一.簡(jiǎn)單復(fù)習(xí)一下KVO的使用

• 定義一個(gè)類(lèi)，繼承自NSObject，并添加一個(gè)name的屬性

#import <Foundation/Foundation.h>

NS_ASSUME_NONNULL_BEGIN

@interface TCPerson : NSObject

@property (nonatomic, copy) NSString *name;

@end

NS_ASSUME_NONNULL_END

• 在ViewController我們簡(jiǎn)單的使用一下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];
}

/// 點(diǎn)擊屏幕出發(fā)改變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";
}

/// 監(jiān)聽(tīng)回調(diào)
/// @param keyPath 監(jiān)聽(tīng)的屬性名字
/// @param object 被監(jiān)聽(tīng)的對(duì)象
/// @param change 改變的新/舊值
/// @param context context description
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context{
    NSLog(@"監(jiān)聽(tīng)到%@對(duì)象的%@發(fā)生了改變%@",object,keyPath,change);
}

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

當(dāng)點(diǎn)擊屏幕的時(shí)候，控制臺(tái)輸出：

2020-09-24 15:53:52.527734+0800 KVO_TC[9255:98204] 監(jiān)聽(tīng)到<TCPerson: 0x600003444d10>對(duì)象的name發(fā)生了改變{
    kind = 1;
    new = "cang lao shi";
    old = "liu yi fei";
}

二.深入剖析KVO的底層

• 在- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
  self.person1.name = @"cang lao shi";
  }我們知道self.person1.name的本質(zhì)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，我們從寫(xiě)setter方法并打斷點(diǎn)，可以看到當(dāng)我們點(diǎn)擊屏幕的時(shí)候，我們發(fā)現(xiàn)進(jìn)入了setter方法：

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

• 在ViewController我們新建一個(gè)person2,代碼變成了：

#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";
}

/// 點(diǎn)擊屏幕出發(fā)改變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";
}

/// 監(jiān)聽(tīng)回調(diào)
/// @param keyPath 監(jiān)聽(tīng)的屬性名字
/// @param object 被監(jiān)聽(tīng)的對(duì)象
/// @param change 改變的新/舊值
/// @param context context description
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context{
    NSLog(@"監(jiān)聽(tīng)到%@對(duì)象的%@發(fā)生了改變%@",object,keyPath,change);
}

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

• 注意：當(dāng)我們點(diǎn)擊屏幕的時(shí)候輸出的結(jié)果是:

2020-09-24 16:10:36.750153+0800 KVO_TC[9313:105906] 監(jiān)聽(tīng)到<TCPerson: 0x600002ce8230>對(duì)象的name發(fā)生了改變{
    kind = 1;
    new = "cang lao shi";
    old = "liu yi fei";
}

• 既然我們改變name的值的時(shí)候走的都是setName:setter方法，按理說(shuō)觀察屬性變化的時(shí)候，person2的值也應(yīng)該被觀察到，為什么它不會(huì)觀察到person2？

三.KVO的isa指向

• 上篇文章中我分析了實(shí)例對(duì)象，類(lèi)對(duì)象，元類(lèi)對(duì)象的isa，既然當(dāng)我們改變屬性值的時(shí)候，其本質(zhì)是調(diào)用setter方法，那么在KVO中，person1和person2的setName方法應(yīng)該存儲(chǔ)在類(lèi)對(duì)象中，我們先來(lái)看看這兩個(gè)實(shí)例對(duì)象的isa指向：
  打開(kāi)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) 

• 從上面的打印我們看到 self.person1的isa指向了NSKVONotifying_TCPerson，而沒(méi)有添加觀察著的self.person2的isa卻指向的是TCPerson
• NSKVONotifying_TCPerson是runtime動(dòng)態(tài)創(chuàng)建的類(lèi)，繼承自TCPerson，其內(nèi)部實(shí)現(xiàn)可以看成（模擬的NSKVONotifying_TCPerson流程，下面代碼不能在xcode中運(yùn)行）:

#import "NSKVONotifying_TCPerson.h"

@implementation NSKVONotifying_TCPerson
//NSKVONotifying_TCPerson的set方法實(shí)現(xiàn)，其本質(zhì)來(lái)自于foundation框架
- (void)setName:(NSString *)name{
    _NSSetIntVaueAndNotify();
}
//改變過(guò)程
void _NSSetIntVaueAndNotify(){
    [self willChangeValueForKey:@"name"];
    [super setName:name];
    [self didChangeValueForKey:@"name"];
}
//通知觀察者
- (void)didChangeValueForKey:(NSString *key){
    [observe  observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context];
}
@end

• 未添加觀察self.person2實(shí)例對(duì)象的isa指向流程圖：

  
  
  elf.person2實(shí)例對(duì)象的isa指向流程圖

• 添加觀察self.person1實(shí)例對(duì)象的isa指向流程圖：

  
  
  self.person1實(shí)例對(duì)象的isa指向流程圖

• 所以KVO其本質(zhì)是動(dòng)態(tài)生成一個(gè)NSKVONotifying_TCPerson類(lèi)，繼承自TCPerson，當(dāng)實(shí)例對(duì)象添加觀察著之后，實(shí)例對(duì)象的isa指向了這個(gè)動(dòng)態(tài)創(chuàng)建的類(lèi)，當(dāng)其屬性發(fā)生改變時(shí)，調(diào)用的是該類(lèi)的setter方法，而不是父類(lèi)的類(lèi)對(duì)象中的setter方法