RACSubscriber 协议
RACSubscriber协议方法
@protocol RACSubscriber <NSObject>
@required //必须实现的
1. - (void)sendNext:(nullable id)value; // 向订阅者(subscribers) 发送的值 (value)
2. - (void)sendError:(nullable NSError *)error;
3. - (void)sendCompleted;
4. - (void)didSubscribeWithDisposable:(RACCompoundDisposable *)disposable;
@end
