利用runtime实现KVO

KVO实现原理

一.关于KVO

KVO(Key-Value Observing)提供一种机制，当指定对象的属性被修改后，就会通知观察者。简单的说就是每次指定的被观察的对象的属性被修改后，KVO就会自动通知相应的观察者了。

KVO其实也是“观察者”设计模式的一种应用。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用法

假设已经有一个Person类，包含name属性，我们可以通过KVO监听到name属性被修改的事件：

_p = [Person new];    
[_p addObserver:self forKeyPath:@"name" options:NSKeyValueObservingOptionNew context:nil];

即当_p对象的name属性被修改后，会调用self的以下方法：

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context{NSLog(@"kvo监听到%@对象的%@属性被修改为%@",object,keyPath,[change objectForKey:@"new"]);
}

三.KVO实现原理

当调用以下函数前：

[_p addObserver:self forKeyPath:@"name" options:NSKeyValueObservingOptionNew context:nil];

我们可以看到_p的isa指针指向类Person：

但是当单步执行过后，_p的isa指针会变为指向NSKVONotifying_Person类
：

即对于上面的代码，系统会执行如下操作：

1、利用runtime动态创建Person类的子类NSKVONotifying_Person，重写name属性的set方法：

-(void)setName:(NSString*)name{
[self willChangeValueForKey:@"name"];
[super setName:name];
[self didChangeValueForKey:@"name"];
}

2、将p对象的isa指针指向新建的子类

当我们修改_p的name属性时，会执行NSKVONotifying_Person类对象的set方法。

四.自己实现KVO

我们可以按照上面的思路，自己用代码实现KVO的基本功能，同时优化为以block的方式进行回调而非使用-observeValueForKeyPath:ofObject:change:context:方法。

1.新建一个NSObjct的Category，声明添加监听和移除监听方法：
NSObject+KVO.h

typedef void(^JXObserveingBlock)(id observer,NSString *key,id oldValue,id newValue);@interface NSObject (KVO)-(void)JX_addObserver:(NSObject *)observer forKeyPath:(NSString *)key block:(JXObserveingBlock*)block;-(void)JX_removeObserver:(NSObject *)observer forKey:(NSString *)key;@end

2.实现JX_addObserver:forKeyPath:block:方法：

-(void)JX_addObserver:(NSObject *)observer forKeyPath:(NSString *)key block:(JXObserveingBlock)block{//1.检查对象是否存在对应的set方法，没有则抛出异常。SEL setterSelector = NSSelectorFromString([self setterForGetter:key]);Method setterMethod = class_getInstanceMethod([self class], setterSelector);if (!setterMethod) {// throw invalid argument exception}//2.检查isa指针指向的类是否已经是KVO的类，不是则新建原类的子类，并把isa指针指向新类。Class class = object_getClass(self);NSString *className = NSStringFromClass(class);if (![className hasPrefix:kJXKVOClassPrefix]) {class = [self makeKvoClassWithOriginalClassName:className];object_setClass(self, class);}//3.检查KVO类是否已经重写了set方法，没有则重写set方法。if (![self hasSelector:setterSelector]) {const char *types = method_getTypeEncoding(setterMethod);class_addMethod(class, setterSelector, (IMP)kvo_setter, types);}//添加观察者JXObservationInfo *info = [[JXObservationInfo alloc] initWithObserver:observer Key:key block:block];NSMutableArray *observers = objc_getAssociatedObject(self, (__bridge const void*)kJXKVOAssociatedObservers);if (!observers) {observers = [NSMutableArray array];objc_setAssociatedObject(self, (__bridge const void*)kJXKVOAssociatedObservers, observers, OBJC_ASSOCIATION_RETAIN_NONATOMIC);}[observers addObject:info];
}

