如何在 flutter 的 build 方法中调用 provider 中的方法？

Question

i have a method in the class which extends changeNotifier which gets the data from API. now I want to call this method whenever page is opened in the build method but when I call that method it is repeatedly called because of the notifyListeners method. I want to know how to call method only once.

ReportProvider.dart

class ReportProvider extends ChangeNotifier
{
  static DateFormat dateFormat = new DateFormat('dd-MM-yyyy');
  static DateFormat actualDateFormat = new DateFormat("yyyy-MM-dd");
  String _toDate = dateFormat.format(new DateTime.now());
  String _actualToDate = actualDateFormat.format(new DateTime.now());
  String _actualFromDate = actualDateFormat.format(new DateTime.now().subtract(new Duration(days: 7)));
  String _fromDate = dateFormat.format(new DateTime.now().subtract(new Duration(days: 7)));
  bool _progressStatuc = false;
  bool _chartVisible = true;
  bool _errorVisible = false;
  String _errorMessage;

  String get errorMessage => _errorMessage;
  bool get errorVisible => _errorVisible;
  bool get chartVisible => _chartVisible;
  bool get progressStatus => _progressStatuc;
  String get toDate => _toDate;
  String get fromDate => _fromDate;

  List<PieData> _data = new List();

  List<PieData> get data => _data;

  Future<void> getReportData() async
  {
    Map<String,dynamic> sessiondata = await new Utilities().getSessionData();
    int shopid = sessiondata['shopid'];
    Map<String,String> reportData = new Map();
    reportData['shopid'] = shopid.toString();
    reportData["fromdate"] = _actualFromDate;
    reportData["todate"] = _actualToDate;
    String token = await new Utilities().getToken();

    Map userHeader = {"token": token};
    print(reportData.toString());

    if(await new Utilities().checkInternet())
    {
      try
      {
        http.Response response = await http.post(EndPointUrl.report,body: reportData,headers: userHeader);
        String message = json.decode(response.body)['message'];
          List<ReportData> data = json.decode(response.body)['data'];
          data.forEach((reportData){
            _data.add(new PieData(reportData.menuname,reportData.itemcount));
          });
          notifyListeners();
      }
      catch(error)
      {
        _errorMessage = "Server error";
        notifyListeners();
      }

    }
    else
    {
      _progressStatuc = false;
      _chartVisible = false;
      _errorVisible = true;
      _errorMessage = "No Internet Connection";
      notifyListeners();
    }
  }




}

Report.dart

class Report extends StatefulWidget
{
  @override
  State<StatefulWidget> createState() {
    return ReportState();
  }

}

class ReportState extends State<Report>
{
  @override
  Widget build(BuildContext context) {

    final reportProvider = Provider.of<ReportProvider>(context);

    reportProvider.getReportData();
    //yprint(reportProvider.data.toString());
    if(reportProvider.errorMessage != null &&  reportProvider.errorMessage.contains("Internet"))
    {
      showDialog(
        context: context,
        builder: (BuildContext context){
          return AlertDialog(
            title: Text("Error"),
            content: Text("${reportProvider.errorMessage}"),
            actions: <Widget>[
              FlatButton(
                child: Text("ok"),
                onPressed: (){
                  Navigator.pop(context);
                },
              )
            ],
          );
        });
    }
   return Stack(
     children: <Widget>[
       Column(
         crossAxisAlignment: CrossAxisAlignment.start,
         children: <Widget>[
           Container(
             margin: EdgeInsets.fromLTRB(8, MediaQuery.of(context).size.height*0.05,0, 0),
             child: Text(
               "Report",
               style: TextStyle(fontSize: 28,color: Colors.black),
             ),
           ),
           Row(
             children: <Widget>[
               Expanded(
                 flex: 3,

                  child: Container(
                    margin: EdgeInsets.fromLTRB(8, 8, 0, 0),
                    child: GestureDetector(
                     onTap: (){
                       reportProvider.selectDate(context, "fromdate");
                     },
                     child: Text(
                       "${reportProvider.fromDate}",
                       style: TextStyle(color: Colors.black,fontSize: 16),
                     ),
                 ),
                  ),
               ),
               Expanded(
                 flex: 1,
                  child: Text(
                   "To",
                   style: TextStyle(fontSize: 16,color: Colors.grey),
                 ),
               ),
               Expanded(
                 flex: 3,
                  child: GestureDetector(
                   onTap: (){
                     reportProvider.selectDate(context, "todate");
                   },
                   child: Text(
                     "${reportProvider.toDate}",
                     style: TextStyle(color: Colors.black,fontSize: 16),
                   ),
                 ),
               ),
               Expanded(
                 flex: 1,
                  child: GestureDetector(
                   onTap: (){},
                   child: Icon(
                     Icons.check,
                     color: Theme.of(context).accentColor,
                   ),
                 ),
               )

             ],
           ),
          //  Visibility(
          //    visible: reportProvider.chartVisible,
          //     child: charts.PieChart<PieData>(

          //    ),
          //  ),
           Expanded(
              child: Visibility(
               visible: reportProvider.errorVisible,
               child: Container(
                 alignment: Alignment.center,
                 child: Column(
                   crossAxisAlignment: CrossAxisAlignment.center,
                   mainAxisAlignment: MainAxisAlignment.center,
                   children: <Widget>[
                     SvgPicture.asset('assets/images/undraw_report.svg',width: MediaQuery.of(context).size.width,height: MediaQuery.of(context).size.height*0.40),
                                    Text(
                                      "No Reports are available",
                                      style: TextStyle(color: Colors.black,fontSize: 20),
                                    )
                   ],
                 ),
               ),
             ),
           )

         ],
       ),

     ],
   );
  }

}

Answer 1

As per the documentation , build() method is going to be called every time something changes. If you want calls to be triggered just once, you could use initState() method and add some helpers methods to update the UI. An example can be found here: https://flutter.dev/docs/get-started/flutter-for/android-devs#what-is-the-equivalent-of-runonuithread-in-flutter

Example for async loading from the previous link, special attention to loadData method:

import 'dart:convert';

import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;

void main() {
  runApp(SampleApp());
}

class SampleApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Sample App',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: SampleAppPage(),
    );
  }
}

class SampleAppPage extends StatefulWidget {
  SampleAppPage({Key key}) : super(key: key);

  @override
  _SampleAppPageState createState() => _SampleAppPageState();
}

class _SampleAppPageState extends State<SampleAppPage> {
  List widgets = [];

  @override
  void initState() {
    super.initState();

    loadData();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Sample App"),
      ),
      body: ListView.builder(
        itemCount: widgets.length,
        itemBuilder: (BuildContext context, int position) {
          return getRow(position);
        },
      ),
    );
  }

  Widget getRow(int i) {
    return Padding(
      padding: EdgeInsets.all(10.0),
      child: Text("Row ${widgets[i]["title"]}"),
    );
  }

  Future<void> loadData() async {
    String dataURL = "https://jsonplaceholder.typicode.com/posts";
    http.Response response = await http.get(dataURL);
    setState(() {
      widgets = json.decode(response.body);
    });
  }
}