使用advance_pdf_viewer包时遭遇late初始化错误

问题根源 #

1. 使用late关键字时，异步加载PDF的loadDocument方法还没完成赋值，build方法就已经执行，导致late变量未初始化错误。
2. 移除late后，Dart非空类型规则要求变量必须在声明时初始化，因此触发document必须初始化的提示。

解决方案1：使用FutureBuilder处理异步加载 #

FutureBuilder能适配异步资源的加载状态，避免变量初始化时机问题，修改后的代码如下：

import 'package:advance_pdf_viewer/advance_pdf_viewer.dart';
import 'package:flutter/material.dart';

class NotesView extends StatefulWidget {
  const NotesView({Key? key, required this.noteId, required this.noteName})
      : super(key: key);
  final int noteId;
  final String noteName;

  @override
  State<NotesView> createState() => _NotesViewState();
}

class _NotesViewState extends State<NotesView> {
  // 定义异步加载方法，返回Future<PDFDocument>
  Future<PDFDocument> loadDocument() async {
    return await PDFDocument.fromURL(
        'http://www.africau.edu/images/default/sample.pdf');
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text("Note View")),
      body: FutureBuilder<PDFDocument>(
        future: loadDocument(),
        builder: (context, snapshot) {
          // 加载中状态
          if (snapshot.connectionState == ConnectionState.waiting) {
            return const Center(child: CircularProgressIndicator());
          }
          // 加载出错状态
          if (snapshot.hasError) {
            return Center(child: Text('加载失败: ${snapshot.error}'));
          }
          // 加载完成，显示PDF
          return PDFViewer(document: snapshot.data!);
        },
      ),
    );
  }
}

解决方案2：可空变量+状态判断 #

如果不想用FutureBuilder，也可以用可空变量配合加载状态管理：

import 'package:advance_pdf_viewer/advance_pdf_viewer.dart';
import 'package:flutter/material.dart';

class NotesView extends StatefulWidget {
  const NotesView({Key? key, required this.noteId, required this.noteName})
      : super(key: key);
  final int noteId;
  final String noteName;

  @override
  State<NotesView> createState() => _NotesViewState();
}

class _NotesViewState extends State<NotesView> {
  PDFDocument? document;
  bool isLoading = true;

  @override
  void initState() {
    super.initState();
    _loadDocument();
  }

  _loadDocument() async {
    final loadedDoc = await PDFDocument.fromURL(
        'http://www.africau.edu/images/default/sample.pdf');
    setState(() {
      document = loadedDoc;
      isLoading = false;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text("Note View")),
      body: Center(
        child: isLoading
            ? const CircularProgressIndicator()
            : PDFViewer(document: document!),
      ),
    );
  }
}

内容的提问来源于stack exchange，提问作者swapnil mane