flatMap(.latest) and delayed inner SignalProducer

[Timotej Papler]

Hello!

The following:

SignalProducer<Int, Never>([ 1, 2, 3, 4 ])
    .flatMap(.latest) { i in
        SignalProducer(value: i)
            .delay(1, on: QueueScheduler.main)
}
    .startWithValues { value in
        print("A", value)
}

Prints nothing 🤔

vs

SignalProducer<Int, Never>([ 1, 2, 3, 4 ])
    .flatMap(.latest) { i in
        SignalProducer.empty
            .delay(1, on: QueueScheduler.main)
            .then(SignalProducer(value: i))
}
    .startWithValues { value in
        print("B", value)
}

prints B 4 🎉

Why is this the case?

[Matt Diephouse]

If you run those exact examples, you won't see the values from the queue scheduler because:

1. You're not waiting for the value to get send on the main queue
2. You may be blocking the main queue

The second example will work because the empty completes immediately, which doesn't require a delay and lets the then run immediately.

[Timotej Papler]

I made command line tool app with main.swift containing:

import Foundation
import ReactiveSwift

let scheduler = QueueScheduler(qos: .userInitiated)
SignalProducer<Int, Never>([ 1, 2, 3, 4 ])
	.flatMap(.latest) { i in
		SignalProducer(value: i).delay(1, on: scheduler)
	}
	.logEvents()
	.startWithCompleted {
		exit(1)
	}

dispatchMain()

in console i get:

[] starting fileName: /Users/tim/development/Lake2/Carthage/Checkouts/ReactiveSwift/test/main.swift, functionName: test, lineNumber: 17
[] started fileName: /Users/tim/development/Lake2/Carthage/Checkouts/ReactiveSwift/test/main.swift, functionName: test, lineNumber: 17
[] completed fileName: /Users/tim/development/Lake2/Carthage/Checkouts/ReactiveSwift/test/main.swift, functionName: test, lineNumber: 17
[] terminated fileName: /Users/tim/development/Lake2/Carthage/Checkouts/ReactiveSwift/test/main.swift, functionName: test, lineNumber: 17

This prints no value, but does complete. I don't think I am blocking scheduler and I am waiting for values. Sorry for insisting with this 🙈

[Matt Diephouse]

That does seem like it should work. 🤔

[Rui Peres]

Hello. 👋 Thanks for opening this issue. Due to inactivity, we will soft close the issue. If you feel that it should remain open, please let us know. 😄