Question

对于以下代码，我收到以下错误。 https://github.com/Avi-nash5/CP-flutter-app/blob/master/lib/PlatformScreen.dart 错误屏幕显示1秒钟，然后按预期方式加载。请帮忙。 error how it is supposed to be

Answer 1

您可以使用完整代码复制粘贴PlatformScreen
使用
时data尚未准备好如果_isLoading尚未准备就绪，请显示data

来控制CircularProgressIndicator

bool _isLoading = true;
...
Future<String> getJsonData() async {
    ...
    setState(() {
      _isLoading = false;
    });
... 
child: _isLoading
      ? Center(child: CircularProgressIndicator(backgroundColor: Colors.white,))
      : ListView.builder(

工作演示

PlatformScreen的完整代码

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

class PlatformScreen extends StatefulWidget {
  final Platform platform;

  PlatformScreen({this.platform});

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

class PlatformScreenState extends State<PlatformScreen> {
  List data;
  int n;
  bool _isLoading = true;

  @override
  void initState() {
    super.initState();
    WidgetsBinding.instance.addPostFrameCallback((_){
      getJsonData();
    });
  }

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

    data = jsonDecode(response.body);

    n = data.length;
    print(data);
    convert();
    setState(() {
      _isLoading = false;
    });
    return "Success";
  }

  void convert() {
    print(n);
    var now = new DateTime.now();
    now = now.toLocal();
    for (int i = 0; i < n; i++) {
      var start = DateTime.parse(data[i]['start_time']);
      var end = DateTime.parse(data[i]['end_time']);
      start = start.toLocal();
      end = end.toLocal();
      data[i]['start_time'] = start.toString();
      data[i]['end_time'] = end.toString();
    }
    print(data);
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Theme.of(context).accentColor,
      body: Column(
        children: <Widget>[
          Expanded(
            child: Container(
              child: Column(
                children: <Widget>[
                  Padding(
                    padding: const EdgeInsets.only(
                      top: 25.0,
                    ),
                    child: Container(
                      margin: EdgeInsets.all(10.0),
                      height: 125,
                      decoration: BoxDecoration(
                        color: Theme.of(context).primaryColor,
                        borderRadius: BorderRadius.circular(30.0),
                      ),
                      child: Stack(
                        children: <Widget>[
                          Hero(
                            tag: widget.platform.imgurl,
                            child: ClipRRect(
                              borderRadius: BorderRadius.circular(30.0),
                              child: Image(
                                height: 125,
                                width: 125,
                                image: AssetImage(widget.platform.imgurl),
                                fit: BoxFit.cover,
                              ),
                            ),
                          ),
                          Padding(
                            padding: const EdgeInsets.fromLTRB(125, 0, 0, 0),
                            child: Container(
                              child: Center(
                                child: Text(
                                  widget.platform.name,
                                  style: TextStyle(
                                    color: Colors.white,
                                    fontSize: 32,
                                  ),
                                ),
                              ),
                            ),
                          ),
                        ],
                      ),
                    ),
                  ),
                  Expanded(
                    child: Container(
                      decoration: BoxDecoration(
                        color: Theme.of(context).accentColor,
                      ),
                      child: _isLoading
                          ? Center(child: CircularProgressIndicator(backgroundColor: Colors.white,))
                          : ListView.builder(
                              scrollDirection: Axis.vertical,
                              itemCount: n,
                              itemBuilder: (BuildContext context, int index) {
                                return Padding(
                                  padding: const EdgeInsets.fromLTRB(
                                      15.0, 15.0, 10.0, 10.0),
                                  child: Container(
                                    height: 150,
                                    decoration: BoxDecoration(
                                        color: Theme.of(context).accentColor,
                                        borderRadius:
                                            BorderRadius.circular(20.0),
                                        boxShadow: [
                                          BoxShadow(
                                            color: Colors.black54,
                                            offset: Offset(0, 10.0),
                                            blurRadius: 10.0,
                                          )
                                        ]),
                                    child: Column(
                                      children: <Widget>[
                                        Center(
                                          child: Text(
                                            data[index]['name'],
                                            style: TextStyle(
                                              color: Colors.white,
                                              fontSize: 18,
                                            ),
                                          ),
                                        ),
                                      ],
                                    ),
                                  ),
                                );
                              }),
                    ),
                  ),
                ],
              ),
            ),
          ),
        ],
      ),
    );
  }
}

Flutter：方法'[]'在null上被调用

1 个答案: