Playground showing the difference between a `PassthroughSubject` and a `CurrentValueSubject`

Combine-PassthroughSubject-CurrentValueSubject.swift

//
//  Combine-PassthroughSubject-CurrentValueSubject.playground
//
//  Created by Felix Mau on 30.07.20.
//  Copyright © 2020 Felix Mau. All rights reserved.
//

import PlaygroundSupport
import Combine

var subscriptions = Set<AnyCancellable>()

// MARK: - PassthroughSubject

/// After subscribing to a `PassthroughSubject` you'll get all the **upcoming elements**.
let passthroughSubject = PassthroughSubject<String, Never>()

passthroughSubject
  .sink(receiveValue: { value in
    print("1st. Subscriber:", value)
  })
  .store(in: &subscriptions)

passthroughSubject.send("One")
passthroughSubject.send("Two")
passthroughSubject.send("Three")

passthroughSubject
  .sink(receiveValue: { value in
    print("2nd. Subscriber:", value)
  })
  .store(in: &subscriptions)

passthroughSubject.send("Four")
passthroughSubject.send("Five")

print("\n", "-- -- -- -- -- -- --", "\n")


// MARK: - CurrentValueSubject

/// After subscribing to a `PassthroughSubject` you'll receive the **most recent element**, as well as all the **upcoming elements**.
/// Therefore a `CurrentValueSubject` needs an initial value!
let currentValueSubject = CurrentValueSubject<String, Never>("Zero (initial value)")

currentValueSubject
  .sink(receiveValue: { value in
    print("1st. Subscriber:", value)
  })
  .store(in: &subscriptions)

currentValueSubject.send("One")
currentValueSubject.send("Two")
currentValueSubject.send("Three")

currentValueSubject
  .sink(receiveValue: { value in
    print("2nd. Subscriber:", value)
  })
  .store(in: &subscriptions)

currentValueSubject.send("Four")
currentValueSubject.send("Five")

The playground will generate the following output:

1st. Subscriber: One
1st. Subscriber: Two
1st. Subscriber: Three
1st. Subscriber: Four
2nd. Subscriber: Four
1st. Subscriber: Five
2nd. Subscriber: Five

 -- -- -- -- -- -- --

1st. Subscriber: Zero (initial value)
1st. Subscriber: One
1st. Subscriber: Two
1st. Subscriber: Three
2nd. Subscriber: Three
1st. Subscriber: Four
2nd. Subscriber: Four
1st. Subscriber: Five
2nd. Subscriber: Five

The second subscriber to the PassthroughSubject will only receive "Four" and "Five" (all upcoming values after the subscription).

The second subscriber to the CurrentValueSubject will receive "Three", "Four" and "Five" (the current value and all upcoming values of the subscription).