断言失败'url！= null'：不是真的错误

Question

我还是 Flutter 的新手，正在学习构建应用程序。我一直在尝试运行此代码，但每次运行它时，我都会在 android 模拟器上看到一个红屏，并出现一个错误，即“断言失败：第 22 行 pos 14 'url != null'：不正确”。

这是我运行应用程序的主文件

import "package:flutter/material.dart";
import "src/app.dart";

voidmain() {
  runApp(MyApp());
}

我的应用文件有这个代码

import 'package:flutter/material.dart';
import 'package:http/http.dart' show get;
import 'models/image_models.dart';
import 'dart:convert';
import 'widgets/image_list.dart';

class MyApp extends StatefulWidget {
  createState() {
    return AppState();
  }
}

class AppState extends State<MyApp> {
  int counter = 0;
  List<ImageModels> images = [];

  void fetchImage() async {
    counter++;
    var response =
        await get('http://jsonplaceholder.typicode.com/photos/$counter');
    var imagemodel = ImageModels.fromjson(json.decode(response.body));

    setState(() {
      images.add(imagemodel);
    });
  }

  Widget build(context) {
    return MaterialApp(
        home: Scaffold(
      body: ImageList(images),
      floatingActionButton: FloatingActionButton(
          child: Icon(Icons.add),
          onPressed: () {
            fetchImage();
          }),
      appBar: AppBar(title: Text("Ola Amigo!")),
    ));
  }
}

这是 ImageModel 类

class ImageModels {
  int id;
  String url;
  String title;

  ImageModels(this.id, this.url, this.title);

  ImageModels.fromjson(Map<String, dynamic> parsedjson) {
    id = parsedjson[id];
    title = parsedjson[title];
    url = parsedjson[url];
  }
}

最后是 ImageList

import 'package:flutter/material.dart';
import '../models/image_models.dart';

class ImageList extends StatelessWidget {
  final List<ImageModels> image;

  ImageList(this.image);

  Widget build(context) {
    return ListView.builder(
        itemCount: image.length,
        itemBuilder: (context, int index) {
          return buildImage(image[index]);
        }); 
  }

  Widget buildImage(ImageModels image) {
    return Container(
      decoration: BoxDecoration(
        border: Border.all(color: Colors.grey)
      ),
      padding: EdgeInsets.all(20.0),
      margin: EdgeInsets.all(20.0),
      child: Image.network(image.url),
    );
  }
}

有人可以看看它并告诉我我做错了什么以及如何解决这个问题。 谢谢

Answer 1

您可以在下面复制粘贴运行完整代码
ImageModels有错误，请修改
代码sn-p

factory ImageModels.fromjson(Map<String, dynamic> parsedjson) => ImageModels(
        parsedjson["id"],
        parsedjson["url"],
        parsedjson["title"],
      );

工作演示

完整代码

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

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

class MyApp extends StatefulWidget {
  createState() {
    return AppState();
  }
}

class AppState extends State<MyApp> {
  int counter = 0;
  List<ImageModels> images = [];

  void fetchImage() async {
    counter++;
    var response =
        await get('https://jsonplaceholder.typicode.com/photos/$counter');
    print(response.body);
    var imagemodel = ImageModels.fromjson(json.decode(response.body));
    print(imagemodel.url);
    setState(() {
      images.add(imagemodel);
      print(images[0].url);
    });
  }

  Widget build(context) {
    return MaterialApp(
        home: Scaffold(
      body: ImageList(images),
      floatingActionButton: FloatingActionButton(
          child: Icon(Icons.add),
          onPressed: () {
            fetchImage();
          }),
      appBar: AppBar(title: Text("Ola Amigo!")),
    ));
  }
}

class ImageModels {
  int id;
  String url;
  String title;

  ImageModels(this.id, this.url, this.title);

  factory ImageModels.fromjson(Map<String, dynamic> parsedjson) => ImageModels(
        parsedjson["id"],
        parsedjson["url"],
        parsedjson["title"],
      );
}

class ImageList extends StatelessWidget {
  final List<ImageModels> image;

  ImageList(this.image);

  Widget build(context) {
    return ListView.builder(
        itemCount: image.length,
        itemBuilder: (context, int index) {
          print(image[index].url);
          return buildImage(image[index]);
        });
  }

  Widget buildImage(ImageModels image) {
    return Container(
      decoration: BoxDecoration(border: Border.all(color: Colors.grey)),
      padding: EdgeInsets.all(20.0),
      margin: EdgeInsets.all(20.0),
      child: Image.network(image.url),
    );
  }
}