ReactiveCocoa zip is not working as expected - objective-c

Question

I want to process data stream with ReactiveCocoa, what I want to do is, I want to calculate average value of the data stream for a period, then use the value to subtract with the average value. The flow char would look like this
Data source -> extract value ----> average---> zip
\ /
\------------/
So you can the extract value signal generates average signal, it also sends to zip to combine with average signal's result.
The code to demonstrate is here
__block id<RACSubscriber> sourceSubscriber;
RACSignal *sourceSignal = [RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
sourceSubscriber = subscriber;
return nil;
}];
RACSignal *extractValueSignal = [sourceSignal map:^id(id value) {
NSLog(#"extractValueSignal %#", value);
return value;
}];
RACSignal *avgSignal = [extractValueSignal scanWithStart:[NSMutableArray array] reduce:^NSMutableArray *(NSMutableArray *array, id next) {
NSLog(#"avgSignal %#", next);
[array addObject:next];
if (array.count > 5) {
[array removeObjectsInRange:NSMakeRange(0, array.count - 5)];
}
return array;
}];
[[RACSignal zip:#[extractValueSignal, avgSignal] reduce:^id(NSNumber *value, NSNumber *avg) {
NSLog(#"zip %#, %#", value, avg);
return value;
}] subscribeNext:^(id x) {
NSLog(#"output %#", x);
}];
[sourceSubscriber sendNext:#1];
[sourceSubscriber sendNext:#2];
[sourceSubscriber sendNext:#3];
[sourceSubscriber sendNext:#4];
[sourceSubscriber sendNext:#5];
The output is
extractValueSignal 1
avgSignal 1
extractValueSignal 2
avgSignal 2
extractValueSignal 3
avgSignal 3
extractValueSignal 4
avgSignal 4
extractValueSignal 5
avgSignal 5
The problem I have here is the zip block never get called, nor the subscribeNext block. I wonder why it doesn't work? Shouldn't I to have one signal in the flow for twice or what? Should I use something like tee or what for extract value signal?

Answer 1

Okay, answering my own question, I noticed that, if you add a NSLog in createSignal block
RACSignal *sourceSignal = [RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
NSLog(#"SUB %#", subscriber);
sourceSubscriber = subscriber;
return nil;
}];
You will notice that it get called twice, as down stream subscribes to it twice, so the subscriber was replaced, so only one side of the signal get sent with next event.
To solve that problem, it's fairly easy, just create a multicast connection
RACMulticastConnection *conn = extractValueSignal.publish;
[conn connect];
then you can subscribe to
conn.signal
as much as you like, it will only call the block once.