSString *)inKey   
error:(out NSError **)outError;        
- (BOOL)validateValue:(inout id _Nullable * _Nonnull)ioValue forKeyPath:(NSString *)  
inKeyPath error:(out NSError **)outError;
获取一个不存在的Key对应的Value会发生crash，重写该方法可避免程序crash
- (nullable id)valueForUndefinedKey:(NSString *)key;
为一个不存在的Key设置Value会发生crash，重写该方法可避免程序crash
- (void)setValue:(nullable id)value forUndefinedKey:(NSString *)key;
将Key对应的value设置为nil会发生crash，重写该方法可避免程序crash
- (void)setNilValueForKey:(NSString *)key;
输入一组key,返回该组key对应的Value，再转成字典返回，用于将Model转到字典。
- (NSDictionary<NSString *, id> *)dictionaryWithValuesForKeys:(NSArray<NSString *> *)keys;
通过给定的字典为对象的属性设置值
- (void)setValuesForKeysWithDictionary:(NSDictionary<NSString *, id> *)keyedValues;

@interface WGTestModel : NSObject
@property(nonatomic, strong) NSString *name;
@property(nonatomic, assign) BOOL isSex;
@end

@implementation WGTestModel
@end
    
- (void)viewDidLoad {
    [super viewDidLoad];
    WGTestModel *model = [[WGTestModel alloc]init];
    //赋值
    [model setValue:@"张三" forKey:@"name"];
    //获取值
    NSString *name = [model valueForKey:@"name"];
    NSLog(@"name:%@",name);
}

输出结果: name:张三

@interface WGTeacher : NSObject
@property(nonatomic, strong) NSString *name;
@property(nonatomic, assign) BOOL isSex;
@end

@interface WGStudent : NSObject
@property(nonatomic, strong) NSString *name;
@property(nonatomic, strong) WGTeacher *teacher;
@end

WGTeacher *tea = [[WGTeacher alloc]init];
WGStudent *stu = [[WGStudent alloc]init];
stu.teacher = tea;
//赋值
[stu setValue:@"小明" forKey:@"name"];
[stu setValue:@"王老师" forKeyPath:@"teacher.name"];
//获取值
NSString *teaName = [stu valueForKeyPath:@"teacher.name"];
NSString *stuName = [stu valueForKey:@"name"];
NSLog(@"学生姓名:%@---老师姓名:%@",stuName,teaName);

打印结果: 学生姓名:小明---老师姓名:王老师

    @interface WGTeacher : NSObject
    @property(nonatomic, strong) NSString *name;
    @property(nonatomic, assign) BOOL isSex;
    @property(nonatomic, assign) int age;
    @end

    WGTeacher *tea = [[WGTeacher alloc]init];
    id age = [tea valueForKey:@"age"];
    id sex = [tea valueForKey:@"isSex"];

WGTeacher *tea = [[WGTeacher alloc]init];
//Implicit conversion of 'int' to 'id _Nullable' is disallowed with ARC
//[tea setValue:18 forKey:@"age"];
[tea setValue:[NSNumber numberWithInt:18] forKey:@"age"];
[tea setValue:[NSNumber numberWithBool:YES] forKey:@"isSex"];
NSLog(@"age:%@----isSex:%@",[tea valueForKey:@"age"],[tea valueForKey:@"isSex"]);
NSLog(@"点语法---age:%d,isSex:%d",tea.age, tea.isSex);

打印结果: age:18----isSex:1
        点语法---age:18,isSex:1

@interface WGTeacher : NSObject
@property(nonatomic, strong) NSString *name;
@property(nonatomic, assign) BOOL isSex;
@property(nonatomic, assign) int age;
@end

@implementation WGTeacher
-(BOOL)validateValue:(inout id  _Nullable __autoreleasing *)ioValue forKey:(NSString *)  
inKey error:(out NSError *__autoreleasing  _Nullable *)outError {
    NSNumber *age = *ioValue;
    if ([age intValue] > 150) { //年龄超过150岁的就说明设置的value是无效的
        return NO;
    }
    return YES;
}
@end


