Swift Combine sink called once at setup?-CodePudding

I am setting up a sink like so:

    name.publisher
        .removeDuplicates()
        .receive(on: RunLoop.main)
        .sink { [weak self] config in
            guard let self = self else { return }

            // this closure gets called right away at setup even though the @Published property `name` was already setup and did not change
            
        }.store(in: &subscribers)

The property is declared like so in an observable object:

 @Published var name:String = ""

So, I'm obviously missing something here. Why is sink called once at setup even though name did not change? I can avoid this behavior by using the dropFirst() operator but, I'd like to understand why the closure is always called once immediately after setup?

Why is that?

CodePudding user response：

Here's a playground that uses debugPrint to show you what you get from name.publisher:

import UIKit
import Combine

//Holds the score
class ViewModel : ObservableObject {
    @Published var name = "0"
}

let viewModel = ViewModel()
debugPrint(viewModel.name.publisher)

What you get is

Combine.Publishers.Sequence<Swift.String, Swift.Never>(sequence: "0")

So you get a sequence publisher that has a single item, "0". It will publish that value once and the sequence will end. Each time a subscriber attaches to that sequence it will get all the items in the sequence (there's only one) and the end.

This is probably not what you want.

Instead I think you want to use $name to access the published property:

import UIKit
import Combine

//Holds the score
class ViewModel : ObservableObject {
    @Published var name = "0"
}

let viewModel = ViewModel()
let subscription = viewModel.$name.sink { print($0) }
viewModel.name = "Alex"

When you subscribe to the published property you will still get a posted event that is the current value of the property. However, by using $name you are attaching to a stream that will send you the current value on subscription AND each subsequent value.