KVO 底层原理详解

基本原理-> 给一个对象的属性添加监听当属性值发生变化时会触发监听器的监听的方法

#import "ViewController.h"
#import "Person.h"
@interface ViewController ()
@property (nonatomic, strong) Person *p;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    self.p=[[Person alloc]init];
    self.p.age=5;
    [self.p addObserver:self forKeyPath:@"age" options:NSKeyValueObservingOptionOld|NSKeyValueObservingOptionNew context:@"额外信息"];
    // Do any additional setup after loading the view.
}
-(void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
    self.p.age=10;
}
-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context{
    NSLog(@"\n%@\n%@\n%@\n%@",keyPath,object,change,context);
    /*
     age
     <Person: 0x600001390970>
     {
         kind = 1;
         new = 10;
         old = 5;
     }
     额外信息
     */
}
-(void)dealloc{
    [self.p removeObserver:self forKeyPath:@"age"];
}

2、在给person 添加监听之后、其实苹果底层通过运行时动态给person添加了一个子类 NSKVONotifying_Person

   NSLog(@"%s",object_getClassName(self.p)); // Person
    [self.p addObserver:self forKeyPath:@"age" options:NSKeyValueObservingOptionOld|NSKeyValueObservingOptionNew context:@"额外信息"];
    NSLog(@"%s",object_getClassName(self.p)); //NSKVONotifying_Person

3、在新生产的类会新生成4个方法

- (void)viewDidLoad {
    [super viewDidLoad];
    self.p=[[Person alloc]init];
    self.p.age=5;
    [self printMethodNamesOfClass:object_getClass(self.p)]; // Person age, setAge:
    [self.p addObserver:self forKeyPath:@"age" options:NSKeyValueObservingOptionOld|NSKeyValueObservingOptionNew context:@"额外信息"];
    [self printMethodNamesOfClass:object_getClass(self.p)];//NSKVONotifying_Person setAge:, class, dealloc, _isKVOA,
}
- (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);
}

4个方法介绍

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

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

- (void)dealloc
{
    // 收尾工作
}

- (BOOL)_isKVOA
{
    return YES;
}

4、在新生成的这个类中修改对象的属性时-> 其实会类似重写Set方法- 会调用Foundation 框架中的

_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:）

5、如要要手动触发一个KVO 监听器的监听方法

调用改对象的 willChangeValueForKey
调用改对象的 didChangeValueForKey

-(void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
//    self.p.age=10;
    [self.p willChangeValueForKey:@"age"];
    [self.p didChangeValueForKey:@"age"];
}
-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context{
    NSLog(@"\n%@\n%@\n%@\n%@",keyPath,object,change,context);
    /*
    age
     <Person: 0x6000019947f0>
     {
         kind = 1;
         new = 5;
         old = 5;
     }
     额外信息
     */
}

6、修改对象的成员变量是不会触发KVO的、KVC赋值是能够触发KVO

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