ReactiveObjC (前身是 ReactiveCocoa 或者 RAC) 是一個 Objective-C 框架，實現了函數響應式編程模式。

最簡單的例子：

//?When?self.username?changes,?logs?the?new?name?to?the?console.
//
//?RACObserve(self,?username)?creates?a?new?RACSignal?that?sends?the?current
//?value?of?self.username,?then?the?new?value?whenever?it?changes.
//?-subscribeNext:?will?execute?the?block?whenever?the?signal?sends?a?value.
[RACObserve(self,?username)?subscribeNext:^(NSString?*newName)?{
????NSLog(@"%@",?newName);
}];

K/V 通知

//?Only?logs?names?that?starts?with?"j".
//
//?-filter?returns?a?new?RACSignal?that?only?sends?a?new?value?when?its?block
//?returns?YES.
[[RACObserve(self,?username)
????filter:^(NSString?*newName)?{
????????return?[newName?hasPrefix:@"j"];
????}]
????subscribeNext:^(NSString?*newName)?{
????????NSLog(@"%@",?newName);
????}];