'Merge3' requires that 'Binding<Double>' conform to 'Publisher'

Question

Minimal Code Used

import SwiftUI
import Combine

struct ContentView: View {
    @State private var value1: Double = 0
    @State private var value2: Double = 0
    @State private var value3: Double = 0

    var body: some View {
        VStack {
            // Your Test UI
        }
        .onReceive(calculateOnChange()) { _ in
            // ...
        }
    }

    private func calculateOnChange() -> AnyPublisher<Void, Never> {
        Publishers.Merge3($value1, $value2, $value3)
           .map { _ in () }
           .eraseToAnyPublisher()
    }
}

The error is

Generic struct 'Merge3' requires that 'Binding' conform to 'Publisher'

I don't understand why I am getting this error. Any help would be appreciated

Answer 1

Let's find out what type $value1 is:

import Combine
import SwiftUI

struct ContentView: View {
    @State private var value1: Double = 0

    var body: some View {
        VStack {
            // Your Test UI
        }
    }

    func printType() {
        print("The type is \(type(of: $value1))")
    }
}

let content = ContentView()
content.printType()

Running this yields

The type is Binding<Double>

So $value1 is not a Publisher, it's a Binding.

If you want to do something similar with publishers you could write it like this:

import SwiftUI
import Combine
import PlaygroundSupport

struct ContentView: View {
    @State var value1 = CurrentValueSubject<Double, Never>(0)
    @State var value2 = CurrentValueSubject<Double, Never>(0)
    @State var value3 = CurrentValueSubject<Double, Never>(0)

    var body: some View {
        VStack {
            Text("Hello")
        }
        .onReceive(calculateOnChange()) { value in
            print("The new value is \(value) ")
        }
    }

    private func calculateOnChange() -> AnyPublisher<Double, Never> {
        Publishers.Merge3(value1, value2, value3)
            .map { value in
                return value
            }
            .eraseToAnyPublisher()
    }
}

let view = ContentView()
PlaygroundSupport.PlaygroundPage.current.liveView = NSHostingView(rootView: view)

view.value1.value = 3
view.value2.value = 4
view.value3.value = 5