flutter - 如何在应用程序启动和使用时下载资产

Question

目前，我的应用程序有许多资产（图像、声音、字体、json、SQL-lite 数据库文件等）。都在pubspec.yaml中定义了

但是，由于要求减小 APK 大小，我需要在应用启动时下载其中的一些并将其保存到存储中，因此下次无需下载。 p>

如果资产还没有准备好，它应该等待一秒钟并显示加载栏圈。

问题是如何做这件事，有什么例子吗？

Answer 1

最简单的方法是将文件下载为 zip（存档文件）并解压到应用程序存储目录的路径getApplicationDocumentsDirectory

您将使用以下软件包列表： archive , http 和 path_provider

pubspec.yaml 看起来像

version: 1.0.0+1

environment:
  sdk: ">=2.1.0 <3.0.0"

dependencies:
  flutter:
    sdk: flutter

  path_provider: ^1.1.0
  http: ^0.12.0+2
  archive: ^2.0.8

dev_dependencies:
  flutter_test:
    sdk: flutter
  uses-material-design: true
  assets:
    - assets/images/

加冕您的应用程序看起来像的 main.dart 文件 请注意，api 是您的文件的 URL，没有文件名。

main.dart

import 'dart:io';

import 'package:archive/archive.dart';
import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;
import 'package:path_provider/path_provider.dart';

import 'data.dart';

const api =
    'https://firebasestorage.googleapis.com/v0/b/playground-a753d.appspot.com/o';

enum AppTheme { candy, cocktail }

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(AppTheme.candy),
    );
  }
}

class MyHomePage extends StatefulWidget {
  final AppTheme theme;

  MyHomePage(this.theme);

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

class _MyHomePageState extends State<MyHomePage> {
  AppTheme _theme;
  String _dir;
  List<String> _images;

  @override
  void initState() {
    super.initState();
    _theme = widget.theme;
    _images = data[_theme];
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      floatingActionButton: FloatingActionButton(
        child: Icon(Icons.style),
        onPressed: () async {
          if (_theme == AppTheme.candy) {
            await _downloadAssets('cocktail');
          }
          setState(() {
            _theme =
                _theme == AppTheme.candy ? AppTheme.cocktail : AppTheme.candy;
            _images = data[_theme];
          });
        },
      ),
      body: ListView.builder(
          itemCount: _images.length,
          itemBuilder: (BuildContext context, int index) {
            return _getImage(_images[index], _dir);
          }),
    );
  }

  Widget _getImage(String name, String dir) {
    if (_theme != AppTheme.candy) {
      var file = _getLocalImageFile(name, dir);
      return Image.file(file);
    }
    return Image.asset('assets/images/$name');
  }

  File _getLocalImageFile(String name, String dir) => File('$dir/$name');

  Future<void> _downloadAssets(String name) async {
    if (_dir == null) {
      _dir = (await getApplicationDocumentsDirectory()).path;
    }

    if (!await _hasToDownloadAssets(name, _dir)) {
      return;
    }
    var zippedFile = await _downloadFile(
        '$api/$name.zip?alt=media&token=7442d067-a656-492f-9791-63e8fc082379',
        '$name.zip',
        _dir);

    var bytes = zippedFile.readAsBytesSync();
    var archive = ZipDecoder().decodeBytes(bytes);

    for (var file in archive) {
      var filename = '$_dir/${file.name}';
      if (file.isFile) {
        var outFile = File(filename);
        outFile = await outFile.create(recursive: true);
        await outFile.writeAsBytes(file.content);
      }
    }
  }

  Future<bool> _hasToDownloadAssets(String name, String dir) async {
    var file = File('$dir/$name.zip');
    return !(await file.exists());
  }

  Future<File> _downloadFile(String url, String filename, String dir) async {
    var req = await http.Client().get(Uri.parse(url));
    var file = File('$dir/$filename');
    return file.writeAsBytes(req.bodyBytes);
  }
}

然后你必须列出所有文件并添加它们（逻辑上到相应的主题）

data.dart：

import 'main.dart' show AppTheme;

const Map<AppTheme, List<String>> data = const {
  AppTheme.candy: [
    'art-background-blue-1289363.jpg',
    'assortment-bright-candy-1043519.jpg',
    'bright-candies-cherry-1405760.jpg',
    'bright-candies-colorful-539447.jpg',
    'bright-candy-chewy-1328885.jpg',
  ],
  AppTheme.cocktail: [
    'alcohol-alcoholic-beverage-beverage-1304540.jpg',
    'alcohol-alcoholic-beverage-beverage-1723638.jpg',
    'alcohol-black-background-close-up-800390.jpg',
    'alcoholic-beverage-beverage-cocktail-970197.jpg',
    'bar-beverage-blur-338713.jpg',
  ]
};

更多信息请查看Github project 和Medium article