KVO的底层原理和自实现KVO

2018-07-27 本文已影响0人强子_4216

一、KVO的概述

KVO : Key-Value Observing（观察者设计模式）

当被观察对象的值改变时，观察者就会受到通知，这种模式有利于两个类间的解耦合，尤其是对于业务逻辑与视图控制这两个功能的解耦合。

二、KVO的实现原理

1、KVO是基于runtime机制实现的；
2、当某个类（Person）的属性被观察时，系统会自动的派生一个该类的子类（NSKVONotifying_Person）
3、为该子类添加setter方法；
4、消息转发，子类（NSKVONotifying_Person）--> 父类（Person）；
备注：苹果粑粑还悄悄的重写了该类的class方法，从而让我们感知不到子类的存在；

三、自实现KVO（观察者模式）

首先看下系统的KVO

//给self.Person属性添加观察者   
[self.person addObserver:self forKeyPath:@"name" options:NSKeyValueObservingOptionNew context:nil];

//通知观察者
-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context{
    if ([keyPath isEqualToString:@"name"]) {
        NSLog(@"通知了观察者，被观察的值改变了");
    }
}

下面我们自己实现KVO
话不多说，代码撸起 Demo

//NSObject+CGQKVO.h

#import  <Foundation/Foundation.h>
#import <objc/message.h>

typedef void (^QZKVOBlock)(id observer, id keyPath, id newValue, id oldValue);
@interface NSObject (CGQKVO)

/**
添加观察者
@param observer 观察者对象
@param keyPath 需要观察的键值
@param handle 回调block
*/
- (void)qz_addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath withHandle:(QZKVOBlock)handle;

/**
移除观察者
@param observer 观察值对象
@param keyPath 观察的键值
*/
-(void)qz_removeObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath;
@end

//创建一个info类来保存观察的数据
@interface QZ_Info : NSObject
@property (nonatomic, weak) id observer;
@property (nonatomic,copy) NSString *keyPath;
@property (nonatomic,copy) QZKVOBlock handle;
-(instancetype)initWhit:(id)observer keyPath:(NSString *)keyPath handle:(QZKVOBlock)handle;
@end

@implementation QZ_Info
-(instancetype)initWhit:(id)observer keyPath:(NSString *)keyPath handle:(QZKVOBlock)handle{
    self = [super init];
    if (self) {
        _observer = observer;
        _keyPath = keyPath;
        _handle = handle;
    }
    return self;
}

@end

//NSObject+CGQKVO.m

#import "NSObject+CGQKVO.h"
static NSString *const QZKVOPrefix = @"QZKVO_";
static NSString *const QZKVOAssicationKey = @"QZKVOAssicationKey";

@implementation NSObject (CGQKVO)
- (void)qz_addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath withHandle:(QZKVOBlock)handle{

    //检查是否是实例变量（实例变量就抛出异常）
    //1.获取当前类
    Class superClass = object_getClass(self);
    //2.检查是否有setter方法
    SEL setterSeletor = NSSelectorFromString(setterFormGetter(keyPath));
    Method setterMethod = class_getInstanceMethod(superClass, setterSeletor);

    if (!setterMethod) {
        @throw [NSException exceptionWithName:NSInvalidArgumentException reason:[NSString stringWithFormat:@"这个key:%@,没有setter:方法",keyPath] userInfo:nil];
    }

    //动态创建子类创建子类
    NSString *superClassName = NSStringFromClass(superClass);
    Class childClass;
    if (![superClassName hasPrefix:QZKVOPrefix]) {
        childClass = [self createClassFromSuperClass:superClassName];
        //把动态创建的子类指向父类
        //isa_swizzling-->黑魔法
        object_setClass(self, childClass);
    }

    //为子类添加setter方法
    /**
    class_addMethod(<#Class  _Nullable __unsafe_unretained cls#>, <#SEL  _Nonnull name#>, <#IMP  _Nonnull imp#>, <#const char * _Nullable types#>)
    1.class -->  给谁添加
    2.SEL  -->  方法编号
    3.IMP  -->  函数指针，指向函数实现
    4.types -->  返回值，参数
    */
    const char *types = method_getTypeEncoding(setterMethod);
    class_addMethod(childClass, setterSeletor, (IMP)QZKVO_Setter, types);
    
    //添加回调，方便调用者处理
    QZ_Info *info = [[QZ_Info alloc] initWhit:observer keyPath:keyPath handle:handle];
    NSMutableArray *Arr = objc_getAssociatedObject(self, &QZKVOAssicationKey);
    if (!Arr) {
        Arr = [[NSMutableArray alloc] initWithCapacity:1];
        objc_setAssociatedObject(self, &QZKVOAssicationKey, Arr, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    }
    [Arr addObject:info];
}

