Creating a publisher that collects values after a trigger

Question

Is there a way to turn on the collect() after it sees a certain value and then turn it off after it sees another?

I tried using map and filter upstream and that's totally wrong.

Here's my playground

import UIKit
import Foundation
import Combine

var subj = PassthroughSubject<Int, Never>()

for iter in 0...10 {
    DispatchQueue.main.asyncAfter(deadline: .now() + .seconds(iter)) {
        print("send \(iter)")
        subj.send(iter)
    }
}

let queue = DispatchQueue(label: "Q")

let cancellable = subj.collect(.byTime(queue, .seconds(2))).sink(receiveCompletion: {
    print("finish")
    print($0)
}, receiveValue: {
    print($0)
})

/* my print
send 0
send 1
send 2
[0, 1]
send 3
send 4
[2, 3]
send 5 
send 6
[4, 5] // is there a way to only have it dispatch all the values after this point? After it sees a value of 5?
send 7
send 8
[6, 7]
send 9
send 10
[8, 9, 10]

*/

So you want it to print all of `[6, 7, 8, 9, 10]` at the very end, and not `[6, 7]`? — Sweeper, Sep 08 '22 at 01:15
One obvious way to do this is to map into a tuple with a Boolean that says whether we've seen a trigger and filter on the boolean (and map back). — matt, Sep 08 '22 at 01:24

score 1 · Accepted Answer · answered Sep 08 '22 at 13:31

One solution to this would be to employ the drop(untilOutputFrom:) and prefix(untilOutputFrom:) operators.

Consider the playground below.

In the code I create a signal startEmitting that doesn't emit anything until it sees the value 5. Then it emits a "true" value and stops.

Similarly the signal stopEmitting waits until it sees a 9 and when it does it emits a single value and stops.

The combined signal drops every value until startEmitting fires, and using prefix only emits values until stopEmitting fires. The net effect is that the combined signal only starts when a 5 is emitted and stops when 9 is emitted.

import UIKit
import Foundation
import Combine

var subj = PassthroughSubject<Int, Never>()

for iter in 0...10 {
    DispatchQueue.main.asyncAfter(deadline: .now() + .seconds(iter)) {
        print("send \(iter)")
        subj.send(iter)
        
        if iter == 10 {
            subj.send(completion: .finished)
        }
    }
}

let startEmitting = subj
    .print("startEmtting")
    .contains(5)

let stopEmitting = subj
    .print("stopEmitting")
    .contains(9)

let cancellable = subj
    .drop(untilOutputFrom: startEmitting)
    .prefix(untilOutputFrom: stopEmitting)
    .collect()
    .sink {
    print("finish")
    print($0)
} receiveValue: {
    print($0)
}

I have another question that you might be interested in https://stackoverflow.com/questions/73655294/custom-publisher-acting-up-when-upstream-is-subscribed-by-multiple-subscribers — Yogurt, Sep 08 '22 at 21:41

Creating a publisher that collects values after a trigger

1 Answers1