iOS KVO底层原理&&KVO的isa指向（一）

一.简单复习一下KVO的使用

• 定义一个类，继承自NSObject，并添加一个name的属性

#import <Foundation/Foundation.h>

NS_ASSUME_NONNULL_BEGIN

@interface TCPerson : NSObject

@property (nonatomic, copy) NSString *name;

@end

NS_ASSUME_NONNULL_END

• 在ViewController我们简单的使用一下KVO

#import "ViewController.h"
#import "TCPerson.h"
@interface ViewController ()
@property (nonatomic, strong) TCPerson *person1;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    self.person1 = [[TCPerson alloc]init];
    self.person1.name = @"liu yi fei";
    [self.person1 addObserver:self forKeyPath:@"name" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:nil];
}

/// 点击屏幕出发改变self.person1的name
/// @param touches touches description
/// @param event event description
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
    self.person1.name = @"cang lao shi";
}

/// 监听回调
/// @param keyPath 监听的属性名字
/// @param object 被监听的对象
/// @param change 改变的新/旧值
/// @param context context description
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context{
    NSLog(@"监听到%@对象的%@发生了改变%@",object,keyPath,change);
}

/// 移除观察者
- (void)dealloc{
    [self.person1 removeObserver:self forKeyPath:@"name"];
}
@end

当点击屏幕的时候，控制台输出：

2020-09-24 15:53:52.527734+0800 KVO_TC[9255:98204] 监听到<TCPerson: 0x600003444d10>对象的name发生了改变{
    kind = 1;
    new = "cang lao shi";
    old = "liu yi fei";
}

二.深入剖析KVO的底层

• 在- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
  self.person1.name = @"cang lao shi";
  }我们知道self.person1.name的本质是[self.person1 setName:@"cang lao shi"];

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
//    self.person1.name = @"cang lao shi";
    [self.person1 setName:@"cang lao shi"];
}

在TCPerson的.m文件，我们从写setter方法并打断点，可以看到当我们点击屏幕的时候，我们发现进入了setter方法：

- (void)setName:(NSString *)name{
    _name = name;
}

• 在ViewController我们新建一个person2,代码变成了：

#import "ViewController.h"
#import "TCPerson.h"
@interface ViewController ()
@property (nonatomic, strong) TCPerson *person1;
@property (nonatomic, strong) TCPerson *person2;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    self.person1 = [[TCPerson alloc]init];
    self.person1.name = @"liu yi fei";
    [self.person1 addObserver:self forKeyPath:@"name" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:nil];
    
    
    self.person2 = [[TCPerson alloc] init];
    self.person2.name = @"yyyyyyyy";
}

/// 点击屏幕出发改变self.person1的name
/// @param touches touches description
/// @param event event description
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
    self.person1.name = @"cang lao shi";
//    [self.person1 setName:@"cang lao shi"];
    
    self.person2.name = @"ttttttttt";
}

/// 监听回调
/// @param keyPath 监听的属性名字
/// @param object 被监听的对象
/// @param change 改变的新/旧值
/// @param context context description
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context{
    NSLog(@"监听到%@对象的%@发生了改变%@",object,keyPath,change);
}

/// 移除观察者
- (void)dealloc{
    [self.person1 removeObserver:self forKeyPath:@"name"];
}
@end

• 注意：当我们点击屏幕的时候输出的结果是:

2020-09-24 16:10:36.750153+0800 KVO_TC[9313:105906] 监听到<TCPerson: 0x600002ce8230>对象的name发生了改变{
    kind = 1;
    new = "cang lao shi";
    old = "liu yi fei";
}

• 既然我们改变name的值的时候走的都是setName:setter方法，按理说观察属性变化的时候，person2的值也应该被观察到，为什么它不会观察到person2？

三.KVO的isa指向

• 上篇文章中我分析了实例对象，类对象，元类对象的isa，既然当我们改变属性值的时候，其本质是调用setter方法，那么在KVO中，person1和person2的setName方法应该存储在类对象中，我们先来看看这两个实例对象的isa指向：
  打开lldb

(lldb) p self.person1.isa
(Class) $0 = NSKVONotifying_TCPerson
  Fix-it applied, fixed expression was: 
    self.person1->isa
(lldb) p self.person2.isa
(Class) $1 = TCPerson
  Fix-it applied, fixed expression was: 
    self.person2->isa
(lldb) 

• 从上面的打印我们看到 self.person1的isa指向了NSKVONotifying_TCPerson，而没有添加观察着的self.person2的isa却指向的是TCPerson
• NSKVONotifying_TCPerson是runtime动态创建的类，继承自TCPerson，其内部实现可以看成（模拟的NSKVONotifying_TCPerson流程，下面代码不能在xcode中运行）:

#import "NSKVONotifying_TCPerson.h"

@implementation NSKVONotifying_TCPerson
//NSKVONotifying_TCPerson的set方法实现，其本质来自于foundation框架
- (void)setName:(NSString *)name{
    _NSSetIntVaueAndNotify();
}
//改变过程
void _NSSetIntVaueAndNotify(){
    [self willChangeValueForKey:@"name"];
    [super setName:name];
    [self didChangeValueForKey:@"name"];
}
//通知观察者
- (void)didChangeValueForKey:(NSString *key){
    [observe  observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context];
}
@end

• 未添加观察self.person2实例对象的isa指向流程图：

  
  elf.person2实例对象的isa指向流程图

• 添加观察self.person1实例对象的isa指向流程图：

  
  self.person1实例对象的isa指向流程图

• 所以KVO其本质是动态生成一个NSKVONotifying_TCPerson类，继承自TCPerson，当实例对象添加观察着之后，实例对象的isa指向了这个动态创建的类，当其属性发生改变时，调用的是该类的setter方法，而不是父类的类对象中的setter方法