Cannot convert value of type 'Published.Publisher' to expected argument type 'Binding'

Question

When trying to compile the following code:

class LoginViewModel: ObservableObject, Identifiable {
    @Published var mailAdress: String = ""
    @Published var password: String = ""
    @Published var showRegister = false
    @Published var showPasswordReset = false

    private let applicationStore: ApplicationStore

    init(applicationStore: ApplicationStore) {
        self.applicationStore = applicationStore
    }

    var passwordResetView: some View {
        PasswordResetView(isPresented: $showPasswordReset) // This is where the error happens
    }
}

Where PasswordResetView looks like this:

struct PasswordResetView: View {
    @Binding var isPresented: Bool
    @State var mailAddress: String = ""
    
    var body: some View {
            EmptyView()
        }
    }
}

I get the error compile error

Cannot convert value of type 'Published<Bool>.Publisher' to expected argument type 'Binding<Bool>'

If I use the published variable from outside the LoginViewModel class it just works fine:

struct LoginView: View {
    @ObservedObject var viewModel: LoginViewModel

    init(viewModel: LoginViewModel) {
      self.viewModel = viewModel
    }
    
    var body: some View {
            PasswordResetView(isPresented: self.$viewModel.showPasswordReset)
    }
}

Any suggestions what I am doing wrong here? Any chance I can pass a published variable as a binding from inside the owning class?

Thanks!

Projected values (something started with $) can be different in different contexts. Binding projected value in your second case is generated by `@ObservedObject`, if first case `@Published` generates publisher projected value. The question is what are you trying to do and why do you put View insider view model? — Asperi, Aug 06 '20 at 11:24
I am following the MVVM principles that were described on the Ray Wenderlich Sitze (https://www.raywenderlich.com/4161005-mvvm-with-combine-tutorial-for-ios). There they put all the routing logic inside the viewmodel of a view (including the instanciation and configuration of views + their models). Thats basically what I am trying to do here. — Jan Koch, Aug 06 '20 at 12:06
Better to follow SwiftUI principals, MVVM isn't really suited to SwiftUI which already solves everything. — malhal, Sep 11 '20 at 07:31

score 119 · Answer 1 · answered Feb 12 '22 at 11:51

119

Not sure why the proposed solutions here are so complex, when there is a very direct fix for this.

Found this answer on a similar Reddit question:

The problem is that you are accessing the projected value of an @Published (which is a Publisher) when you should instead be accessing the projected value of an @ObservedObject (which is a Binding), that is: you have globalSettings.$tutorialView where you should have $globalSettings.tutorialView.

answered Feb 12 '22 at 11:51

Big_Chair

2,781
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7

This should be the accepted answer! – obevan Feb 25 '22 at 15:47
2

That makes so much sense I don't know why I didn't think about it. lol – Jayson Mar 14 '22 at 09:37
1

Can't believe this actually worked – Hajji Daoud Mar 27 '22 at 19:05
2

Most people probably have `globalSettings.$tutorialView` because that's what XCode suggests while you're typing. – Parth Mehrotra May 11 '22 at 15:31
Besides other answers giving more ideas, this one is a direct answer to the question asked. – Dren Jul 25 '22 at 14:15
Also don't forget to add `@ObservedObject` to the object (view model) – Oleg Sherman Dec 25 '22 at 10:44
1

You are a god among men – robhasacamera Apr 22 '23 at 21:10

score 12 · Answer 2 · answered Aug 06 '20 at 11:44

12

** Still new to Combine & SwiftUI so not sure if there is better way to approach **

You can initalize Binding from publisher.

https://developer.apple.com/documentation/swiftui/binding/init(get:set:)-6g3d5

let binding = Binding(
    get: { [weak self] in
        (self?.showPasswordReset ?? false)
    },
    set: { [weak self] in
        self?.showPasswordReset = $0
    }
)

PasswordResetView(isPresented: binding)

answered Aug 06 '20 at 11:44

k-thorat

4,873
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2

Thanks. I saw that as well but it looks a bit clunky. But I will try it out now :-) – Jan Koch Aug 06 '20 at 12:07
2

I get Cannot find 'self' in scope – Ahmadreza May 07 '21 at 15:10
I was about to give up on using binding in favor of some closure, but this was exactly what I needed! I had a view model with a published property that it needed to pass as a binding to something else and this did the trick. Thank you! – luismgb Feb 07 '23 at 15:56

score 9 · Answer 3 · answered Apr 25 '21 at 00:13

I think the important thing to understand here is what "$" does in the Combine context.

What "$" does is to publish the changes of the variable "showPasswordReset" where it is being observed.

when it precedes a type, it doesn't represent the type you declared for the variable (Boolean in this case), it represents a publisher, if you want the value of the type, just remove the "$".

"$" is used in the context where a variable was marked as an @ObservedObject, (the ObservableObject here is LoginViewModel and you subscribe to it to listen for changes in its variables market as publishers)

struct ContentView: View {
       @ObservedObject var loginViewModel: LoginViewModel...

in that context (the ContentView for example) the changes of "showPasswordReset" are going to be 'Published' when its value is updated so the view is updated with the new value.

score 4 · Accepted Answer · answered Aug 06 '20 at 17:48

Here is possible approach - the idea to make possible observation in generated view and avoid tight coupling between factory & presenter.

Tested with Xcode 12 / iOS 14 (for older systems some tuning might be needed)

protocol ResetViewModel {
    var showPasswordReset: Bool { get set }
}

struct PasswordResetView<Model: ResetViewModel & ObservableObject>: View {
    @ObservedObject var resetModel: Model

    var body: some View {
        if resetModel.showPasswordReset {
            Text("Show password reset")
        } else {
            Text("Show something else")
        }
    }
}

class LoginViewModel: ObservableObject, Identifiable, ResetViewModel {
    @Published var mailAdress: String = ""
    @Published var password: String = ""
    @Published var showRegister = false
    @Published var showPasswordReset = false

    private let applicationStore: ApplicationStore

    init(applicationStore: ApplicationStore) {
        self.applicationStore = applicationStore
    }

    var passwordResetView: some View {
        PasswordResetView(resetModel: self)
    }
}

That's a very smart idea by just putting the ObservedObject in the view and with that preventing any type issues. I use it to update a progress bar which is monitoring a published progress value in another thread and works perfectly. — B Porr, Feb 16 '21 at 22:31

score -1 · Answer 5 · answered Oct 20 '20 at 15:42

For error that states: "Cannot convert value of type 'Binding' to expected argument type 'Bool'" solution is to use wrappedValue as in example below.

If you have MyObject object with property isEnabled and you need to use that as vanilla Bool type instead of 'Binding' then do this myView.disabled($myObject.isEnabled.wrappedValue)

Cannot convert value of type 'Published.Publisher' to expected argument type 'Binding'

5 Answers5