CODEWITHSUNDEEP
iosswiftcombine
JIE WANG
JIE WANG

Reputation: 1934

`Sink.receiveValue` is not called after `publisher.receive(on: backgroundQueue)`

With the following code, after I put receive(on: backgroundQueue), the receiveCompletion will be called 100%, but the receiveValue block is not.

xxxxPublisher
    .xxxx()
    .receive(on: backgroundQueue)
    .xxxx()
    .receive(on: DispatchQueue.main)
    .sink(receiveCompletion: { completion in
        // completion code
    }, receiveValue: { value in
        // receive value code
    }).store(in: &cancellables)

This seems not a good behavior, are we supposed to not use receive<S>(on scheduler: S, options: S.SchedulerOptions? = nil) in this way? Am I missing something?

Here is the code to reproduce this bug, if you run this code, you will see that receiveCompletion is called exactly 300 times, but receiveValue is called less than 300.

import Foundation
import Combine

private var cancellables: Set<AnyCancellable> = []
let backgroundQueue = DispatchQueue.global(qos: .background)
for i in 1...300 {
    runPublisher(i)
}
var sinkCompletedIndices = Set<Int>()
var sinkOutputIndices = Set<Int>()

func runPublisher(_ index: Int) {
    [1].publisher
    .receive(on: backgroundQueue)
    .receive(on: DispatchQueue.main)
    .sink(receiveCompletion: { completion in
        NSLog("sink receiveCompletion")
        sinkCompletedIndices.insert(index)
    }, receiveValue: { value in
        NSLog("sink receiveValue")
        sinkOutputIndices.insert(index)
    }).store(in: &cancellables)
}
DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + 2) {
    let diff = sinkCompletedIndices.filter { !sinkOutputIndices.contains($0) }
    NSLog("Difference between completions and outputs \(diff)")
}
RunLoop.main.run()

Upvotes: 1

Views: 1968

Answers (1)

LuLuGaGa
LuLuGaGa

Reputation: 14418

The problem is that Combine expect schedulers to operate as serial queues, but DispatchQueue.global is concurrent.

If you declare you background queue like this:

let backgroundQueue = DispatchQueue(label: "any name will do", qos: .background)

Everything will work as expected.

There is an interesting discussion about it on Swift forums:

https://forums.swift.org/t/runloop-main-or-dispatchqueue-main-when-using-combine-scheduler/26635

Upvotes: 8

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