WGTeacher *tea = [[WGTeacher alloc]init];
NSNumber *age = @151;
NSError *error;
BOOL isEffectValue = [tea validateValue:&age forKey:@"age" error:&error];
if (isEffectValue) { //如果设置的Value不超过150
    [tea setValue:age forKey:@"age"];
}else {
    NSLog(@"设置的值不满足条件");
}
NSLog(@"老师的年龄是:%@",[tea valueForKey:@"age"]);

打印结果: 设置的值不满足条件
        老师的年龄是:0

@interface WGTeacher : NSObject
@property(nonatomic, strong) NSString *name;
@property(nonatomic, assign) int age;
@end

WGTeacher *tea1 = [[WGTeacher alloc]init];
tea1.name = @"张老师";
tea1.age = 32;
WGTeacher *tea2 = [[WGTeacher alloc]init];
tea2.name = @"王老师";
tea2.age = 22;
WGTeacher *tea3 = [[WGTeacher alloc]init];
tea3.name = @"李老师";
tea3.age = 43;
WGTeacher *tea4 = [[WGTeacher alloc]init];
tea4.name = @"武老师";
tea4.age = 22;
NSArray *teacherArr = @[tea1, tea2, tea3, tea4];
    
//集合运算符
NSNumber *sumAge = [teacherArr valueForKeyPath:@"@sum.age"];
NSNumber *countAge = [teacherArr valueForKeyPath:@"@count.age"];
NSNumber *maxAge = [teacherArr valueForKeyPath:@"@max.age"];
NSNumber *minAge = [teacherArr valueForKeyPath:@"@min.age"];
NSNumber *avgAge = [teacherArr valueForKeyPath:@"@avg.age"];
NSLog(@"\n年龄和:%d\n总个数:%d\n最大年龄是:%d\n最小年龄:%d\n平均年龄:%f",[sumAge intValue],  
[countAge intValue],[maxAge intValue],[minAge intValue],[avgAge floatValue]);
    
//对象运算符
//返回对应属性的值(返回的元素都是唯一的，是去重以后的结果)
NSArray *ageArr1 = [teacherArr valueForKeyPath:@"@distinctUnionOfObjects.age"];
for (NSNumber *age in ageArr1) {
    NSLog(@"ageArr1--年龄:%d",[age intValue]);
}
//返回对应属性的值(返回元素的全集)
NSArray *ageArr2 = [teacherArr valueForKeyPath:@"@unionOfObjects.age"];
for (NSNumber *age in ageArr2) {
    NSLog(@"ageArr2--年龄:%d",[age intValue]);
}

打印结果: 年龄和:119
                总个数:4
                最大年龄是:43
                最小年龄:22
                平均年龄:29.750000
                ageArr1--年龄:43
                ageArr1--年龄:22
                ageArr1--年龄:32
                ageArr2--年龄:32
                ageArr2--年龄:22
                ageArr2--年龄:43
                ageArr2--年龄:22

@interface WGTeacher : NSObject
@property(nonatomic, strong) NSString *name;
@property(nonatomic, assign) int age;
@end

WGTeacher *tea = [[WGTeacher alloc]init];
tea.name = @"张老师";
tea.age = 32;
//传进去一组Key数组，返回key对应的value,并组装成字典返回
NSDictionary *dic1 = [tea dictionaryWithValuesForKeys:@[@"name",@"age"]];
NSString *name = [dic1 objectForKey:@"name"];
NSNumber *age = [dic1 objectForKey:@"age"];
NSLog(@"name:%@,age:%d",name,[age intValue]);

//传进去一个字典，用来修改对象中对应Key的value（修改对象中属性的值）
NSDictionary *dic2 = @{@"name": @"张三", @"age": @18};
[tea setValuesForKeysWithDictionary:dic2];
NSLog(@"name:%@----age:%d",tea.name, tea.age);