-(void)qz_removeObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath{
    NSMutableArray *Arr = objc_getAssociatedObject(self, &QZKVOAssicationKey);
    QZ_Info *tmp;
    for (QZ_Info *info in Arr) {
        if ([info.keyPath isEqualToString:keyPath]) {
            tmp = info;
            break;
        }
    }
    if (tmp) {
        [Arr removeObject:tmp];
    }
}

/**
通过父类创建子类
@param superClassName 父类类名
@return 子类
*/
- (Class)createClassFromSuperClass:(NSString *)superClassName{

    //获取父类
    Class superClass = object_getClass(self);

    //创建新类
    //1.获取子类类名
    NSString *childClassName = [QZKVOPrefix stringByAppendingString:NSStringFromClass(superClass)];

    //2.检查是否创建过该子类
    Class childClass = NSClassFromString(childClassName);
    //创建过就直接返回改类
    if (childClass) return childClass;

    //没创建过就开始创建
    /**
    1.superClass --> 父类
    2.childClassName --> 创建的类名
    3.size_t extraBytes -->开辟的内存空间
    */
    childClass = objc_allocateClassPair(superClass, childClassName.UTF8String, 0);

    /**
    添加子类动态添加的class方法
    class_addMethod(Class  _Nullable __unsafe_unretained cls, SEL  _Nonnull name, IMP  _Nonnull imp, const char * _Nullable types)
    1.class --> 给谁添加
    2.SEL  --> 方法编号
    3.IMP  --> 函数指针，指向函数的实现
    4.types --> 返回值/参数
    */
    Method childClassMethod = class_getInstanceMethod(superClass, @selector(class));//获取父类的class方法
    const char *types = method_getTypeEncoding(childClassMethod);
    class_addMethod(childClass, @selector(class),(IMP)QZKVO_Class, types);

    //注册
    objc_registerClassPair(childClass);

    return childClass;
}

#pragma mark - 函数区域

static void QZKVO_Setter(id self, SEL _cmd, id newValue){
    NSString *setterName = NSStringFromSelector(_cmd);
    NSString *getterName = getterFormSetter(setterName);
    if (!getterName) {
        @throw [NSException exceptionWithName:NSInvalidArgumentException reason:[NSString stringWithFormat:@"%@没有getter:%@方法",self,getterName] userInfo:nil];
    }

    //取出旧值
    id oldValue = [self valueForKey:getterName];

    //即将改变值
    [self willChangeValueForKey:getterName];

    //消息转发 子类-->父类
    /**
    objc_msgSendSuper(void * struct objc_super *super, SEL op, ... *)
    1.super --> void * struct objc_super 结构体
    2.op    --> SEL方法编号
    */
    void (*qzkvo_megSendSuper)(void *, SEL, id) = (void*)objc_msgSendSuper;

    struct objc_super qz_objcSuper = {
        /// Specifies an instance of a class.
        .receiver = self,
        .super_class = class_getSuperclass(object_getClass(self)),
    };

    qzkvo_megSendSuper(&qz_objcSuper, _cmd, newValue);

    //已经改变值
    [self didChangeValueForKey:getterName];

    NSArray *Arr = objc_getAssociatedObject(self, &QZKVOAssicationKey);
    for (QZ_Info *info in Arr) {
        if ([info.keyPath isEqualToString:getterName]) {
            info.handle(info.observer, info.keyPath, newValue, oldValue);
        }
    }
}

static Class QZKVO_Class(id self){
    return class_getSuperclass(object_getClass(self));
}

//name --> setName:
static NSString *setterFormGetter(NSString * getter){

    if (getter.length == 0)   return nil;

    NSString *firstString = [[getter substringToIndex:1] uppercaseString];
    NSString *lastString = [getter substringFromIndex:1];
    return [NSString stringWithFormat:@"set%@%@:",firstString,lastString];
}

//setName: --> name
static NSString *getterFormSetter(NSString * setter){

    if (setter.length <= 0 || ![setter hasPrefix:@"set"] || ![setter hasSuffix:@":"]) {
        return nil;
    }
    NSRange range = NSMakeRange(3, setter.length-4);
    NSString *getterStr = [setter substringWithRange:range];
    NSString *firstString = [[getterStr substringToIndex:1] lowercaseString];
    return [getterStr stringByReplacingCharactersInRange:NSMakeRange(0, 1) withString:firstString];
}

@end

总结

KVO实际上就是观察对象的setter方法，对于没有setter方法对象和不调用setter方法（_name = @"张三"）时，值的改变，是无法触发KVO机制的；
KVO机制能够很方便的提供两个对象间的同步（像view和model之间的同步）
自己完成实现KVO后，会对KVO底层有一个较深入的了解，增加了block回调也能增加代码的可读性和可维护性;

KVO的底层原理和自实现KVO

一、KVO的概述

二、KVO的实现原理

三、自实现KVO（观察者模式）

总结

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