Why are the functions called several times within the FlatMaps?

Question

I have the following code:

func myFunction() -> AnyPublisher<MyObject, MyError> {
   return self.globalPublisher
      // FlatMap 1
      .flatMap { value1 -> AnyPublisher<Object1, Error1 > in 
         return self.function1(value1)
      }
      // FlatMap 2
      .flatMap { value2 -> AnyPublisher<Object2, Error2 > in 
         return self.function1(value2)
      }
      // FlatMap3
      .flatMap { value3 -> AnyPublisher<Object3, Error3 > in 
         return self.function1(value3)
      }
      .eraseToAnyPublisher()
   })
}

myFunction has only one subscriber (checked with debug). globalPublisher can be triggered multiple times and at any time. This triggers the whole flatMap logic.

When the globalPublisher is triggered for the first time, everything is fine: every function in every flatMap block is called once. But the second time happens something strange. The globalPublisher is triggered only once. The function in FlatMap 1 is also triggered only once and returns only one value (checked with debug). But the function in FlatMap 2 is suddenly triggered twice and returns two values. The function in FlatMap 3 is then triggered 6 times.

The same thing happens for the third and further times: globalPublisher and the function in the FlatMap 1 are triggered once and the function1() returns only one value. The rest is triggered several times and the number of triggers is getting bigger and bigger.

Could someone tell me what could be the reason for such strange behavior of FlatMaps? I have already gone through my code several times and debugged it. Actually, everything must work. I suppose it's possible that the global publisher is storing somehow the "subscriptions" of the FlatMaps? But I don't think it works that way. Do you have some ideas?

I suspect that the problem lies in the combination of a global publisher and all the FlatMaps.

Thanks in advance.

Answer 1

I see two options here.

Either

So flatmap has a maxPublishers parameter check here. The parameter is set default to
.unlimited which is controlling how many publishers the method can accept.

That's why your flatmaps are publishing these values unlimited. You need to change the parameter and set it to something like flatMap(maxPublishers: .max(1))

Or
You can use switchToLatest which will always use the most recent provided publisher.