Flutter - JSON and Time Series Charts

问题:

I am trying to display some data in a time-series chart, I found an example "https://google.github.io/charts/flutter/example/time_series_charts/simple.html" but the data is not request from internet. The problem is that how to apply the JSON data into a time-series data. The code of a time-series data as the following:

final data = [
      // How to apply the JSON data in TimeSeriesSales ?
      new TimeSeriesSales(new DateTime(2017, 9, 19), 5),
      new TimeSeriesSales(new DateTime(2017, 9, 26), 25),
      new TimeSeriesSales(new DateTime(2017, 10, 3), 100),
      new TimeSeriesSales(new DateTime(2017, 10, 10), 75),
    ];

There is a completed simple code for reference. Please feel free to comment. Thank you.

import 'dart:async';
import 'dart:convert';
import 'package:http/http.dart' as http;

import 'package:charts_flutter/flutter.dart' as charts;
import 'package:flutter/material.dart';

/// Sample time series data type.
class TimeSeriesSales {
  final DateTime time;
  final int sales;

  TimeSeriesSales(this.time, this.sales);
}

class ItemDetailsPage extends StatefulWidget {
  @override
  _ItemDetailsPageState createState() => new _ItemDetailsPageState();
}

class _ItemDetailsPageState extends State<ItemDetailsPage> {
  String url =
      "https://min-api.cryptocompare.com/data/histoday?fsym=BTC&tsym=USD&limit=1&aggregate=1&allData=true";

  List dataJSON;

  Future<String> getCoinsTimeSeries() async {
    var response = await http
        .get(Uri.encodeFull(url), headers: {"Accept": "application/json"});

    setState(() {
      var extractdata = json.decode(response.body);
      dataJSON = extractdata["Data"];
    });
  }

  @override
  void initState() {
    this.getCoinsTimeSeries();
  }

  @override
  Widget build(BuildContext context) {
    final data = [
      // How to apply the JSON data in TimeSeriesSales ?
      new TimeSeriesSales(new DateTime(2017, 9, 19), 5),
      new TimeSeriesSales(new DateTime(2017, 9, 26), 25),
      new TimeSeriesSales(new DateTime(2017, 10, 3), 100),
      new TimeSeriesSales(new DateTime(2017, 10, 10), 75),
    ];

    var series = [
      new charts.Series<TimeSeriesSales, DateTime>(
        id: 'Sales',
        colorFn: (_, __) => charts.MaterialPalette.blue.shadeDefault,
        domainFn: (TimeSeriesSales sales, _) => sales.time,
        measureFn: (TimeSeriesSales sales, _) => sales.sales,
        data: data,
      )
    ];

    var chart = new charts.TimeSeriesChart(
      series,
      animate: true,
    );

    var chartWidget = new Padding(
      padding: new EdgeInsets.all(32.0),
      child: new SizedBox(
        height: 200.0,
        child: chart,
      ),
    );

    return Scaffold(
      appBar: new AppBar(title: new Text("Details")),
      body: new Center(
        child: new Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            new Text(
              'You have pushed the button this many times:',
            ),
            chartWidget,
          ],
        ),
      ),
    );
  }
}

回答1:

The first thing to do is to refactor TimeSeriesSales so that it makes sense in your application, for example:

class TimeSeriesPrice {
  final DateTime time;
  final double price;
  TimeSeriesSales(this.time, this.price);
}

Next, you need to build data.

List<TimeSeriesPrice> data = [];
// populate data with a list of dates and prices from the json
for (Map m in dataJSON) {
  data.add(TimeSeriesPrice(m['date'], m['price']);
}
var series = ... 

You don't give an example of the json format, so this is a guess. (You are likely to have to parse the json string date into a Dart DateTime.)

回答2:

@Richard Heap, thanks for your prompt reply. According to your advice. I modified the source code as the following.

1. I encountered the NULL pointer of dataJSON. To overcome this problem, applied some exceptional handling.
2. When draw the chart, I encountered that the data(dataJSON) was delayed, I added a dummy list to handle the exception.

Please feel free to comment.

import 'dart:async';
import 'dart:convert';
import 'package:http/http.dart' as http;

import 'package:charts_flutter/flutter.dart' as charts;
import 'package:flutter/material.dart';

// JSON format
// {"Response":"Success","Type":100,"Aggregated":false,"Data":
// [{"time":1279324800,"close":0.04951,"high":0.04951,"low":0.04951,"open":0.04951,"volumefrom":20,"volumeto":0.9902},

/// Time-series data type.
class TimeSeriesPrice {
  final DateTime time;
  final double price;
  TimeSeriesPrice(this.time, this.price);
}

class ItemDetailsPage extends StatefulWidget {
  @override
  _ItemDetailsPageState createState() => new _ItemDetailsPageState();
}

class _ItemDetailsPageState extends State<ItemDetailsPage> {
  String url =
      "https://min-api.cryptocompare.com/data/histoday?fsym=BTC&tsym=USD&limit=1&aggregate=1&allData=true";

  List dataJSON;

  Future<String> getCoinsTimeSeries() async {
    var response = await http
        .get(Uri.encodeFull(url), headers: {"Accept": "application/json"});

    if (this.mounted) {
      this.setState(() {
        var extractdata = json.decode(response.body);
        dataJSON = extractdata["Data"];
      });
    }
  }

  @override
  void initState() {
    this.getCoinsTimeSeries();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: new AppBar(title: new Text("Details")),
      body: chartWidget(),
    );
  }

  Widget chartWidget() {
    List<TimeSeriesPrice> tsdata = [];
    if (dataJSON != null) {
      for (Map m in dataJSON) {
        try {
          tsdata.add(new TimeSeriesPrice(
              new DateTime.fromMillisecondsSinceEpoch(m['time'] * 1000, isUtc: true), m['close']+.0));
        } catch (e) {
          print(e.toString());
        }
      }
    } else {
      // Dummy list to prevent dataJSON = NULL
      tsdata.add(new TimeSeriesPrice(new DateTime.now(), 0.0));
    }

    var series = [
      new charts.Series<TimeSeriesPrice, DateTime>(
        id: 'Price',
        colorFn: (_, __) => charts.MaterialPalette.blue.shadeDefault,
        domainFn: (TimeSeriesPrice coinsPrice, _) => coinsPrice.time,
        measureFn: (TimeSeriesPrice coinsPrice, _) => coinsPrice.price,
        data: tsdata,
      ),
    ];

    var chart = new charts.TimeSeriesChart(
      series,
      animate: true,
    );

    return new Container(
      child: new Column(
        mainAxisAlignment: MainAxisAlignment.center,
        children: <Widget>[
          new Padding(
            padding: new EdgeInsets.all(32.0),
            child: new SizedBox(
              height: 200.0,
              child: chart,
            ),
          ),
        ],
      ),
    );
  }
}