enum SelectItemType {
   case one
   case two
   case three
}
@State var selectType = SelectItemType.three

var body: some View {
   Picker("选择数字", selection: $selectType) {
        Text("一").tag(SelectItemType.one)
        Text("二").tag(SelectItemType.two)
        Text("三").tag(SelectItemType.three)
   }.onReceive([self.selectType].publisher.first()) { value in
       // 选项改变时 回调
       print("value = \(value)")
   }
}