ContentView.swift

    // 父层级
    struct ContentView : View {
      @State private var brain: CalculatorBrain = .left("0")
      var body: some View {
    			// ...
    	    CalculatorButtonPad(brain: $brain)
          .padding(.bottom)
      }
    }
    
    // 子层级
    struct CalculatorButtonPad: View {
      @Binding var brain: CalculatorBrain
      // ...
    	 var body: some View {
    	 VStack(spacing: 8) {
          ForEach(pad, id: \.self) { row in
            CalculatorButtonRow(row: row, brain: self.$brain)
          }
    	 }
    }