了解KVO实现的原理和本质

KVO我们经常使用,KVO的全称是: Key-Value Observing ,俗称"键值监听",可以用来监听某个对象的属性改变.今天我们就来研究一下 KVO 实现的本质,看看它是怎么做到属性发生改变的时候通知外部的.

为了接下来的研究,我们创建一个HHPerson类,类中添加一个age属性,然后在ViewController中创建person1,person2两个属性,给person1添加观察者,如下:

@interface ViewController ()

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

@end


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

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
    self.person1.age = 11;
    self.person2.age = 22;
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context{
    NSLog(@"%@ 对象的 %@ 属性发生了 %@ 的变化",object,keyPath,change);
}

我们点击屏幕输出如下:

2019-02-28 15:59:47.648955+0800 KVO本质_01[2656:334677] <HHPerson: 0x600001e4e7c0> 对象的 age 属性发生了 {
    kind = 1;
    new = 11;
    old = 11;
} 的变化

发现监听到了person1属性值得变化,而person2并没有变化,为什么会这样呢?我们看看他们给age赋值的方法:

    self.person1.age = 11;
    self.person2.age = 22;

其实赋值方法的本质就是调用age的setter方法,如下:

[self.person1 setAge:11];
[self.person2 setAge:22];

person1和person2的setter方法都是一样的,因为都是系统生成的,那为什么结果会有这么大的差别呢?
首先问题肯定不会在setter方法上,因为两个setter都是一样的,问题就是出在person1和person2两个对象上,我们打印person1和person2的isa看看:

(lldb) p/x self.person1->isa
(Class) $0 = 0x00006000039343f0 NSKVONotifying_HHPerson
(lldb) p/x self.person2->isa
(Class) $1 = 0x000000010a6b8f88 HHPerson

会发现person2的类对象还是HHPerson,而person1的类对象却变成了NSKVONotifying_HHPerson.
这是因为我们给person1添加 KVO 之后,rutime会动态的给person1生成一个新的类对象NSKVONotifying_HHPerson,这个类对象的结构如下:

我们对上图做一个解释:
实例对象person1的isa指针指向NSKVONotifying_HHPerson类,在NSKVONotifying_HHPerson类中有两个成员变量isa和superClass和4个方法setAge:,class,dealloc,_isKVO.其中superClass指向HHPerson类,所以NSKVONotifying_HHPerson其实是HHPerson的子类.NSKVONotifying_HHPerson重写父类的setAge:方法内部调用了Foundation框架的_NSSetIntValueAndNotify方法.通过方法名我们可以大概猜测出在这个方法内部先设置了属性的值,然后再通知外部.它的伪代码大概如下:

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

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

void didChangeValueForKey(NSString *)key{
    //通知监听器,属性值发生了改变
    [obser observeValueForKeyPath: key ofObject:self change:change context:context];
}

现在我们终于明白了KVO的大概原理,我们在给一个对象添加KVO时,runtime会动态的生成一个相关联的派生类,然后重写了setter方法,在setter方法内部进行了一些的操作,达到监听的目的.

下面开始验证我们上面所说的结论.
首选我们在person1添加KVO之前和之后分别打印person1和person2的类对象看看:

NSLog(@"person1添加KVO之前,person1的类对象是:%@  , person2的类对象是:%@",object_getClass(self.person1),object_getClass(self.person2));
NSKeyValueObservingOptions options = NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew;
[self.person1 addObserver:self forKeyPath:@"age" options:options context:nil];
NSLog(@"person1添加KVO之之后,person1的类对象是:%@  , person2的类对象是:%@",object_getClass(self.person1),object_getClass(self.person2));
================打印结果================
2019-02-28 17:30:45.941806+0800 KVO本质_01[2818:406318] person1添加KVO之前,person1的类对象是:HHPerson  , person2的类对象是:HHPerson
2019-02-28 17:30:45.942208+0800 KVO本质_01[2818:406318] person1添加KVO之之后,person1的类对象是:NSKVONotifying_HHPerson  , person2的类对象是:HHPerson

可以看到在添加KVO之后,person1的类对象的确从HHPerson变成了NSKVONotifying_HHPerson.

