KVC是什么？

KVC是Key Value Coding的缩写,意思是键值编码. 在iOS中,Provides a method by using the name of the property(也就是Key)Methods to indirectly access object properties,This method can not passgetter/settermethods to access object properties. 用KVCA mechanism for indirectly accessing object properties.通常我们使用valueForKey 来替代getter 方法,setValue:forKey来代替setter方法.

常用API：

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

setValue：forKey：和valueForKey：Can only be used to access properties of the current object,而keyPathCan access the attributes of the by the current object,Can be put down layer by layer.

简单举个例子：

#import <Foundation/Foundation.h>
#import "Car.h"
NS_ASSUME_NONNULL_BEGIN

@interface Person : NSObject 
@property (nonatomic, assign) NSInteger age;

@property (nonatomic, strong) Car* car;

@end

NS_ASSUME_NONNULL_END```
#import <Foundation/Foundation.h>

NS_ASSUME_NONNULL_BEGIN

@interface Car : NSObject 

@property (nonatomic, strong) NSString* name;
@end
#import "ViewController.h"
#import "Person.h"
@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad {
    
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    Person* person = [[Person alloc] init];
    person.car = [[Car alloc] init];
    // KVC
    [person setValue:@"20" forKey:@"age"];
    NSLog(@"%@", [person valueForKey:@"age"]);
    [person setValue:@111 forKeyPath:@"car.name"];
    NSLog(@"%@", [person valueForKeyPath:@"car.name"]);
}

KVC的底层原理

KVC赋值

keypath的底层和key是同理的,下面分析一个key的流程,On an online understand chart.

Analyze the steps below：

• 首先第一步,Find the class firstsetter方法,首先会把参数key的首字母大写,如何根据Key去查找对应的setter方法,包括setKey和_setKey,If found, call this method and pass the parametersvalue,To modify the value,如果没有找到setter方法,Just go back and look at this class+accessInstanceVariablesDirectlyreturn value of class method（Can you directly access member variables?,默认为YES）,如果不能直接访问,就会报错,并且触发“setValue:forUndefinedKey:"方法.
• If you can access member variables,The corresponding member variables will be searched in order,_key,_isKey,key,isKey,If the corresponding member variable is found,Just assign a value directly to the member variable,（如果设置了KVO的话,此时会触发KVO,因为KVC在此处,也调用了willChangeValueForKey：和didChangeValueForKey：方法）,If the member variable is not found,就会报错,并且触发“setValue:forUndefinedKey:"方法.

KVC取值

• Similar to the above process,just search firstgetter方法,按照getKey,key,isKey,_key顺序查找
• 如果没有找到getter方法,Just go back and look at this class+accessInstanceVariablesDirectlyreturn value of class method（Can you directly access member variables?,默认为YES）,如果不能直接访问,就会报错,并且触发“valueForUndefinedKey:"方法.
• If you can access member variables,The corresponding member variables will be searched in order,_key,_isKey,key,isKey,如果没找到,就会报错,并且触发“valueForUndefinedKey:"方法.

KVO？

KVO的全称是Key-Value Observing,俗称“键值监听”,可以用于监听某个对象属性值的改变.

使用KVOThere are only three key points：who was observed：That is, you want to observe which object and which attribute value changes.;who is the observer：who observes this object,After determining may be added to the objectKVOAnd remove the objectKVO了;Observer callback methods：Method triggered after object property changes.
举个例子：

#import "ViewController.h"
#import "Person.h"
@interface ViewController ()
@property (nonatomic, strong) Person* person;
@end

@implementation ViewController

- (void)viewDidLoad {
    
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    self.person = [[Person alloc] init];
    self.person.car = [[Car alloc] init];
    // KVO
    // 给person添加观察者
    [self.person addObserver:self forKeyPath:@"age" options:(NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew) context:nil];
    // KVC
    [self.person setValue:@"20" forKey:@"age"];
    NSLog(@"%@", [self.person valueForKey:@"age"]);
    [self.person setValue:@111 forKeyPath:@"car.name"];
    NSLog(@"%@", [self.person valueForKeyPath:@"car.name"]);
}

- (void)dealloc {
    
    // 移除person对象的KVO
    [self.person removeObserver:self forKeyPath:@"age"];
}
// 回调方法
-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context {
    
    NSLog(@"被观察对象%@的%@属性发生变化：%@", object, keyPath, change);
}
@end

For specific use and parameter meanings, please refer to my previous blog——简单使用KVO

KVO底层原理

Since it is usedRuntime实现的KVO,就来探索一下,到底是如何实现的.
先下结论.

• First of all in use at runtimeRuntimeTo this class to create a new subclass,And will be usedKVO的instance的isaPointer to the new class.At this point, the instance object is no longer the original class.,but belongs to the newly created subclass.
• Next, this class will override the original class andNSObject类的部分方法,结合之前KVC找setter方法,The most important thing we can think of is overriding instance methodssetter.
• 重写后的setterThe main three processes：首先调用willChangeValueForKey：Indicates that the value is about to be modified,then find the objectsetter方法：To actually modify the value of the member variable,最后调用didChangeValueForKey：And in this method implement the observer to call the observer callback method.
  About pseudo code can look at it：

@interface NSKVONotifying_Person : Person
@end

@implementation NSKVONotifying_Person

- (void)setAge:(int)age
{
    
    _NSSetIntValueAndNotify();
}

// 伪代码
void _NSSetIntValueAndNotify()
{
    
    [self willChangeValueForKey:@"age"];
    [super setAge:age];
    [self didChangeValueForKey:@"age"];
}

- (void)didChangeValueForKey:(NSString *)key
{
    
    // 通知监听器,某某属性值发生了改变
    [oberser observeValueForKeyPath:key ofObject:self change:nil context:nil];
}

// 屏幕内部实现,隐藏了NSKVONotifying_MJPerson类的存在
- (Class)class
{
    
    return [Person class];
}

- (void)dealloc
{
    
    // 收尾工作
}

- (BOOL)_isKVOA
{
    
    return YES;
}

@end