Combine define scheduler for *first* publisher

Using Swift
#1

Hi,

I know there is the receiveOn call for defining the scheduler for all downstream publishers, but how can I tell combine on wich scheduler to execute the first publisher? Example:

Say I want to read a file from a background thread. Since there is no special publisher for that, I use a Future:

extension Data {
    
    static func read(fileAt url: URL) -> Future<Data, Error> {
        return Future { promise in
            do {
                let data = try Data(contentsOf: url)
                promise(Result<Data, Error>.success(data))
            } catch (let error) {
                promise(Result<Data, Error>.failure(error))
            }
        }
    }
    
}

Now I can use this as a publisher of the data at that file url, and its easy to execute the downstream on another thread:

let disposal = Data.read(fileAt: myFileUrl)
            .receive(on: myBackgroundthread)
            .decode(type: MyType.self, decoder: JSONDecoder())
            .sink(...)

But how can I tell combine on wich scheduler to perform the first publisher? I know I could run the promise in the future on another queue, but I wonder if there is a way doing it with the combine api.

Greetings,
Johannes

#2

Hello Johannes Auer,

The operator that you are looking for is subscribe(on:options:). This changes the execution context of the upstream messages.

Data.read(fileAt: myFileUrl)
        .subscribe(on: globalScheduler)
        .receive(on: myBackgroundthread)
        .decode(type: MyType.self, decoder: JSONDecoder())
        .sink(...)

Couple of side notes

  1. Future is an eager publisher. This means, the closure within Future will get fired when read(fileAt url: URL) is called instead of when attaching a subscriber. You can mitigate this by wrapping Future within a Deferred Publisher.

  2. func read(fileAt url: URL) -> Future<Data, Error>. I would suggest erasing to AnyPublisher so to hide the implementation detail (nit).

#3

Hi Roshan,

thanks for your answer and suggestions. As you said, Future executes its closure immediately, so the code above only works when the Future is wrapped in a Deferred publisher. Seems like a lot of code for such a simple thing. This wrapper could be handy:

func Publish<S, Output, Failure>(on scheduler: S, value: @escaping () -> Result<Output, Failure>) -> AnyPublisher<Output, Failure> where S: Scheduler {
    return Future { promise in
        scheduler.schedule {
            let result = value()
            promise(result)
        }
    }.eraseToAnyPublisher()
}

Greetings,
Johannes

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