iOS-底层原理(4)-KVO原理详解

面试题

1. iOS用什么方式实现对一个对象的KVO？(KVO的本质是什么？)

利用Runtime动态生成一个子类，并且让instance对象的isa指向这个全新的子类(即这个子类的superclass指向原来的那个类)
当修改instance对象的属性时，会调用Foundation的_NSSetXXXValueAndNotify函数
- willChangeValueForKey:
- 父类原来的setter
- didChangeValueForKey:
- 内部会触发监听器（Oberser）的监听方法observeValueForKeyPath:ofObject:change:context:

2. 如何手动触发KVO？

手动调用willChangeValueForKey:和didChangeValueForKey:

代码佐证

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event{
    [self.person1 willChangeValueForKey:@"age"];
    [self.person1 didChangeValueForKey:@"age"];
}

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

运行结果

image.png

3. 直接修改成员变量会触发KVO么？

会触发KVO

代码佐证

@interface Person : NSObject {
@public int _age;
}
@property(nonatomic,assign)int age;
@end

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

手动点击触发 - 调用成员变量

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    // 直接修改成员变量会触发KVO么
    self.person1.age = 10;
}

运行结果

image.png

手动点击触发 - 调用带下划线的成员变量

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    NSLog(@"调用带下划线的成员变量");
    self.person1->_age = 10;
}

运行结果

image.png

可知，直接调用带下下划线的成员变量不会触发KVO，因为没有调用set方法

序言

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。

image.png

代码佐证

// 创建一个新类
@interface Person : NSObject
@property (assign, nonatomic) int age;
@end

@implementation Person
- (void)setAge:(int)age {
    _age = age;
}
@end

// 调用
- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    
    self.person1 = [[Person alloc] init];
    self.person1.age = 1;
    
    self.person2 = [[Person alloc] init];
    self.person2.age = 2;
    
    // 给person1对象添加KVO监听
    NSKeyValueObservingOptions options = NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld;
    [self.person1 addObserver:self forKeyPath:@"age" options:options context:@"123"];
}

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

- (void)dealloc {
    [self.person1 removeObserver:self forKeyPath:@"age"];
}

// 触摸屏幕改变值
- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    // NSKVONotifying_MJPerson是使用Runtime动态创建的一个类，是MJPerson的子类
    
    // self.person1.isa == NSKVONotifying_MJPerson
    [self.person1 setAge:21];
    
    // self.person2.isa = MJPerson
    [self.person2 setAge:22];
    NSLog(@"person1 age = %d,person2 age = %d",self.person1.age,self.person2.age);
}

新建NSKVONotifying_Person子类，使用伪代码推测其底层实现

#import "Person.h"
@interface NSKVONotifying_Person : Person
@end

#import "NSKVONotifying_Person.h"
@implementation NSKVONotifying_Person

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

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

// 伪代码
- (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];
}

@end

打印结果

image.png

底层分析

未使用KVO监听的对象

image.png

使用了KVO监听的对象

image.png

底层原理解析

先看看添加监听前后实例对象是否发生变化

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    
    self.person1 = [[Person alloc] init];
    self.person1.age = 1;
    
    self.person2 = [[Person alloc] init];
    self.person2.age = 2;
    
    NSLog(@"person1添加KVO监听之前 - %@ %@",object_getClass(self.person1),object_getClass(self.person2));
    
    // 给person1对象添加KVO监听
    NSKeyValueObservingOptions options = NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld;
    [self.person1 addObserver:self forKeyPath:@"age" options:options context:@"123"];
    
    NSLog(@"person1添加KVO监听之后 - %@ %@",object_getClass(self.person1),object_getClass(self.person2));
}

打印结果

image.png

通过打印结果，可以发现person1在添加监听之后class类型发生了变化

添加监听前后方法是否发生变化

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    
    self.person1 = [[Person alloc] init];
    self.person1.age = 1;
    
    self.person2 = [[Person alloc] init];
    self.person2.age = 2;
    
//    NSLog(@"person1添加KVO监听之前 - %@ %@",object_getClass(self.person1),object_getClass(self.person2));
    NSLog(@"person1添加KVO监听之前 - %p %p",
          [self.person1 methodForSelector:@selector(setAge:)],
          [self.person2 methodForSelector:@selector(setAge:)]);
    
    // 给person1对象添加KVO监听
    NSKeyValueObservingOptions options = NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld;
    [self.person1 addObserver:self forKeyPath:@"age" options:options context:@"123"];
    
