KVO的本质分析

作者: SKY_Tang | 来源:发表于2018-09-14 18:32 被阅读0次

KVO全程就是“key-value observing”，俗称“键值监听”，用于对象属性值改变的监听。

上代码

#import "ViewController.h"
#import "MJPerson.h"

@interface ViewController ()

@property(nonatomic, strong) MJPerson *person1;
@property(nonatomic, strong) MJPerson *person2;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.person1 = [[MJPerson alloc] init];
    self.person2 = [[MJPerson alloc] init];
    
    self.person1.age = 2;
    self.person2.age = 3;
    
    NSKeyValueObservingOptions options = NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld;
    [self.person1 addObserver:self forKeyPath:@"age" options:options context:@"123"];
    
}

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
    self.person1.age = 6;
    self.person2.age = 7;
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context
{
    NSLog(@"监听到%@的%@属性值改变了 - %@ - %@", object, keyPath, change, context);
}

@end

点击屏幕改变person1和person2的age的值，但是只对person1的age做了KVO，运行起来看打印结果

打印结果

打印看出只有person1的age的值改变有打印，但是点击屏幕改变了person1和person2的age的值，那猜测是不是age的set方法有不同，重写MJPerson的setAge:方法。

#import "MJPerson.h"

@implementation MJPerson

- (void)setAge:(int)age
{
    _age = age;
    
    NSLog(@"newAge == %i", _age);
}

@end

打印结果

打印看出，点击屏幕改变person1和person2的age的值，都走了MJPerson的setAge:方法，也就是在setAge:方法上没有什么不同。

现在我们猜测是不是person1和person2的instance对象的不同？

在person1的age做了KVO前后分别打印person1和person2的class对象

#import "ViewController.h"
#import "MJPerson.h"
#import <objc/runtime.h>

@interface ViewController ()

@property(nonatomic, strong) MJPerson *person1;
@property(nonatomic, strong) MJPerson *person2;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.person1 = [[MJPerson alloc] init];
    self.person2 = [[MJPerson alloc] init];
    
    self.person1.age = 2;
    self.person2.age = 3;
    
    NSLog(@"监听前-----person1Class------%@", object_getClass(self.person1));
    NSLog(@"监听前-----person2Class------%@", object_getClass(self.person2));
    
    NSKeyValueObservingOptions options = NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew;
    [self.person1 addObserver:self forKeyPath:@"age" options:options context:@"123"];
    
    NSLog(@"监听前-----person1Class------%@", object_getClass(self.person1));
    NSLog(@"监听前-----person2Class------%@", object_getClass(self.person2));
}

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
    self.person1.age = 6;
    self.person2.age = 7;
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context
{
    NSLog(@"监听到%@的%@属性值改变了 - %@ - %@", object, keyPath, change, context);
}

@end

打印结果

打印看出在person1的age实现KVO后，person1的Class对象改变为NSKVONotifying_MJPerson。也就是KVO时，利用RuntimeAPI动态生成一个NSKVONotifying_MJPerson的类，并且让person1对象的isa指向这个全新的class对象。

接下来探索NSKVONotifying_MJPerson的class对象有哪些方法实现

#import "ViewController.h"
#import "MJPerson.h"
#import <objc/runtime.h>

@interface ViewController ()

@property(nonatomic, strong) MJPerson *person1;

@end

@implementation ViewController

- (void)printMethodNamesOfClass:(Class)cls
{
    unsigned int count;
    // 获取方法数组
    Method *methodList = class_copyMethodList(cls, &count);
    // 存储方法名
    NSMutableString *methodNames = [NSMutableString new];
    // 遍历所有方法
    for (int i=0; i<count; i++) {
        // 获得方法
        Method method = methodList[i];
        // 获得方法名
        NSString *methodName = NSStringFromSelector(method_getName(method));
        // 拼接方法名
        [methodNames appendString:methodName];
        [methodNames appendString:@", "];
    }
    // 释放
    free(methodList);
    // 打印方法名
    NSLog(@"%@ %@", cls, methodNames);
}

- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.person1 = [[MJPerson alloc] init];
    
    self.person1.age = 2;
    
    [self printMethodNamesOfClass:object_getClass(self.person1)];
    
    NSKeyValueObservingOptions options = NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew;
    [self.person1 addObserver:self forKeyPath:@"age" options:options context:@"123"];
    
