渲染 Text() 时，小部件内的 Flutter If 语句

Question

I was wondering how to use if statements within widgets in Flutter.

The code below prompts the user to choose a date range using the DateRange package. I am trying to receive the date input, and print it using Text(...).

However, the if statement that I am using:

        Text((() {
          if(condition){
            return habitDates[0].toString() + "to " + habitDates[1].toString();}
          return "Pick Date";
        })()),

does not seem to update (it's constantly "Pick Date"), and I assume it is an issue regarding the widget's state.

Below is the full code:

class DateRange extends StatefulWidget {
  @override
  _DateRangeState createState() => _DateRangeState();
}

class _DateRangeState extends State<DateRange> {

  List<DateTime> habitDates = [];

  @override
  Widget build(BuildContext context) {
    return Column(
      children: <Widget>[
        MaterialButton(
        color: Colors.grey[700],
        onPressed: () async {
          final List<DateTime> picked = await DateRagePicker.showDatePicker(
              context: context,
              initialFirstDate: new DateTime.now(),
              initialLastDate: (new DateTime.now()),
              firstDate: new DateTime(2020),
              lastDate: new DateTime(2030),
          );
          if (picked != null && picked.length == 2) {
            picked.forEach((date) {
              habitDates.add(date);
              print(habitDates);
              }
            );
          }
        },
        child: Text("Pick date range", style: TextStyle(color: Colors.white),)
        ),
        Text((() {
          if(habitDates != null && habitDates.length == 2){
            return habitDates[0].toString() + "to " + habitDates[1].toString();}
          return "Pick Date";
        })()),
      ],
    );
  }
}

Answer 1

You can call setState(() {...}); for update your Text Widget. Like this.

if (picked != null && picked.length == 2) {
  setState(() {
    picked.forEach((date) {
      habitDates.add(date);
      print(habitDates);
    });
  });
}