I'm trying to understand Combine Future

Question

Created Jul ’21

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I'm using playground to experiment with Combine. I found this example on a blog. I expect it to create the Future publisher, wait a couple seconds then send the promise and complete.

import Foundation

import Combine

import SwiftUI

let future = Future<Int, Never> { promise in

    print("Creating")

    DispatchQueue.global().asyncAfter(deadline: .now() + 2) {

        print("sending promise")

        promise(.success(1))

    }
}

future
    .sink(receiveCompletion: { print("receiveCompletion:\($0)") },
            receiveValue: { print("receiveValue:\($0)") })

print("end")

The output I expect:

Creating
end
sending promise
receiveValue: ...
receiveCompletion: ...

The output I get:

Creating
end
sending promise

I don't see an indication the promise was executed. What am I doing wrong?

Answered by OOPer in 683073022

Please try something like this:

import Foundation
import Combine
import SwiftUI

let future = Future<Int, Never> { promise in
    print("Creating")
    DispatchQueue.global().asyncAfter(deadline: .now() + 2) {
        print("sending promise")
        promise(.success(1))
    }
}

let cancellable = future //<-
    .sink(receiveCompletion: { print("receiveCompletion:\($0)") },
            receiveValue: { print("receiveValue:\($0)") })

print("end")

It seems the result of future.sink needs to be held in a strong reference until the operation is completed.

Boost

Answer 1

OOPer OP

Jul ’21

Accepted Answer

Please try something like this:

import Foundation
import Combine
import SwiftUI

let future = Future<Int, Never> { promise in
    print("Creating")
    DispatchQueue.global().asyncAfter(deadline: .now() + 2) {
        print("sending promise")
        promise(.success(1))
    }
}

let cancellable = future //<-
    .sink(receiveCompletion: { print("receiveCompletion:\($0)") },
            receiveValue: { print("receiveValue:\($0)") })

print("end")

It seems the result of future.sink needs to be held in a strong reference until the operation is completed.

0

Answer 2

ronm OP

Jul ’21

That did it. It makes sense that it needs to be held. Thanks!

0