我们上面说了,添加KVO后,执行的setAge:方法是NSKVONotifying_HHPerson中的setAge:方法,也就是说peson1和person2的setAge:方法实现并不一样,我们来验证一下:

NSLog(@"person1添加KVO之前,person1的setAge方法实现是:%p  , person2的setAge方法实现是:%p",[self.person1 methodForSelector:@selector(setAge:)],[self.person2 methodForSelector:@selector(setAge:)]);

NSKeyValueObservingOptions options = NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew;
[self.person1 addObserver:self forKeyPath:@"age" options:options context:nil];
    
NSLog(@"person1添加KVO之后,person1的setAge方法实现是:%p  , person2的setAge方法实现是:%p",[self.person1 methodForSelector:@selector(setAge:)],[self.person2 methodForSelector:@selector(setAge:)]);

================打印结果================

2019-02-28 17:37:46.009609+0800 KVO本质_01[2842:413393] person1添加KVO之前,person1的setAge方法实现是:0x10f1dce50  , person2的setAge方法实现是:0x10f1dce50
2019-02-28 17:37:46.009940+0800 KVO本质_01[2842:413393] person1添加KVO之后,person1的setAge方法实现是:0x10f536cf2  , person2的setAge方法实现是:0x10f1dce50

可以发现,添加KVO之前setAge:方法实现都相同,添加KVO之后,person1的setAge:方法实现发生改变了. 那我们怎么知道的setAge:方法中调用了_NSSetIntValueAndNotify方法的呢?
我们在LLDB中打印这两个地址的实现看看:

(lldb) p (IMP)0x10f1dce50
(IMP) $2 = 0x000000010f1dce50 (KVO本质_01`-[HHPerson setAge:] at HHPerson.h:15)
(lldb) p (IMP)0x10f536cf2
(IMP) $3 = 0x000000010f536cf2 (Foundation`_NSSetIntValueAndNotify)

现在已经很清晰了,person1添加KVO后,他的setAge:方法实现就是_NSSetIntValueAndNotify.

思考一下,添加KVO后的person1的类对象和元类对象和person2一样吗?我们直接打印看一下:

Class person1_class = object_getClass(self.person1);
Class person1_metaClass = object_getClass(person1_class);
Class person2_class = object_getClass(self.person2);
Class person2_metaClass = object_getClass(person2_class);
NSLog(@"person1的类对象:%@,元类对象:%@",person1_class,person1_metaClass);
NSLog(@"person2的类对象:%@,元类对象:%@",person2_class,person2_metaClass);

================打印结果================

2019-02-28 17:57:10.776202+0800 KVO本质_01[2873:430403] person1的类对象:NSKVONotifying_HHPerson,元类对象:NSKVONotifying_HHPerson
2019-02-28 17:57:10.776349+0800 KVO本质_01[2873:430403] person2的类对象:HHPerson,元类对象:HHPerson

通过打印结果可以看出,添加KVO后类对象和元类对象都是runtime动态生成的,跟之前的并不一样.

我们之前用伪代码演示了_NSSetIntValueAndNotify方法内部大概如下:

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

接下来我们验证一下_NSSetIntValueAndNotify方法的执行顺序是否如我们所说,在HHPerson类中分别重写willChangeValueForKey:,didChangeValueForKey:两个方法,然后打印:

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

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

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

================打印结果================

2019-03-01 11:43:17.636388+0800 KVO本质_01[3362:398737] willChangeValueForKey
2019-03-01 11:43:17.636529+0800 KVO本质_01[3362:398737] age:
2019-03-01 11:43:17.636774+0800 KVO本质_01[3362:398737] <HHPerson: 0x6000005606b0> 对象的 age 属性发生了 {
    kind = 1;
    new = 11;
    old = 1;
} 的变化
2019-03-01 11:43:17.636931+0800 KVO本质_01[3362:398737] didChangeValueForKey

从打印结果可以看出来的确是先调用willChangeValueForKey,然后又调用了[super setAge:age],但是为什么先发出通知后调用didChangeValueForKey呢?这跟我们上面说的不一样呀!上面我们说是在didChangeValueForKey中出的通知.我们把didChangeValueForKey中的代码改成下面这样再打印:

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

================打印结果================

2019-03-01 11:48:06.587903+0800 KVO本质_01[3378:403190] willChangeValueForKey
2019-03-01 11:48:06.588084+0800 KVO本质_01[3378:403190] age:
2019-03-01 11:48:06.588187+0800 KVO本质_01[3378:403190] didChangeValueForKey---begin
2019-03-01 11:48:06.588385+0800 KVO本质_01[3378:403190] <HHPerson: 0x6000012b7320> 对象的 age 属性发生了 {
    kind = 1;
    new = 11;
    old = 11;
} 的变化
2019-03-01 11:48:06.588507+0800 KVO本质_01[3378:403190] didChangeValueForKey---end

从结果可以看到,的确是先进入didChangeValueForKey方法,然后再发出通知.