SwiftUI/Combine: subscribe to value change of @Binding

Question

I have a view with a view model, and actions in this view can change the view model. To be able to break out logic into reusable pieces, I have part of the view as its own view, with a @Binding to the values it needs to have. Now, I want to be able to perform some logic based on the value changes, not necessarily only view changes. How can I do that? If it was a regular property, I'd implement a didSet, but that gets me nowhere. I wanted to use Combine to and treat the @Binding as a publisher, but I couldn't find a way to do that either. Suggestions?

Here's the code:

class ViewModel: ObservableObject {
    @Published var counter: Int = 0
}

struct Greeter: View {
    @Binding var counter: Int {
        didSet {
            // this isn't printed....
            print("Did set -> \(counter)")
        }
    }
    
    init(counter: Binding<Int>) {
        self._counter = counter
        
        // ...so how about setting up a subscription to the @Binding counter above here?
    }
    
    var body: some View {
        Text("Hello, world #\(counter)!")
            .padding()
    }
}

struct ContentView: View {
    
    @ObservedObject var viewModel: ViewModel
    
    var body: some View {
        VStack {
            Greeter(counter: $viewModel.counter)
            Button("Go!") {
                viewModel.counter += 1
            }
        }
    }
}

So I want to retain the structure where the data is in a ViewModel, and that only parts of it is being passed down to the subview. And it is in the subview (Greeter) I want to be able to do something (let's say print the value like in the didSet)

Martijn Pieters · Accepted Answer · 2022-08-23 13:41:03Z

27

Here is possible approach. Tested with Xcode 11.4 / iOS 13.4 (SwiftUI 1.0+)

struct Greeter: View {
    @Binding var counter: Int

    var body: some View {
        Text("Hello, world #\(counter)!")
            .padding()
            .onReceive(Just(counter)) {    // << subscribe
                print(">> greeter: \($0)") // << use !!
            }
    }
}

Note for SwiftUI 2.0+: you can use .onChange(of: counter) { instead (which is actually built-in replacement for above)

edited Aug 23, 2022 at 13:41

Martijn Pieters

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answered Aug 31, 2020 at 11:34

Asperi

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6 Comments

niklassaers Over a year ago

Thanks for the suggestion. onChange has the added benefit of only reacting when the value changed, as opposed to onReceive that also fires when the view loads

Logan Over a year ago

took me forever to find this to support ios13, a perfect onChange replacement, thanks :)

Miroslav Kuťák Over a year ago

It just gets called on every refresh of the view, doesn't it?

zdravko zdravkin Over a year ago

Yes it is, is stack in the loop

Orkhan Alikhanov Over a year ago

For me .onReceive(Just(binding)) works but .onChange(of: binding) does not.

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