博客> KVC/KVO实现原理
KVC/KVO实现原理
2019-09-18 20:42 评论:0 阅读:469 yahib
ios kvo kvc

一、kvo实现原理

一个对象的属性被观察时系统动态创建了一个子类,并且改变了原有对象的isa指针指向,指向动态创建的子类,子类中重写了被观察属性的set方法,在使用点方法和set方法给属性赋值时,最终调用的是子类中的set方法。 注意:查看isa指针指向,如果断点执行过程中isa指针没有变化, 需要关闭xcode重新打开

相关代码:在addObserver处设置断点观察对象isa指针变化:

- (void)viewDidLoad {
    [super viewDidLoad];
    People *p = [[People alloc] init];
    [p addObserver:self forKeyPath:@"name" options:NSKeyValueObservingOptionNew context:nil];
    p.name = @"123";
}
-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey> *)change context:(void *)context{
    NSLog(@"%@",change);
}

通过断点观察isa指针的指向 被观察前isa指针指向的是原始类如图:

 image.png

执行代码被观察后指针指向的是NSKVONotifying_People类:

 image.png

二、自定义KVO

创建一个分类新增一个方法HBaddObserver,在方法中创建子类注册并指向子类,再为子类添加set方法既可。

主要使用函数: 1、创建一个子类

 objc_allocateClassPair(Class _Nullable superclass, const char * _Nonnull name,
 size_t extraBytes)
 superclass:设置新类的父类
 name:新类名称
 extraBytes:额外字节数设置为0

2、注册该类

 objc_registerClassPair(Class _Nonnull cls)
 cls:当前要注册的类,注册后才可以使用

3、设置当前对象指向其他类

 object_setClass(id _Nullable obj, Class _Nonnull cls)
 obj:要设置的对象
 cls:指向的类

4、动态添加一个方法

 class_addMethod(Class _Nullable cls, SEL _Nonnull name, IMP _Nonnull imp,
 const char * _Nullable types)
 cls:设置添加方法对应的类
 name:选择子(选择器)名称,描述了方法的格式,并不会指向方法
 imp:函数名称(函数指针),和选择子一一对应,指向方法实现的地址

通过分类添加新的观察者添加方法:

-(void)HBaddObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(void *)context{
    NSString *oldName = NSStringFromClass(self.class);
    NSString *newName = [@"HBKVO_" stringByAppendingString:oldName];
    //1、创建一个子类
    Class newClass = objc_allocateClassPair(self.class, newName.UTF8String, 0);
    //2、注册该类
    objc_registerClassPair(newClass);
    //3、指向子类
    object_setClass(self, newClass);
    //4、动态添加一个方法
    NSString *first = [keyPath substringWithRange:NSMakeRange(0, 1)];
    NSString *other = [keyPath substringFromIndex:1];
    NSString *setName = [NSString stringWithFormat:@"set%@%@:",first.uppercaseString,other];//设置一个属性名首字母大写的方法
    Method method = class_getInstanceMethod(self.class, sel_registerName(setName.UTF8String));
    const char *types = method_getTypeEncoding(method);
    class_addMethod(newClass, sel_registerName(setName.UTF8String), (IMP)setValue, types);
    //class_addMethod(newClass, sel_registerName(setMethod.UTF8String), (IMP)setName, "v@:@");

    //设置关联数据
    //获取元类旧值使用
    objc_setAssociatedObject(self, "keyPath", keyPath, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    //设置新值的时候使用
    objc_setAssociatedObject(self, "setName", setName, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    //通知值变化
    objc_setAssociatedObject(self, "observer", observer, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    //传进来的内容需要回传
    objc_setAssociatedObject(self, "context", (__bridge id _Nullable)(context), OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

设置属性值的新调用方法:

void setValue(id self,SEL _cmd,NSString *newValue){
    NSLog(@"newValue:%@",newValue);
    NSString *keyPath = objc_getAssociatedObject(self, "keyPath");
    NSString *setName = objc_getAssociatedObject(self, "setName");
    id observer = objc_getAssociatedObject(self, "observer");
    id context = objc_getAssociatedObject(self, "context");
    //存储新类
    Class newClass = [self class];
    //指向父类获取旧值
    object_setClass(self, class_getSuperclass(newClass));
    NSString *oldValue = objc_msgSend(self,sel_registerName(keyPath.UTF8String));
    //对原始类属性或成员变量复制
    objc_msgSend(self, sel_registerName(setName.UTF8String),newValue);
    NSMutableDictionary *change = [NSMutableDictionary dictionary];
    if (oldValue) {
        change[NSKeyValueChangeOldKey] = oldValue;
    }
    if (newValue) {
        change[NSKeyValueChangeNewKey] = newValue;
    }
    //调用observer的回调方法
    objc_msgSend(observer, @selector(observeValueForKeyPath:ofObject:change:context:),keyPath,observer,change,context);
    //操作完成后指回动态创建的新类
    object_setClass(self, newClass);
}