    [self printMethodNamesOfClass:object_getClass(self.person1)];
}

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
    self.person1.age = 6;
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context
{
    NSLog(@"监听到%@的%@属性值改变了 - %@ - %@", object, keyPath, change, context);
}

@end

打印结果

看打印结果可以看出变化如图

KVO 前

KVO 后

通过打印可以看出，KVO的实现监听主要NSKVONotifying_MJPerson的class对象的setAge:方法里，可以用methodForSelector:来打印查看setAge:的实现

#import "ViewController.h"
#import "MJPerson.h"
#import <objc/runtime.h>

@interface ViewController ()

@property(nonatomic, strong) MJPerson *person1;

@end

@implementation ViewController

- (void)printMethodNamesOfClass:(Class)cls
{
    unsigned int count;
    // 获取方法数组
    Method *methodList = class_copyMethodList(cls, &count);
    // 存储方法名
    NSMutableString *methodNames = [NSMutableString new];
    // 遍历所有方法
    for (int i=0; i<count; i++) {
        // 获得方法
        Method method = methodList[i];
        // 获得方法名
        NSString *methodName = NSStringFromSelector(method_getName(method));
        // 拼接方法名
        [methodNames appendString:methodName];
        [methodNames appendString:@", "];
    }
    // 释放
    free(methodList);
    // 打印方法名
    NSLog(@"%@ %@", cls, methodNames);
}

- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.person1 = [[MJPerson alloc] init];
    
    self.person1.age = 2;
    
//    [self printMethodNamesOfClass:object_getClass(self.person1)];
    
    NSLog(@"KVO前----%p", [self.person1 methodForSelector:@selector(setAge:)]);
    
    NSKeyValueObservingOptions options = NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew;
    [self.person1 addObserver:self forKeyPath:@"age" options:options context:@"123"];
    
//    [self printMethodNamesOfClass:object_getClass(self.person1)];
    
    NSLog(@"KVO后----%p", [self.person1 methodForSelector:@selector(setAge:)]);
}

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
    self.person1.age = 6;
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context
{
    NSLog(@"监听到%@的%@属性值改变了 - %@ - %@", object, keyPath, change, context);
}

@end

断点打印

可以看到KVO后NSKVONotifying_MJPerson的class对象的setAge:变成了(Foundation _NSSetIntValueAndNotify)。Foundation框架的_NSSetIntValueAndNotify函数，因为苹果Foundation没开源，只能用逆向研究。

越狱手机缓存

抽取后找到Foundation的编译文件，用工具hopper反编译Foundation的编译文件

hopper工具搜索

命令搜索

看到不光有_NSSetIntValueAndNotify，还有其他类型，猜测一下把age换成 double类型是不是NSKVONotifying_MJPerson的setAge:实现相应变为_NSSetDoubleValueAndNotify？

#import <Foundation/Foundation.h>

@interface MJPerson : NSObject

@property(nonatomic, assign) double age;

@end

断点打印

打印看出把age换成 double类型后，NSKVONotifying_MJPerson的setAge:实现确实相应变为_NSSetDoubleValueAndNotify。

研究NSKVONotifying_MJPerson和MJPerson的关系

通过《isa、superclass的细节》可知，通过结构体mj_objc_class来强制转化class类

#import "ViewController.h"
#import "MJPerson.h"
#import <objc/runtime.h>

struct mj_objc_class {
    Class isa;
    Class superclass;
};

@interface ViewController ()

@property(nonatomic, strong) MJPerson *person1;

@end

@implementation ViewController

- (void)printMethodNamesOfClass:(Class)cls
{
    unsigned int count;
    // 获取方法数组
    Method *methodList = class_copyMethodList(cls, &count);
    // 存储方法名
    NSMutableString *methodNames = [NSMutableString new];
    // 遍历所有方法
    for (int i=0; i<count; i++) {
        // 获得方法
        Method method = methodList[i];
        // 获得方法名
        NSString *methodName = NSStringFromSelector(method_getName(method));
        // 拼接方法名
        [methodNames appendString:methodName];
        [methodNames appendString:@", "];
    }
    // 释放
    free(methodList);
    // 打印方法名
    NSLog(@"%@ %@", cls, methodNames);
}

- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.person1 = [[MJPerson alloc] init];
    
    self.person1.age = 2;
    