其中setterForGetter:方法是获取属性对应的set方法名：

-(NSString*)setterForGetter:(NSString*)key{if (key.length <= 0) {return nil;}//1.首字母大写key = [key capitalizedStringWithLocale:[NSLocale currentLocale]];//2.添加set前缀和:后缀NSString *setter = [NSString stringWithFormat:@"set%@:",key];return setter;
}

hasSelector:方法判断是否已经重写了set方法

- (BOOL)hasSelector:(SEL)selector{Class clazz = object_getClass(self);unsigned int methodCount = 0;Method* methodList = class_copyMethodList(clazz, &methodCount);for (unsigned int i = 0; i < methodCount; i++) {SEL thisSelector = method_getName(methodList[i]);if (thisSelector == selector) {free(methodList);return YES;}}free(methodList);return NO;
}

makeKvoClassWithOriginalClassName:方法根据类名返回kvo类：

- (Class)makeKvoClassWithOriginalClassName:(NSString *)originalClazzName{NSString *kvoClassName = [kJXKVOClassPrefix stringByAppendingString:originalClazzName];Class clazz = NSClassFromString(kvoClassName);if (clazz) {return clazz;}//如果还没创建对应的kvo类，则创建Class originalClazz = object_getClass(self);Class kvoClazz = objc_allocateClassPair(originalClazz, kvoClassName.UTF8String, 0);//重写class方法Method clazzMethod = class_getInstanceMethod(originalClazz, @selector(class));const char *types = method_getTypeEncoding(clazzMethod);class_addMethod(kvoClazz, @selector(class), (IMP)kvo_class, types);objc_registerClassPair(kvoClazz);return kvoClazz;
}static Class kvo_class(id self, SEL _cmd)
{return class_getSuperclass(object_getClass(self));
}

最后，重写setter 方法：

static void kvo_setter(id self, SEL _cmd, id newValue)
{NSString *setterName = NSStringFromSelector(_cmd);NSString *getterName = getterForSetter(setterName);if (!getterName) {NSString *reason = [NSString stringWithFormat:@"Object %@ does not have setter %@", self, setterName];@throw [NSException exceptionWithName:NSInvalidArgumentExceptionreason:reasonuserInfo:nil];return;}id oldValue = [self valueForKey:getterName];struct objc_super superclazz = {.receiver = self,.super_class = class_getSuperclass(object_getClass(self))};// cast our pointer so the compiler won't complainvoid (*objc_msgSendSuperCasted)(void *, SEL, id) = (void *)objc_msgSendSuper;// call super's setter, which is original class's setter methodobjc_msgSendSuperCasted(&superclazz, _cmd, newValue);// look up observers and call the blocksNSMutableArray *observers = objc_getAssociatedObject(self, (__bridge const void *)(kJXKVOAssociatedObservers));for (JXObservationInfo *each in observers) {if ([each.key isEqualToString:getterName]) {dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{each.block(self, getterName, oldValue, newValue);});}}
}static NSString * getterForSetter(NSString *setter)
{if (setter.length <=0 || ![setter hasPrefix:@"set"] || ![setter hasSuffix:@":"]) {return nil;}// 移除‘set’前缀以及后面的‘:’NSRange range = NSMakeRange(3, setter.length - 4);NSString *key = [setter substringWithRange:range];// 首字母小写NSString *firstLetter = [[key substringToIndex:1] lowercaseString];key = [key stringByReplacingCharactersInRange:NSMakeRange(0, 1)withString:firstLetter];return key;
}

五.测试实现的KVO

把原来的：

[_p addObserver:self forKeyPath:@"name" options:NSKeyValueObservingOptionNew context:nil];

修改为：

[_p JX_addObserver:self forKeyPath:@"name" block:^(id observer, NSString *key, id oldValue, id newValue) {NSLog(@"kvo监听到%@对象的%@属性被修改为%@",observer,key,newValue);}];

在修改_p的name属性之后，会打印出修改信息。

完整代码下载：http://download.csdn.net/detail/dolacmeng/9849000

参考：http://tech.glowing.com/cn/implement-kvo/