iOS-runtime自定义KVO----分类添加关键属性

1. KVO原理

2. runtime自定义KVO

3. runtime给分类添加关联属性

• 我们注册监听的时候，会对注册者动态的创建一个子类对象，然后底层找方法的的isa指针就变成指向新创建的子类对象。当改变注册对象某个属性的时候,就重写属性的set方法来进行监听。这么说可能理解上不是很明白，下面我们结合代码来分析：

• 自定义

#import <Foundation/Foundation.h>

NS_ASSUME_NONNULL_BEGIN

@interface NSObject (KVO)

@property (nonatomic, strong) NSObject *test;

@property (nonatomic, strong) NSTimer *timer;

- (void)kvo_addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(nullable void *)context;

@end

NS_ASSUME_NONNULL_END


/***********************************************/

#import "NSObject+KVO.h"
#import <objc/message.h>

// timer 与 test 是runtime添加关联属性代码, 与自定义kvo无关
static const char *key_test = "test";
static const char *key_observer = "objc";
static const char *key_timer = "timer";

@implementation NSObject (KVO)

- (void)kvo_addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(nullable void *)context {

    NSString *oldClassName = NSStringFromClass([self class]);
    NSString *newClassName = [NSString stringWithFormat:@"KVC_%@", oldClassName];
    const char *newName = [newClassName UTF8String];
    Class newClass = objc_allocateClassPair([self class], newName, 0);
    class_addMethod(newClass,@selector(setAge:), (IMP)setAge, "v@:i");
    objc_registerClassPair(newClass);
    
   // (__bridge const void *)@"objc"
    object_setClass(self, newClass);
    objc_setAssociatedObject(self, key_observer, observer, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    
    Class nclass = NSClassFromString(@"KVO_ViewController");
    
}

- (NSObject *)test {
    NSLog(@"分类添加属性值test_get:%@",objc_getAssociatedObject(self, key_test));
    return objc_getAssociatedObject(self, key_test);
}

- (void)setTest:(NSObject *)test {
    NSLog(@"分类添加属性值test_set前:%@",objc_getAssociatedObject(self, key_test));
    objc_setAssociatedObject(self, key_test, test, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    NSLog(@"分类添加属性值test_set后:%@",objc_getAssociatedObject(self, key_test));
}

- (void)setTimer:(NSTimer *)timer {
    objc_setAssociatedObject(self, key_timer, timer, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

- (NSTimer *)timer {
    return objc_getAssociatedObject(self, key_timer);
}

void setAge(id self, SEL _cmd, int age) {
    //保存当前类
    Class myclass = [self class];

    //将self的isa指针指向父类
    object_setClass(self, class_getSuperclass([self class]));

    //调用父类
    objc_msgSend(self,@selector(setAge:),age);
    
    //拿出观察者 (__bridge const void *)@"objc"
    id objc = objc_getAssociatedObject(self, key_observer);
    
    //通知观察者
    objc_msgSend(objc,@selector(observeValueForKeyPath:ofObject:change:context:),@"age",self,@{},nil);

    //改为子类
    object_setClass(self, myclass);

}

@end

• 使用

#import "ViewController.h"
#import "NSObject+KVO.h"

@interface ViewController ()


@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    // [self addObserver:(nonnull NSObject *) forKeyPath:(nonnull NSString *) options:(NSKeyValueObservingOptions) context:(nullable void *)];
    self.view.backgroundColor = [UIColor whiteColor];
    
    [self kvo_addObserver:self forKeyPath:@"age" options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld context:nil];
    
}

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
    self.age = 20;
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context {
    NSLog(@"自定义kvo监听到属性改变");
}

- (void)setAge:(int)age {
    _age = age;
    NSLog(@"方法没有被覆盖:%s", __func__);
}

@end

• 详解

1. 动态添加一个当前类的子类

    NSString *oldClassName = NSStringFromClass([self class]);

    NSString *newClassName = [@"AWKVO_" stringByAppendingString:oldClassName];

    const char * newName = [newClassName UTF8String];

    Class myclass = objc_allocateClassPair([self class], newName, 0);

    

    //添加setter方法，相当于重写setter方法， "v@:i" 含义 @: id   : SEL     v : void  

      OC(消息发送机制),方法由两部分组成，方法编号@selector和方法实现(imp方法指针)，先找方法编号再得到方法的指针，再执行方法的代码块。

    class_addMethod(myclass, @selector(setAge:), (IMP)setAge, "v@:i");

    

    //注册新添加的这个类

    objc_registerClassPair(myclass);

    

    //修改被观察这的isa指针，isa指针指向Person类改成指向myclass这个类

    object_setClass(self, myclass);


    //将观察者的属性保存到当前类里面去

    objc_setAssociatedObject(self, (__bridge const void *)@"objc", observer, OBJC_ASSOCIATION_RETAIN_NONATOMIC);

}




//相当于重写父类的方法

void setAge(id self, SEL _cmd, int age) {

    

    //保存当前类

    Class myclass = [self class];

    

    //将self的isa指针指向父类

    object_setClass(self, class_getSuperclass([self class]));

    

    //调用父类

    objc_msgSend(self, @selector(setAge:),age);

    

    //拿出观察者

    objc_getAssociatedObject(self, (__bridge const void *)@"objc");

    

    //通知观察者

    objc_msgSend(objc,@selector(observeValueForKeyPath:ofObject:change:context:),self,age,nil,nil);

    

    //改为子类

    object_setClass(self, myclass);

}


这样就可以回调到ViewContoller中的监听方法observeValueForKeyPath:ofObject:change:context:中去，而且相关的值也传递过去了。

总结自定义一个KVO思路：

1.自定义一个类，继承 [self class]的一个子类

2.重写父类的属性setter方法

3.调用observeValueForKeyPath:ofObject:change:context:方法，回调到ViewController中去。

• 附加：
  • 苹果为什么要用子类(就是C语言创建的那个子类)监听setter方法，而不用分类(Person+AWKVO)呢？
  • 回答：原因是当你用分类监听setter方法的时候，Person类中setter方法就不会走了，这样不好，所以苹果使用了子类监听setter方法。