打印结果: name:张老师,age:32
        name:张三----age:18

NSArray *nameArr = @[@"zhangsan",@"lisi",@"wangwu",@"liy"];
//将capitalizedString属性传递给nameArr中的每一个元素，让每个元素都能响应capitalizedString的功能
NSArray *resultNameArr = [nameArr valueForKey:@"capitalizedString"];
NSLog(@"将字符串首字符大写:\nresultNameArr---:%@",resultNameArr);

//将uppercaseString属性传递给nameArr中的没一个元素，让每个元素都能响应uppercaseString的功能
NSArray *resultNameArr1 = [nameArr valueForKey:@"uppercaseString"];
NSLog(@"将字符串全部大写:\nresultNameArr1---:%@",resultNameArr1);

打印结果: 将字符串首字符大写:
        resultNameArr---:(
            Zhangsan,
            Lisi,
            Wangwu,
            Liy
        )
        将字符串全部大写:
        resultNameArr1---:(
            ZHANGSAN,
            LISI,
            WANGWU,
            LIY
        )

@interface WGTeacher : NSObject

@property(nonatomic, strong) NSString *name;
@property(nonatomic, assign) int age;
- (void)eat;
- (int)answerQuestionNum;

@end

@implementation WGTeacher
- (void)eat {
    NSLog(@"%@:开始吃饭吧",_name);
}

-(int)answerQuestionNum {
    if ([_name hasPrefix:@"张"]) {
        return 30;
    }else if ([_name hasPrefix:@"赵"]) {
        return 20;
    }else {
        return 10;
    }
}

//私有方法
-(void)run {
    NSLog(@"%@:开始起来跑步了",_name);
}
@end

WGTeacher *tea1 = [[WGTeacher alloc]init];
tea1.name = @"张老师";
WGTeacher *tea2 = [[WGTeacher alloc]init];
tea2.name = @"赵老师";
WGTeacher *tea3 = [[WGTeacher alloc]init];
tea3.name = @"王老师";
NSArray *teacherArr = @[tea1,tea2,tea3];
//公开方法 无返回值 无参数 eatResultArr数组中存放的NSNull类型的空置，因为无返回值
NSArray *eatResultArr = [teacherArr valueForKey:@"eat"];
//公开方法 有返回值 无参数 answerQuestionNumArr数组中存放的是多个元素响应answerQuestionNum方法的返回值
NSArray *answerQuestionNumArr = [teacherArr valueForKey:@"answerQuestionNum"];
//私有方法 无返回值 无参数
NSArray *runResultArr = [teacherArr valueForKey:@"run"];
NSLog(@"\neatResultArr:%@\nanswerQuestionNumArr:%@\nrunResultArr:%@",  
eatResultArr,answerQuestionNumArr, runResultArr);

打印结果: 张老师:开始吃饭吧
        赵老师:开始吃饭吧
        王老师:开始吃饭吧
        张老师:开始起来跑步了
        赵老师:开始起来跑步了
        王老师:开始起来跑步了
        eatResultArr:(
            "<null>",
            "<null>",
            "<null>"
        )
        answerQuestionNumArr:(
            30,
            20,
            10
        )
        runResultArr:(
            "<null>",
            "<null>",
            "<null>"
        )

设置属性值
setValue:(nullable id) forKey:(nonnull NSString *)
setValue:(nullable id) forKeyPath:(nonnull NSString *)
获取属性值
valueForKey:(nonnull NSString *)
valueForKeyPath:(nonnull NSString *)

//Person.h文件
@interface Person : NSObject
@property(nonatomic, assign)int age;
@end

- (void)viewDidLoad {
    [super viewDidLoad];
    Person *p = [[Person alloc]init];
    //1. 点语法给属性赋值
    p.age = 10;
    //2. 通过KVC赋值 注意这里的value是id类型，所以需要转为OC对象NSNumber
    //[p setValue:@10 forKey:@"age"];
    [p setValue:[NSNumber numberWithInt:10] forKey:@"age"];
    NSLog(@"\n属性age的值:%d\n", p.age);
}
打印结果: 属性age的值: 10

