//创建
RACSubject *subject = [RACSubject subject];
// [subject subscribeNext:^(id _Nullable x) {
// NSLog(@"%@",x);
// }];
[[subject distinctUntilChanged] subscribeNext:^(id _Nullable x) {
NSLog(@"%@",x);
}];
//请求回来
[subject sendNext: @"1"];
[subject sendNext: @"1"];
[subject sendNext: @"1"];
[subject sendNext: @"2"];
打印：只有 1 和 2