//    NSLog(@"person1添加KVO监听之后 - %@ %@",object_getClass(self.person1),object_getClass(self.person2));
    NSLog(@"person1添加KVO监听之后 - %p %p",
          [self.person1 methodForSelector:@selector(setAge:)],
          [self.person2 methodForSelector:@selector(setAge:)]);
}

打印结果

image.png

通过打印我们知道添加监听之后，调用setAge方法发生了变化

添加监听前后isa是否发生变化

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    
    self.person1 = [[Person alloc] init];
    self.person1.age = 1;
    
    self.person2 = [[Person alloc] init];
    self.person2.age = 2;
   
    // 给person1对象添加KVO监听
    NSKeyValueObservingOptions options = NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld;
    [self.person1 addObserver:self forKeyPath:@"age" options:options context:@"123"];

    NSLog(@"类对象 - %@ %@",
          object_getClass(self.person1),  // self.person1.isa
          object_getClass(self.person2)); // self.person2.isa

    NSLog(@"元类对象 - %@ %@",
          object_getClass(object_getClass(self.person1)), // self.person1.isa.isa
          object_getClass(object_getClass(self.person2))); // self.person2.isa.isa
}

运行结果

image.png

可以得出在添加监听之后，实例对象了类对象的isa发生了变化

_NSSet*ValueAndNotify的内部实现

image.png

调用方法的顺序
调用willChangeValueForKey:
调用原来的setter实现
调用didChangeValueForKey:
- didChangeValueForKey:内部会调用observer的
- observeValueForKeyPath:ofObject:change:context:方法

代码例子佐证

#import "Person.h"

@implementation Person

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

- (void)willChangeValueForKey:(NSString *)key {
    [super willChangeValueForKey:key];
    
    NSLog(@"willChangeValueForKey");
}

- (void)didChangeValueForKey:(NSString *)key {
    NSLog(@"didChangeValueForKey - begin");
    
    [super didChangeValueForKey:key];
    
    NSLog(@"didChangeValueForKey - end");
}

@end

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    [self.person1 setAge:21];
}

运行结果

_NSSet*ValueAndNotify.png

_NSSet*ValueAndNotify的内部实现 - Class

#import "NSKVONotifying_Person.h"

@implementation NSKVONotifying_Person

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

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

- (void)dealloc {
    // 收尾工作
}

- (BOOL)_isKVOA {
    return YES;
}
@end

- (void)viewDidLoad {
    [super viewDidLoad];

    self.person1 = [[Person alloc] init];
    self.person1.age = 1;
    
    self.person2 = [[Person alloc] init];
    self.person2.age = 2;
    
    // 给person1对象添加KVO监听
    NSKeyValueObservingOptions options = NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld;
    [self.person1 addObserver:self forKeyPath:@"age" options:options context:@"123"];
 
    NSLog(@"类对象 - %@ %@",[self.person1 class],[self.person2 class]);
    NSLog(@"类对象 - %@ %@",object_getClass(self.person1),object_getClass(self.person2));
}

运行结果

class.png

打印 _NSSet*ValueAndNotify所有方法

// 打印class所有方法列表
- (void)printMethodNamesOfClass:(Class)cls {
    unsigned int count;
    // 获得方法数组
    Method *methodList = class_copyMethodList(cls, &count);
    
    // 存储方法名
    NSMutableString *methodNames = [NSMutableString string];
    
    // 遍历所有的方法
    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];
    // Do any additional setup after loading the view, typically from a nib.
    
    self.person1 = [[Person alloc] init];
    self.person1.age = 1;
    
    self.person2 = [[Person alloc] init];
    self.person2.age = 2;

    // 给person1对象添加KVO监听
    NSKeyValueObservingOptions options = NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld;
    [self.person1 addObserver:self forKeyPath:@"age" options:options context:@"123"];
    
    [self printMethodNamesOfClass:object_getClass(self.person1)];
    [self printMethodNamesOfClass:object_getClass(self.person2)];
}

运行结果

methodList.png

本文参考借鉴MJ的教程视频，非常感谢.

项目连接地址 - kvo-detail

iOS-底层原理(4)-KVO原理详解

面试题

1. iOS用什么方式实现对一个对象的KVO？(KVO的本质是什么？)

2. 如何手动触发KVO？

3. 直接修改成员变量会触发KVO么？

序言

底层分析

底层原理解析

_NSSet*ValueAndNotify的内部实现

_NSSet*ValueAndNotify的内部实现 - Class

打印 _NSSet*ValueAndNotify所有方法

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