概述

KVO官網(wǎng)
KVO鍵值觀察是一種機(jī)制猜揪，它允許對象被通知對其他對象的指定屬性的更改.

KVO使用

- (void)viewDidLoad {
    [super viewDidLoad];
    // NSKeyValueObservingOptionNew = 0x01,       //返回新值
    //NSKeyValueObservingOptionOld = 0x02,       //返回舊值
    //NSKeyValueObservingOptionInitial           //注冊的時(shí)候就會(huì)發(fā)一次通知鸽素，改變的時(shí)候也會(huì)發(fā)通知
   // NSKeyValueObservingOptionPrior         //改變之前發(fā)一次，改變之后發(fā)一次
    
    QHPerson *person = [[QHPerson alloc] init];
    person.name = @"123";
    [person addObserver:self forKeyPath:@"name" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:nil];
    
    _p = person;
}
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
    
    _p.name = @"qinhan";
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context
{
    NSLog(@"change:%@",change);
}

上面的是自動(dòng)發(fā)送通知，kvc也提供了手動(dòng)發(fā)送通知

//默認(rèn)是自動(dòng)
+ (BOOL)automaticallyNotifiesObserversForKey:(NSString *)key
{
    return NO;
}
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
    [_p willChangeValueForKey:@"name"];
    _p.name = @"qinhan";
    [_p didChangeValueForKey:@"name"];
}

如果我們想見觀察多個(gè)屬性

 QHPerson *person = [[QHPerson alloc] init];
 person.name = @"123";
    //[person addObserver:self forKeyPath:@"name" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:nil];
 [person addObserver:self forKeyPath:@"son" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:nil];

+(NSSet<NSString *> *)keyPathsForValuesAffectingValueForKey:(NSString *)key
{
    NSSet *keypaths = [super keyPathsForValuesAffectingValueForKey:key];
    if ([key isEqualToString:@"son"]) {
        
        NSArray *arr = @[@"_son.nickName",@"_son.age"];
        keypaths = [keypaths setByAddingObjectsFromArray:arr];
    }
     return keypaths;
}

這樣我們可以觀察person屬性的son實(shí)例對象

kvo探索

1.我們分別定義了屬性、成員變量添瓷，然后分別觀察

 [person addObserver:self forKeyPath:@"nickName" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:nil];
[person addObserver:self forKeyPath:@"name" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:nil];
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
    
    self.p.name =@"qinhan";
    self.p->nickName = @"nickName";
}
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context
{
    NSLog(@"change:%@",change);
}

打印結(jié)果

2021-06-05 22:22:14.983537+0800 001---KVC基礎(chǔ)使用[12792:12309284] change:{
 kind = 1;
new = qinhan;

發(fā)送我們只監(jiān)聽到屬性鹅髓、沒有監(jiān)聽到成員變量瑟押∠萸蓿可見kvo實(shí)際上時(shí)觀察setter方法
2.探索addObserver
在調(diào)用addObserver前先打一個(gè)斷點(diǎn)魁索，

截屏2021-06-05 下午10.26.58.png

截屏2021-06-05 下午10.27.25.png

- (void)printClasses:(Class)cls{

    // 注冊類的總數(shù)
    int count = objc_getClassList(NULL, 0);
    // 創(chuàng)建一個(gè)數(shù)組， 其中包含給定對象
    NSMutableArray *mArray = [NSMutableArray arrayWithObject:cls];
    // 獲取所有已注冊的類
    Class* classes = (Class*)malloc(sizeof(Class)*count);
    objc_getClassList(classes, count);
    for (int i = 0; i<count; i++) {
        if (cls == class_getSuperclass(classes[i])) {
            [mArray addObject:classes[i]];
        }
    }
    free(classes);
    NSLog(@"classes = %@", mArray);
}

結(jié)果如下圖：

截屏2021-06-05 下午10.41.44.png

模擬KVO實(shí)現(xiàn)

注冊自定義addobserver方法

  [person qh_addObserver:self forKeyPath:@"name" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:nil];

自定義一個(gè)NSObject 分類

- (void)qh_addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(nullable void *)context
{
    //自定義NSKVONotify_object 子類
    NSString *oldClassName = NSStringFromClass(self.class);
    NSString *newClassName = [@"QHKVO_" stringByAppendingString:oldClassName];
    //創(chuàng)建一個(gè)類
    Class myclass = objc_allocateClassPair(self.class, newClassName.UTF8String, 0);
    
    //注冊新類
    objc_registerClassPair(myclass);
    
    //動(dòng)態(tài)修改 isa 指向中間類
    object_setClass(self, myclass);
    
    //給中間類重新添加setter方法
    class_addMethod(myclass, @selector(setName:), (IMP)setName, "v@:@");
    
    //將觀察者綁定到對象上面
    objc_setAssociatedObject(self, (__bridge  const void *)@"objc", observer,OBJC_ASSOCIATION_ASSIGN );
}
void setName(id self,SEL _cmd ,NSString *newName){
    struct objc_super person1 = {self,class_getSuperclass([self class])};
    //調(diào)用父類的方法
    objc_msgSendSuper(&person1, _cmd,newName);
    NSLog(@"newName:%@",newName);
    
    //拿到觀察者
  id observer =  objc_getAssociatedObject(self, (__bridge  const void *)@"objc");
    
    //調(diào)用observer valueforkey 發(fā)送通知
    objc_msgSend(observer, @selector(observeValueForKeyPath:ofObject:change:context:),@"name",self,@{@"name":newName});
}

總結(jié)KVO

• 驗(yàn)證是否存在setter方法
• 動(dòng)態(tài)生成子類盼铁，需要重寫class、setter方法
• 在子類的setter方法中向父類發(fā)消息
• 調(diào)用通知方法尝偎，把修改后的信息返回回去