//    [self printMethodNamesOfClass:object_getClass(self.person1)];
//    NSLog(@"KVO前----%p", [self.person1 methodForSelector:@selector(setAge:)]);
    
    // 强制转换 KVO 前 person1 的 class 对象
    struct mj_objc_class *person1Class = (__bridge struct mj_objc_class *)(object_getClass(self.person1));
    
    NSKeyValueObservingOptions options = NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew;
    [self.person1 addObserver:self forKeyPath:@"age" options:options context:@"123"];
    
//    [self printMethodNamesOfClass:object_getClass(self.person1)];
//    NSLog(@"KVO后----%p", [self.person1 methodForSelector:@selector(setAge:)]);
    
    // 强制转换 KVO 后 person1 的 class 对象
    struct mj_objc_class *person1KVOClass = (__bridge struct mj_objc_class *)(object_getClass(self.person1));
    
    NSLog(@"person1Class-------------%p", person1Class);
    NSLog(@"person1KVOClass-------------%p", person1KVOClass);
}

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
    self.person1.age = 6;
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context
{
    NSLog(@"监听到%@的%@属性值改变了 - %@ - %@", object, keyPath, change, context);
}

@end

断点打印

打印看到MJPerson和NSKVONotifying_MJPerson的class对象不一样。

断点打印

打印看到MJPerson和NSKVONotifying_MJPerson的meta-class对象不一样。

断点打印

打印看到NSKVONotifying_MJPerson的父类是MJPerson，MJPerson的父类是NSObject。

根据上面我们可以得到这个逻辑图

`MJPerson`和`NSKVONotifying_MJPerson`关系图

现在来研究NSKVONotifying_MJPerson的setAge:内部的实现

通过hopper能看到_NSSetIntValueAndNotify的实现，下面写一下伪代码

#import "NSKVONotifying_MJPerson.h"

@implementation NSKVONotifying_MJPerson

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

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

- (void)didChangeValueForKey:(NSString *)key
{
    [observer observeValueForKeyPath:key ofObject:self change:nil context:nil];
}

@end

自定义NSKVONotifying_MJPerson，KVO 的时候会失败报错，所以我们在父类MJPerson中重写方法

#import "MJPerson.h"

@implementation MJPerson

- (void)setAge:(int)age
{
    _age = age;

    NSLog(@"setAge:");
}

- (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:-----end");
}

@end

#import "ViewController.h"
#import "MJPerson.h"
#import <objc/runtime.h>

struct mj_objc_class {
    Class isa;
    Class superclass;
};

@interface ViewController ()

@property(nonatomic, strong) MJPerson *person1;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.person1 = [[MJPerson alloc] init];
    
    self.person1.age = 2;
    
    NSKeyValueObservingOptions options = NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew;
    [self.person1 addObserver:self forKeyPath:@"age" options:options context:@"123"];
}

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
    self.person1.age = 6;
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context
{
    NSLog(@"监听到%@的%@属性值改变了 - %@ - %@", object, keyPath, change, context);
}

@end

点击屏幕看打印顺序

点击屏幕打印

`_NSSet*ValueAndNotify`内部实现

打印顺序可以看出_NSSetXXXValueAndNotify的内部实现

调用willChangeValueForKey:
调用原来的setter实现
调用didChangeValueForKey:

内部会调用observer的observeValueForKeyPath:ofObject:change:context:方法

面试题

iOS用什么方式实现对一个对象的KVO？(KVO的本质是什么？)

利用RuntimeAPi动态生成一个NSKVONotifying_XXX子类，并且让instance对象的isa指向NSKVONotifying_XXX。
当调用instance对象的set方法时，会调用foundation框架的 _NSSetXXXValueAndNotify函数
- willChangeValueForKey:
- 父类的set:方法
- didChangeValueForKey:
  
  内部会调用监控器（Observer）的监听方法（observeValueForKeyPath:ofObject:change:context:）

如何手动触发KVO？

手动调用willChangeValueForKey:和didChangeValueForKey:

直接修改成员变量会触发KVO么？

不会触发KVO（没有触发set:方法）

KVO的本质分析

面试题

iOS用什么方式实现对一个对象的KVO？(KVO的本质是什么？)

如何手动触发KVO？

直接修改成员变量会触发KVO么？

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