通过断点观察isa指针的指向 被观察前isa指针指向的是原始类如图:

 image.png

执行代码被观察后指针指向的是HBKVO_Person类:

 image.png

1、主控制器代码如下:

#import "ViewController.h"
#import <objc>
#import "Person.h"
#import "NSObject+HBKVO.h"

@interface ViewController (){
    Person *p;
}
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    p = [[Person alloc] init];
    [p HBaddObserver:self forKeyPath:@"Name" options:NSKeyValueObservingOptionNew context:nil];

}
-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey> *)change context:(void *)context{
    NSLog(@"change:%@",change);
}
-(void)touchesBegan:(NSSet<UITouch> *)touches withEvent:(UIEvent *)event{
    static int num = 0;
    p.Name = [NSString stringWithFormat:@"%d",num++];
}
@end

2、person类头文件代码(.m中无相关代码):

#import <Foundation>

NS_ASSUME_NONNULL_BEGIN

@interface Person : NSObject
@property (nonatomic,strong)NSString *Name;
@end

NS_ASSUME_NONNULL_END

3、自定义KVO相关代码(NSObject分类):

NSObject+HBKVO.h

#import <Foundation>
NS_ASSUME_NONNULL_BEGIN
@interface NSObject (HBKVO)
- (void)HBaddObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(nullable void *)context;
@end
NS_ASSUME_NONNULL_END

NSObject+HBKVO.m


#import "NSObject+HBKVO.h"
#import <objc>

@implementation NSObject (HBKVO)
-(void)HBaddObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(void *)context{
    NSString *oldName = NSStringFromClass(self.class);
    NSString *newName = [@"HBKVO_" stringByAppendingString:oldName];
    //1、创建一个子类
    Class newClass = objc_allocateClassPair(self.class, newName.UTF8String, 0);
    //2、注册该类
    objc_registerClassPair(newClass);
    //3、指向子类
    object_setClass(self, newClass);
    //4、动态添加一个方法
    NSString *first = [keyPath substringWithRange:NSMakeRange(0, 1)];
    NSString *other = [keyPath substringFromIndex:1];
    NSString *setName = [NSString stringWithFormat:@"set%@%@:",first.uppercaseString,other];//设置一个属性名首字母大写的方法
    Method method = class_getInstanceMethod(self.class, sel_registerName(setName.UTF8String));
    const char *types = method_getTypeEncoding(method);
    class_addMethod(newClass, sel_registerName(setName.UTF8String), (IMP)setValue, types);
    //class_addMethod(newClass, sel_registerName(setMethod.UTF8String), (IMP)setName, "v@:@");

    //设置关联数据
    //获取元类旧值使用
    objc_setAssociatedObject(self, "keyPath", keyPath, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    //设置新值的时候使用
    objc_setAssociatedObject(self, "setName", setName, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    //通知值变化
    objc_setAssociatedObject(self, "observer", observer, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    //传进来的内容需要回传
    objc_setAssociatedObject(self, "context", (__bridge id _Nullable)(context), OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}
void setValue(id self,SEL _cmd,NSString *newValue){
    NSLog(@"newValue:%@",newValue);
    NSString *keyPath = objc_getAssociatedObject(self, "keyPath");
    NSString *setName = objc_getAssociatedObject(self, "setName");
    id observer = objc_getAssociatedObject(self, "observer");
    id context = objc_getAssociatedObject(self, "context");
    //存储新类
    Class newClass = [self class];
    //指向父类获取旧值
    object_setClass(self, class_getSuperclass(newClass));
    NSString *oldValue = objc_msgSend(self,sel_registerName(keyPath.UTF8String));
    //对原始类属性或成员变量复制
    objc_msgSend(self, sel_registerName(setName.UTF8String),newValue);
    NSMutableDictionary *change = [NSMutableDictionary dictionary];
    if (oldValue) {
        change[NSKeyValueChangeOldKey] = oldValue;
    }
    if (newValue) {
        change[NSKeyValueChangeNewKey] = newValue;
    }
    //调用observer的回调方法
    objc_msgSend(observer, @selector(observeValueForKeyPath:ofObject:change:context:),keyPath,observer,change,context);
    //操作完成后指回动态创建的新类
    object_setClass(self, newClass);
}
@end
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