Flutter Future：构建函数返回null

Question

I want to retrieve data from firebase to future builder.I use this function for that.

Future getAllData() async {
  ReseviorDataModel resModel;
  DatabaseReference ref = FirebaseDatabase.instance.reference();
  DataSnapshot childed =await 
  ref.child("Reservoir/${widget.placeName}").once();
  Map<dynamic, dynamic> values;
  values = childed.value;
  resModel = ReseviorDataModel.customConstrcutor(values["sensor1"], 
  values["sensor2"], values["sensor3"]);
  return resModel;
}

I invoke this function inside my future builder.

  child: FutureBuilder(
         future: getAllData(),
         builder: (context, snapshot) {
         Center(child: Text(snapshot.data));
  }

but it keeps throwing this "A build function returned null." error .i cant figure out what is the problem here

Answer 1

To clear things up here, your builder always has to return a widget to display. You can never return null, as the error message describes.

The problem in this case was that the OP didn't return anything from the builder, so just adding a return worked out fine.

Some things to keep in mind when using FutureBuilder. Always check the snapshot.hasData property and return a UI accordingly. This will prevent cases where the snapshot.data is null causing a new error to be thrown in your Widget taking in the null.

Example:

child: FutureBuilder(
     future: getAllData(),
     builder: (context, snapshot) {
     if(!snapshot.hasData) {
         // show loading while waiting for real data
        return CircularProgressIndicator();
     }

     return Center(child: Text(snapshot.data));
}