如何解决Flutter开发中返回null的报错？投诉应用ImagePicker功能异常修复

错误触发的3个核心原因 #

• 1. _decideImageView 函数声明返回值为非空Widget类型，但else分支仅创建了Image组件未添加return关键字，函数走else分支时会返回null，不符合非空返回要求
• 2. 用late修饰imageFile的同时判断它是否为null：late关键字声明的变量默认不允许为null，你需要将其改为可空类型File? imageFile才能做空判断
• 3. ImagePicker().pickImage返回的是XFile类型，直接强转为io.File会抛出类型转换异常，需要通过XFile.path构造File对象，同时要处理用户取消选择时返回null的情况

修正后的完整代码 #

import 'dart:io';
import 'package:flutter/material.dart';
import 'package:image_picker/image_picker.dart';

class ComplaintNo1 extends StatefulWidget {
  @override
  _ComplaintNo1State createState() => _ComplaintNo1State();
}

class _ComplaintNo1State extends State<ComplaintNo1> {
  // 修改为可空类型，允许为空
  File? imageFile;

  _openGallery(BuildContext context) async{
    final XFile? picture = await ImagePicker().pickImage(source: ImageSource.gallery);
    // 处理用户未选择图片的空情况
    if(picture != null) {
      setState(() {
        // 用XFile的path构造File对象
        imageFile = File(picture.path);
      });
    }
    Navigator.of(context).pop();
  }

  _openCamera(BuildContext context) async{
    final XFile? picture = await ImagePicker().pickImage(source: ImageSource.camera);
    if(picture != null) {
      setState(() {
        imageFile = File(picture.path);
      });
    }
    Navigator.of(context).pop();
  }


  Future<void> _showChoiceDialog(BuildContext context) {
    return showDialog(context: context, builder: (BuildContext context) {
      return AlertDialog(
        title: const Text('Make a choice!'),
        content: SingleChildScrollView(
          child: ListBody(
            children: <Widget>[
              GestureDetector(
                child: const Text('Gallery'),
                onTap: () {
                  _openGallery(context);
                },
              ),
              const Padding(padding: EdgeInsets.all(8.0)),
              GestureDetector(
                child: const Text('Camera'),
                onTap: () {
                  _openCamera(context);
                },
              )
            ],
          ),
        ),
      );
    });
  }

  Widget _decideImageView() {
    if(imageFile == null) {
      return const Text('No Image Selected!');
    }
    else {
      // 补全return关键字
      return Image.file(imageFile!, width: 100, height: 200);
    }
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Column(
        mainAxisAlignment: MainAxisAlignment.center,
        children: <Widget>[
          const Icon(
            Icons.camera_alt,
          ),
          ElevatedButton(onPressed: () {
            _showChoiceDialog(context);
          }, child: const Text('Select Image'),
          ),
          _decideImageView(),
          const SizedBox(
            width: 20.0,
            height: 20.0,
          ),
          TextFormField(
            decoration: const InputDecoration(
              border: OutlineInputBorder(),
              labelText: 'Title of complaint',
              prefixIcon: Icon(Icons.title),
              labelStyle: TextStyle(
                fontSize: 20.0,
                color: Colors.blueAccent,
              ),
            ),
          ),
          const SizedBox(
            width: 20.0,
            height: 20.0,
          ),
          TextFormField(
            decoration: const InputDecoration(
              border: OutlineInputBorder(),
              labelText: 'Time',
              prefixIcon: Icon(Icons.access_time),
              labelStyle: TextStyle(
                fontSize: 20.0,
                color: Colors.blueAccent,
              ),
            ),
          ),
          const SizedBox(
            width: 20.0,
            height: 20.0,
          ),
          TextFormField(
            decoration: const InputDecoration(
              border: OutlineInputBorder(),
              labelText: 'Starting date',
              prefixIcon: Icon(Icons.date_range),
              labelStyle: TextStyle(
                fontSize: 20.0,
                color: Colors.blueAccent,
              ),
            ),
          ),
          const SizedBox(
            width: 20.0,
            height: 20.0,
          ),
          TextFormField(
            decoration: const InputDecoration(
              border: OutlineInputBorder(),
              labelText: 'Description of complaint',
              prefixIcon: Icon(Icons.description),
              labelStyle: TextStyle(
                fontSize: 20.0,
                color: Colors.blueAccent,
              ),
            ),
            maxLines: 5,
          ),
        ],
      ),
    );
  }
}

内容的提问来源于stack exchange，提问作者suk