Combine: “Lazy” CurrentValueSubject

This is a technique suggested to me by Jordan Gustafson (@minnesota_gus) on how to make a CurrentValueSubject that doesn’t require an initial value. This should probably be developed into an actual type which conforms to Subject, but I haven’t figured that out yet.

	import Foundation
	import Combine

	class Example {
	private var index = 1
	private let valuePublisher = CurrentValueSubject<Int?, Never>(nil)
	var value: AnyPublisher<Int, Never> {
	valuePublisher.compactMap({ $0 }).eraseToAnyPublisher()
	}

	func doSomething() {
	print("[Example] sending value: \(index)")
	valuePublisher.send(index)
	index += 1
	}
	}

	let example = Example()

	print("First subscription.")
	let subscription1 = example.value.sink { print("sub1: \($0)") }

	example.doSomething()

	print("Second subscription.")
	let subscription2 = example.value.sink { print("sub2: \($0)") }

	example.doSomething()

view raw LazyCurrentValueSubject.swift hosted with ❤ by GitHub

This will produce the following output:

First subscription. [Example] sending value: 1 sub1: 1 Second subscription. sub2: 1 [Example] sending value: 2 sub2: 2 sub1: 2

As you can see, the first subscription did not receive a value until the first time doSomething() was called. Conversely, the second subscription—which was added after the first value had been sent—received a value immediately. Both subscriptions receive all values thereafter.