当 SwiftUI 拒绝时使用可选绑定

Question

The problem

TL;DR: A String I'm trying to bind to inside TextField is nested in an Optional type, therefore I cannot do that in a straightforward manner. I've tried various fixes listed below.

I'm a simple man and my use case is rather simple - I want to be able to use TextField to edit my object's name .
The difficulty arises due to the fact that the object might not exist .

The code

Stripping the code bare, the code looks like this.
Please note that that the example View does not take Optional into account

model

struct Foo {
  var name: String
}

extension Foo {
  var sampleData: [Foo] = [
    Foo(name: "Bar")
  ]
}

view

again, in the perfect world without Optionals it would look like this

struct Ashwagandha: View {
  @StateObject var ashwagandhaVM = AshwagandhaVM()
  var body: some View {
    TextField("", text: $ashwagandhaVM.currentFoo.name)
  }
}

view model

I'm purposely not unwrapping the optional, making the currentFoo: Foo?

class AshwagandhaVM: ObservableObject {
  @Published var currentFoo: Foo?

  init() {
    self.currentFoo = Foo.sampleData.first
  }
}

The trial and error

Below are the futile undertakings to make the TextField and Foo.name friends, with associated errors.

Optional chaining

The 'Xcode fix' way

TextField("", text: $ashwagandhaVM.currentFoo?.name)
gets into the cycle of fixes on adding/removing "?"/"!"

The desperate way

TextField("Change chatBot's name", text: $(ashwagandhaVM.currentFoo..name)
"'$' is not an identifier; use backticks to escape it"

Forced unwrapping

The dumb way

TextField("", text: $ashwagandhaVM.currentFoo..name)
"Cannot force unwrap value of non-optional type 'Binding<Foo?>'"

The smarter way

if let asparagus = ashwagandhaVM.currentFoo.name {
  TextField("", text: $asparagus.name)
}

"Cannot find $asparagus in scope"

Workarounds

My new favorite quote's way

No luck, as the String is nested inside an Optional ; I just don't think there should be so much hassle with editing a String .

The rationale behind it all

ie why this question might be irrelevant

I'm re-learning about the usage of MVVM, especially how to work with nested data types. I want to check how far I can get without writing an extra CRUD layer for every property in every ViewModel in my app. If you know any better way to achieve this, hit me up.

Answer 1

Folks in the question comments are giving good advice. Don't do this: change your view model to provide a non-optional property to bind instead.

But... maybe you're stuck with an optional property, and for some reason you just need to bind to it. In that case, you can create a Binding and unwrap by hand:

class MyModel: ObservableObject {
    @Published var name: String? = nil
    
    var nameBinding: Binding<String> {
        Binding {
            self.name ?? "some default value"
        } set: {
            self.name = $0
        }
    }
}

struct AnOptionalBindingView: View {
    @StateObject var model = MyModel()
    
    var body: some View {
        TextField("Name", text: model.nameBinding)
    }
}

That will let you bind to the text field. If the backing property is nil it will supply a default value. If the backing property changes, the view will re-render (as long as it's a @Published property of your @StateObject or @ObservedObject ).

Answer 2

I think you should change approach, the control of saving should remain inside the model, in the view you should catch just the new name and intercept the save button coming from the user:

class AshwagandhaVM: ObservableObject {
    @Published var currentFoo: Foo?

    init() {
        self.currentFoo = Foo.sampleData.first
    }
    func saveCurrentName(_ name: String) {
        if currentFoo == nil {
            Foo.sampleData.append(Foo(name: name))
            self.currentFoo = Foo.sampleData.first(where: {$0.name == name})
        }
        else {
            self.currentFoo?.name = name
        }
    }
}

struct ContentView: View {
    @StateObject var ashwagandhaVM = AshwagandhaVM()
    @State private var textInput = ""
    @State private var showingConfirmation = false
    
    var body: some View {
        VStack {
            TextField("", text: $textInput)
                .padding()
                .textFieldStyle(.roundedBorder)
            Button("save") {
                showingConfirmation = true
            }
            .padding()
            .buttonStyle(.bordered)
            .controlSize(.large)
            .tint(.green)
            .confirmationDialog("are you sure?", isPresented: $showingConfirmation, titleVisibility: .visible) {
                Button("Yes") {
                    confirmAndSave()
                }
                Button("No", role: .cancel) { }
            }
            //just to check
            if let name = ashwagandhaVM.currentFoo?.name {
                Text("in model: \(name)")
                    .font(.largeTitle)
            }
        }
        .onAppear() {
            textInput = ashwagandhaVM.currentFoo?.name ?? "default"
        }
    }
    
    func confirmAndSave() {
        ashwagandhaVM.saveCurrentName(textInput)
    }
}

UPDATE

do it with whole struct

struct ContentView: View {
    @StateObject var ashwagandhaVM = AshwagandhaVM()
    @State private var modelInput = Foo(name: "input")
    @State private var showingConfirmation = false
    
    var body: some View {
        VStack {
            TextField("", text: $modelInput.name)
                .padding()
                .textFieldStyle(.roundedBorder)
            Button("save") {
                showingConfirmation = true
            }
            .padding()
            .buttonStyle(.bordered)
            .controlSize(.large)
            .tint(.green)
            .confirmationDialog("are you sure?", isPresented: $showingConfirmation, titleVisibility: .visible) {
                Button("Yes") {
                    confirmAndSave()
                }
                Button("No", role: .cancel) { }
            }
            //just to check
            if let name = ashwagandhaVM.currentFoo?.name {
                Text("in model: \(name)")
                    .font(.largeTitle)
            }
        }
        .onAppear() {
            modelInput = ashwagandhaVM.currentFoo ?? Foo(name: "input")
        }
    }
    
    func confirmAndSave() {
        ashwagandhaVM.saveCurrentName(modelInput.name)
    }
}

Answer 3

struct ContentView: View {
   @StateObject var store = Store()

   var body: some View {
    if let nonOptionalStructBinding = Binding($store.optionalStruct)
        TextField("", text: nonOptionalStructBinding.name)
    }
    else {
        Text("optionalStruct does not exist")
    }
  }
}

Also, MVVM in SwiftUI is a bad idea because the View data struct is better than a view model object.

Answer 4

I've written a couple nice generic optional Binding helpers to address cases like this. See this thread .