SwiftUI ObservedObject Binding KeyPath subscript error

Question

I get a compiler error trying to specify Binding on property of @ObservedObject using KeyPath for a generic type T.

I am trying to create a generic SwiftUI View that will work for a class that has several properties of the same type. The SwiftUI View needs to be generic; however I cannot determine how to create a Binding to a property of the main object that I access through a KeyPath. The code below returns an error of "Value of type 'T' has no subscripts"

UPDATE: I identified the error in my code instead of $model[keyPath: kp] I should have used $model[dynamicMember: kp].

import SwiftUI

class TestClass: ObservableObject {
    @Published public var str1: String = "one"
    @Published public var str2: String = "two"
}

struct ContentView: View { 

    @ObservedObject var test = TestClass()
    var body: some View {
        VStack {
            HStack{
                Text("String One").padding(20)
                StringView(model: test, kp: \TestClass.str1).padding(20)
            }
            HStack{
                Text("String Two").padding(20)
                StringView(model: test, kp: \TestClass.str2).padding(20)
            }
        }
    }
}

struct StringView<T>: View where T: ObservableObject {

    @ObservedObject var model: T

    var kp: ReferenceWritableKeyPath<T,String>

    init(model: T, kp: ReferenceWritableKeyPath<T,String>) {
        self.model = model
        self.kp = kp
    }

    var body: some View {
        HStack{
        TextField("String", text: $model[keyPath: kp])   <--- COMPILER ERROR HERE
        Text(model[keyPath: kp])
        }
    }
}

Is it possible to do this?

Muukii · Accepted Answer · 2019-09-18 06:47:06Z

1

I think you can do it with followings. But my idea is a little bit verbosely.

At least, TextField requires Binder. So KeyPath's target Type should be this.

class TestClass: ObservableObject {
  @Published public var str1: String = "one"
  @Published public var str2: String = "two"
}

struct _ContentView: View {

  @ObservedObject var test = TestClass()

  init() {

  }
  var body: some View {
    VStack {
      HStack{
        Text("String One").padding(20)
        StringView(model: test, kp: \ObservedObject<TestClass>.Wrapper.str1).padding(20)
      }
      HStack{
        Text("String Two").padding(20)
        StringView(model: test, kp: \ObservedObject<TestClass>.Wrapper.str2).padding(20)
      }
    }
  }
}

struct StringView<T>: View where T: ObservableObject {

  @ObservedObject var model: T

  var kp: ReferenceWritableKeyPath<ObservedObject<T>.Wrapper, Binding<String>>

  init(model: T, kp: ReferenceWritableKeyPath<ObservedObject<T>.Wrapper, Binding<String>>) {
    self.model = model
    self.kp = kp
  }

  var body: some View {
    HStack{
      TextField("String", text: $model[keyPath: kp])
      Text($model[keyPath: kp].wrappedValue)
    }
  }
}

answered Sep 18, 2019 at 6:47

Muukii

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1 Comment

G J Over a year ago

I updated the original post. I was misread the subscript definition for the ObserveObject. Instead of $model[keyPath: kp] it should be $model[dynamicMember: kp]

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