Combine: Add alongside(with block:(T) -> Void) publisher

Evolution Discussion
#1

Consider following Service that captures data.

class Service {
  var captured: String = ""
  func save(_ captured: String) -> String {
    self.captured = data
    return captured
  }

  func myPublisher() -> AnyPublisher<String, Error> {
    // ... configure publisher ...
    return publisher.map(self.save)
  }
}

Example above doesn't work, because it causes memory leak if I understand map() correctly.

Could we add .alongside publisher which captures [weak self] and behaves exactly as map which captures non-escaping closure:

// kind of..
func alongside<Value, P: Publisher>(value: Value, block: (Value) -> Void) -> P {
   block(value) // waiting
   return value
}

OR

We could add publisher that accepts functions instead like assign:

.alongside(\.save, on: self)
#2

Combine is not a part of the open-source Swift project and is not therefore within the scope of the Swift Evolution process. I think there may be ways to offer suggestions to Apple.

2 Likes
#3

I don't see any memory leak in your example. You return a publisher that retains the Service, but you didn't include any code that makes the Service retain the publisher.

1 Like
#4

Interesting point. Maybe I should rethink current Service model.

However, in outer code it would be used as:

class ViewModel {
  private var service: Service = .init()
  var subscription: AnyCancellable?

  func setup() {
    self.subscription = self.service.myPublisher().sink{ (value) in
      // process value from publisher.
    }    
  }
}
#5

You probably need to use [weak self] in the “process value from publisher” closure, if you use self in that closure.

#6

Do you mean that everything is fine in this example, because view model retains service and also publisher retains service?

But it seems a bit weird, because publisher can't retain service, no?

#7

Why not?

#8

Because Service produces publisher...
Maybe I don't understand common patterns with Combine / Reactive.

So, this implementation allows following tricky code:

// We don't bother about lifetime of service.
// Service's publisher retains service.
self.subscription = Service.init().myPublisher().sink{ 
  [weak self] (value) in
  self?.update(newValue: value)
}
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