title: Textfield not updating
body: <p>the following code loads fine, the text and the double field both show 300. however when I click submit the text only changes to 0. you will see in the print output that the init was called with the new value of 0 but it does not update the double field? any suggestions?</p>
  
  <pre><code>import SwiftUI
  
  struct ContentView: View {
      @State var amount:Double = 300
      var body: some View {
          VStack {
              Text("\(self.amount)")
              DoubleField(value: $amount)
              Button(action: {self.amount=0}) {Text("Submit")}
          }
          .padding()
          .frame(maxWidth: .infinity, maxHeight: .infinity)
      }
  }
  
  struct DoubleField:View {
  
      @Binding var doubleValue:Double
      @State var stringValue:String
  
      init(value: Binding&lt;Double&gt;) {
          print("DoubleField.init(value: \(value.wrappedValue))")
          self._doubleValue = value
          self._stringValue = State(initialValue: "\(value.wrappedValue)")
      }
  
      var body: some View {
          TextField("0.00", text: $stringValue)
      }
  
  }
  </code></pre>