调用setValue:forKey:方法
        |            NO                                      NO  抛出异常
按照setKey:/:-->+(BOOL)accessInstanceVariablesDirectly-->setValue:forUndefinedKey:
   _setKey顺序查找方法                       |YES
        | YES             按照_key、_isKey、key、isKey  NO   抛出异常
若找到，传递参数，调用方法               顺序查找成员变量  ------>setValue:forUndefinedKey:
                                           |YES
                                        直接赋值

调用valueForKey:方法
        |                  NO                                      NO   抛出异常
按照getKey/key/isKey/-->+(BOOL)accessInstanceVariablesDirectly-->valueForUndefinedKey:
   _key:顺序查找方法                            |YES
        | YES                    按照_key、_isKey、key、isKey  NO    抛出异常
若找到，调用方法                             顺序查找成员变量    ------>valueForUndefinedKey:
                                              |YES
                                            直接赋值

//Person.h文件
@interface Person : NSObject
@property(nonatomic, assign)int age;
@end
    
//Person.m文件
@implementation Person
//重写监听属性的setter方法
-(void)setAge:(int)age {
    NSLog(@"setAge方法---");
    _age = age;
}
-(void)willChangeValueForKey:(NSString *)key {
    NSLog(@"willChangeValueForKey---begin");
    [super willChangeValueForKey: key];
    NSLog(@"willChangeValueForKey---end");
}

-(void)didChangeValueForKey:(NSString *)key {
    NSLog(@"didChangeValueForKey---begin");
    [super didChangeValueForKey:key];
    NSLog(@"didChangeValueForKey---begin");
}
@end
    
- (void)viewDidLoad {
    [super viewDidLoad];
    Person *p = [[Person alloc]init];
    //为对象p的age属性添加观察者
    [p addObserver:self forKeyPath:@"age" options:NSKeyValueObservingOptionNew   
    | NSKeyValueObservingOptionOld context:nil];
    // 1. 点语法对属性进行赋值，是会触发KVO的
    //p.age = 10;
    // 2. 通过KVC对属性进行赋值
    [p setValue:@10 forKeyPath:@"age"];
}
//观察者实现监听方法
-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object  
change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context {
    NSLog(@"监听到%p的%@发生改变了---%@",object,keyPath,change);
}
    
打印结果:  willChangeValueForKey---begin
          willChangeValueForKey---end
          setAge方法---
          didChangeValueForKey---begin
          监听到0x61000000ffb0的age发生改变了---{
            kind = 1;
            new = 10;
            old = 0;
          }
          didChangeValueForKey---begin

@interface Person : NSObject
{
    @public
    int age;
}
@end

@implementation Person
-(void)willChangeValueForKey:(NSString *)key {
    NSLog(@"willChangeValueForKey---begin");
    [super willChangeValueForKey: key];
    NSLog(@"willChangeValueForKey---end");
}
-(void)didChangeValueForKey:(NSString *)key {
    NSLog(@"didChangeValueForKey---begin");
    [super didChangeValueForKey:key];
    NSLog(@"didChangeValueForKey---begin");
}
@end

- (void)viewDidLoad {
    [super viewDidLoad];
    Person *p = [[Person alloc]init];
    //为对象p的age属性添加观察者
    [p addObserver:self forKeyPath:@"age" options:NSKeyValueObservingOptionNew   
    | NSKeyValueObservingOptionOld context:nil];
    //1.直接通过对象->成员变量是不会触发KVO的
    //p->age = 100;
    //2. 通过KVC对成员变量进行赋值，是可以触发KVO的
    [p setValue:@10 forKey:@"age"];
}

//观察者实现监听方法
-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object  
change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context {
    NSLog(@"监听到%p的%@发生改变了---%@",object,keyPath,change);
}
    
打印结果:  willChangeValueForKey---begin
          willChangeValueForKey---end
          didChangeValueForKey---begin
          监听到0x61000000ffb0的age发生改变了---{
            kind = 1;
            new = 10;
            old = 0;
          }
          didChangeValueForKey---begin