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iOS KVO底层原理&&KVO的isa指向

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

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TCPersonNSKVONotifying_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:

97cc8f27a5f568fce9cf23819ce6586a.png

添加观察self.person1实例对象的isa指向流程图

6a75f5641252b26718c666d1048a29fe.png

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


作者:枫紫_6174
链接:https://www.jianshu.com/p/0b6083b91b04

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