Timer.periodic

[K Wilde]

I'm hitting a web endpoint and getting JSON data back for display.  I'm relying on a periodic timer to do updates.  I'd like updates at around 10 seconds.  However, since I'm relying on the timer to trigger the read, I have to wait initially for the timer to cycle.  Is there a way I can trigger when a page 1st opens or a way to change the timer's periodic rate?  Say start at a small rate and change to 10 seconds after the 1st read?  Thanks

[K Wilde]

Well I guess I'll answer my own question:

One thing I found was using a periodic timer and calling it in the Widget build section, caused a new periodic timer to be created each update.  This obviously led to problems with so many calls occurring after a while.  What I ended up doing is creating a bool value called _initialUpdate set to false initially.  On the 1st build, it calls the async funtion _doUpdate which gets the data then the page is built.  After the first go at _doUpdate, the _initialUpdate variable is set to true and a regular timer is called (not periodic) with a callback of _doUpdate.  At the end of _doUpdate, the timer is recreated.  See code attached:

import 'package:flutter/material.dart';
import 'dart:convert';
import 'dart:async';
import 'package:http/http.dart' as http;
import 'data_item_stateful.dart';
import 'page_tag_defs.dart' as tagDefs;
import 'types.dart' as userTypes;
import 'globals.dart' as globals;

int _cnt = 0;
bool _initialUpdate = false;

void main() => runApp(new MyApp());

class MyApp extends StatelessWidget {
  // This widget is the root of your application.

  @override
  Widget build(BuildContext context) {

    return new MaterialApp(
      title: 'Mobile SCADA Test',
      theme: new ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: new MainPage(title: 'Mobile SCADA Test'),
    );
  }
}

class MainPage extends StatefulWidget {
  MainPage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MainPageState createState() => new _MainPageState();

}



class _MainPageState extends State<MainPage> {

  IconData _appBarIcon;
  MaterialColor _appBarColor;
  String _appBarMessage = "";
  TextStyle _appBarMessageStyle;

  _doUpdate() async{
    String url = Uri.encodeFull(globals.mainURL);
    var response = await http.get(url,headers:{"Accept":"application.json"});
    globals.commsStatusCode = response.statusCode;
    if (response.statusCode == 200) {
      // valid...
      globals.commsOK = true;
      Map<String, dynamic> data = json.decode(response.body);
      List  tl = data["tagList"];
      List <userTypes.Tag> tLst = [];
      tagDefs.tagsMainPage.clear();
      double val;
      for (int i = 0;i < tl.length;i++) {
        if (tl[i]["quality"] == "Good"){
          val = tl[i]["value"];
        }else{
          val = 0.0;
        }
        userTypes.Tag tTag = new userTypes.Tag(
            tl[i]["tagPath"], tl[i]["label"], val, tl[i]["units"], tl[i]["inAlarm"]);
        // build the tag list...
        tLst.add(tTag);
      }
      setState(() {
        tagDefs.tagsMainPage = tLst;
      });

    }else{
      // error..
      print(response.statusCode.toString());
      globals.commsOK = false;
      new Timer(new Duration(seconds:10),_doUpdate);
    }

  }

  @override
  Widget build(BuildContext context) {

    // first pass through get data every other time set a timer to trigger the update...
    if (_initialUpdate == false){
      _doUpdate();
      _initialUpdate = true;
    }else{
      new Timer(new Duration(seconds:10),_doUpdate);
    }

    IconData _appBarIcon;
    MaterialColor _appBarColor;
    String _appBarMessage = "";
    TextStyle _appBarMessageStyle;

//    new Timer.periodic(new Duration(seconds:1), _doUpdate);

    if (globals.commsOK){
      _appBarIcon = Icons.check;
      _appBarColor = Colors.blue;
      _appBarMessage = "";
      _appBarMessageStyle = new TextStyle(fontSize: 1.0);

    }else{
      _appBarIcon = Icons.access_alarm;
      _appBarColor = Colors.red;
      _appBarMessage = "Communications error! Code: " + globals.commsStatusCode.toString();
      _appBarMessageStyle = new TextStyle(fontSize: 16.0);
    }

    return new Scaffold(
        appBar: new AppBar(
            title: new Text(widget.title, style: TextStyle(fontSize: 24.0),),
            backgroundColor: _appBarColor,
            leading: new Icon(
              _appBarIcon,
            ),
            bottom: PreferredSize(
              child: new Text(_appBarMessage, style: _appBarMessageStyle
              ),
            )
        ),

        body: new ListView.builder(
            padding: new EdgeInsets.all(8.0),
            itemExtent: 50.0,
            itemCount: tagDefs.tagsMainPage.length,
            itemBuilder: (BuildContext context, int index) {
              return new DataItem(key: new ObjectKey(index.toString()),
                  tag: tagDefs.tagsMainPage[index],
                  color: Colors.blueAccent);
            }
        )
    );
  }
}