skip:跳跃前面几个信号值
RACSubject *subject = [RACSubject subject];
// 跳跃几个值
[[subject skip:2] subscribeNext:^(id _Nullable x) {
NSLog(@"%@",x);
}];
[subject sendNext:@"11111"];
[subject sendNext:@"11211"];
[subject sendNext:@"11311"];
[subject sendNext:@"11411"];
打印： 11311 + 11411
