KVO
KVO的全称是Key-Value Observing，俗称“键值监听”，可以用于监听某个对象属性值的改变
KVO 使用方式
1、添加观察者
// 摘自AFNetworking
for (NSString *keyPath in AFHTTPRequestSerializerObservedKeyPaths()) {
if ([self respondsToSelector:NSSelectorFromString(keyPath)]) {
[self addObserver:self forKeyPath:keyPath options:NSKeyValueObservingOptionNew context:AFHTTPRequestSerializerObserverContext];
}
}
2、实现 observeValueForKeyPath:ofObject:change:context: 方法
// 摘自AFNetworking
- (void)observeValueForKeyPath:(NSString *)keyPath
ofObject:(__unused id)object
change:(NSDictionary *)change
context:(void *)context
{
if (context == AFHTTPRequestSerializerObserverContext) {
if ([change[NSKeyValueChangeNewKey] isEqual:[NSNull null]]) {
[self.mutableObservedChangedKeyPaths removeObject:keyPath];
} else {
[self.mutableObservedChangedKeyPaths addObject:keyPath];
}
}
}
3、移除观察者 removeObserver:forKeyPath:
- (void)dealloc {
for (NSString *keyPath in AFHTTPRequestSerializerObservedKeyPaths()) {
if ([self respondsToSelector:NSSelectorFromString(keyPath)]) {
[self removeObserver:self forKeyPath:keyPath context:AFHTTPRequestSerializerObserverContext];
}
}
}
KVO 禁用
// 摘自AFNetworking
+ (BOOL)automaticallyNotifiesObserversForKey:(NSString *)key {
if ([AFHTTPRequestSerializerObservedKeyPaths() containsObject:key]) {
return NO;
}
return [super automaticallyNotifiesObserversForKey:key];
}
KVO 设置依赖
+ (NSSet *)keyPathsForValuesAffectingValueForKey:(NSString *)key {
if ([key isEqualToString:@"reachable"] || [key isEqualToString:@"reachableViaWWAN"] || [key isEqualToString:@"reachableViaWiFi"]) {
return [NSSet setWithObject:@"networkReachabilityStatus"];
}
return [super keyPathsForValuesAffectingValueForKey:key];
}
KVO原理
1:动态生成子类:NSKVONotifiy_A
2:给动态子类添加Setter方法
3:消息转发给父类(runtime消息转发)
image-20220601140719614
image-20220601140706480
_NSSet*ValueAndNotify的内部实现
调用willChangeValueForKey:
调用原来的setter实现
调用didChangeValueForKey:
- didChangeValueForKey:内部会调用observer的observeValueForKeyPath:ofObject:change:context:方法
KVO存在问题
1、必须成对出现
2、重复添加，重复几次，执行几次响应
3、重复移除，移除超过添加次数后，NSRangeException报错。
在移除观察的时候NSKVONotifying_XXX 是否移除? + isa 是否会回来?
答：NSKVONotifying_XXX不会移除，但是实例对象的isa是会重新指向原类
