Flutter FutureBuilder返回null

Question

当使用FutureBuilder快照列出本地数据库中的数据时，返回null。我没有列出我的数据。当我检查连接状态时，它正在等待。当使用debugprint dbHelper.randevuListesiGetir（）方法时，我的数据监听控制台会滚动，但不会监听futurebuilder。

class _RandevularState extends State<Randevular> {
  DatabaseHelper dbHelper;
  List<Randevu> tumRandevular;
  @override
  void initState() {
    // TODO: implement initState
    super.initState();

      dbHelper=DatabaseHelper();
     tumRandevular=List<Randevu>();
  }

  @override
  Widget build(BuildContext context) {
    return FutureBuilder(
      future:dbHelper.randevuListesiGetir() ,
      builder: (context,AsyncSnapshot<List<Randevu>> snapshot){
        if(snapshot.connectionState==ConnectionState.done){
          tumRandevular=snapshot.data;
          print("data"+snapshot.data.toString());
        }else if(snapshot.connectionState==ConnectionState.waiting){
          print("waiting");
        }else{
          print("error");
        }

      },
    );
  }

}

Answer 1

对于您的问题，您需要先检查快照中是否包含数据，然后再尝试对snapshot.data进行任何操作。最后我有几个技巧。

con = file(file.path("my_texts", "text1"), encoding = "macintosh")
on.exit(close(con)) # Always make sure to close connections!
contents = readLines(con)

• 这是一个构建器方法，您必须从中返回小部件。
• 在import 'package:flutter/material.dart'; class _RandevularState extends State<Randevular> { DatabaseHelper dbHelper; List<Randevu> tumRandevular; Future<List<Randevu>> randevuFuture; @override void initState() { super.initState(); dbHelper = DatabaseHelper(); randevuFuture = dbHelper.randevuListesiGetir(); tumRandevular = List<Randevu>(); } @override Widget build(BuildContext context) { return FutureBuilder( future: randevuFuture, builder: (context, AsyncSnapshot<List<Randevu>> snapshot) { if (snapshot.hasData) { print("Data: ${snapshot.data}"); final randevuList = snapshot.data; return ListView.builder( itemCount: randevuList.length, itemBuilder: (context, index) { final currentRandevu = randevuList[index]; return ListTile( title: Text("${currentRandevu.name}"), ); }); } // Also check for possible errors if (snapshot.hasError) { print("An error occurred"); return Text("An error occurred"); } // You probably want to show loading indicator before you have data return CircularProgressIndicator(); }, ); } } 中进行API调用，而不是将来：部分。由于您在initState中进行了函数调用，因此future:将在每个版本中都进行调用。

Answer 2

databasehelper类中的这些方法

  Future<List<Map<String,dynamic>>> randevuGetir() async {
    var db = await _getDatabase();
    var sonuc = await db.query("randevu");
    print("sonuc:$sonuc");
    return sonuc;
  }

  Future<List<Randevu>> randevuListesiGetir() async {
    var randevuMapListesi = await randevuGetir();
    var randevuListesi = List<Randevu>();
    for(Map map in randevuMapListesi){
      randevuListesi.add(Randevu.fromMap(map));
    }
    print(randevuListesi);
    return randevuListesi;

  }

这些编码我的行列页面

import 'dart:io';
import 'package:flutter/material.dart';
import 'package:kuaforapp/models/musteri.dart';
import 'package:kuaforapp/models/randevu.dart';
import 'package:kuaforapp/utils/database_helper.dart';

class Randevular extends StatefulWidget {
  @override
  _RandevularState createState() => _RandevularState();
}

class _RandevularState extends State<Randevular> {
  List<Randevu> tumRAndevular;
  DatabaseHelper databaseHelper;

  @override
  void initState() {
    // TODO: implement initState
    super.initState();
    tumRAndevular = List<Randevu>();
    databaseHelper = DatabaseHelper();
  }

  @override
  Widget build(BuildContext context) {
    return FutureBuilder(
      future: databaseHelper.randevuListesiGetir(),
      builder: (context, AsyncSnapshot<List<Randevu>> snapshot) {
        if (snapshot.connectionState == ConnectionState.done) {
          tumRAndevular = snapshot.data;
          return ListView.builder(
              itemCount: tumRAndevular.length,
              itemBuilder: (context, index) {
                return ListTile(
                  title: Text(tumRAndevular[index].randevuId.toString()),
                );
              });
        } else {
          return Center(
            child: CircularProgressIndicator(),
          );
        }
      },
    );
  }
}