SwiftUI ViewModel published property and binding

Question

My question is probably the result of a misunderstanding but I can't figure it out, so here it is:

When using a component like a TextField or any other component requiring a binding as input

TextField(title: StringProtocol, text: Binding<String>)

And a View with a ViewModel, I naturally thought that I could simply pass my ViewModel @Published properties as binding :

class MyViewModel: ObservableObject { 
     @Published var title: String
     @Published var text: String
}

// Now in my view
var body: some View {
    TextField(title: myViewModel.title, text: myViewModel.$text)
}

But I obviously can't since the publisher cannot act as binding. From my understanding, only a @State property can act like that but shouldn't all the @State properties live only in the View and not in the view model? Or could I do something like that :

class MyViewModel: ObservableObject { 
     @Published var title: String
     @State var text: String
}

And if I can't, how can I transfer the information to my ViewModel when my text is updated?

sElanthiraiyan · Accepted Answer · 2023-01-19 04:26:28Z

86

You were almost there. You just have to replace myViewModel.$text with $myViewModel.text.

class MyViewModel: ObservableObject {
    
    var title: String = "SwiftUI"
    
    @Published var text: String = ""
}

struct TextFieldView: View {
    
    @StateObject var myViewModel: MyViewModel = MyViewModel()
    
    var body: some View {
        TextField(myViewModel.title, text: $myViewModel.text)
    }
}

TextField expects a Binding (for text parameter) and StateObject property wrapper takes care of creating bindings to MyViewModel's properties using dynamic member lookup.

edited Jan 19, 2023 at 4:26

answered Apr 28, 2020 at 3:11

sElanthiraiyan

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3 Comments

MegaManX Over a year ago

Can you elaborate how this works? How is $myViewModel.text able to satisfy @Binding?

adri567 Over a year ago

@MegaManX You actually have 4 different spellings. First, you have $myViewModel which is is the whole struct as an ObservedObject. The second one is the myViewModel.text, which is simply a normal string to display on a view. For the third one, we have a binding. Since we want to bind our text to a textField we are using $myViewModel.text. The fourth one is myViewModel.$text. We are using this if we want the published values that this variable can give us. We could subscribe to this with combine.

Daniel Over a year ago

Shouldn't it be @StateObject, not @ObservedObject since you are instantiating it inside of the view?

Robert Dresler · Accepted Answer · 2025-02-18 09:43:35Z

2

If you want to change the text by calling a method, you can create a Binding yourself and then use it in a View.

var body: some View {
    TextField(title: ..., text: text)
}

private var text: Binding<String> {
    Binding(
        get: { myViewModel.text },
        set: { myViewModel.setText($0) }
    )
}

edited Feb 18 at 9:43

answered Jan 30 at 17:31

Robert Dresler

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1 Comment

CouchDeveloper Apr 28 at 8:30

This approach is unidirectional - and it is clearly superior to "two-way-bindings" as demonstrated in the question. It's more like the MVI pattern. A unidirectional approach preserves encapsulation of the mutable state in the view model. Also, due to the need of the view model to observe the Two-way-bindings itself - actually reacting on the view's changes - it can quickly lead to unnecessary complicated code in the view model.

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