这是我处理您问题的方法。这基于here中http包的实现。

核心思想如下。

1. 创建一个抽象类来定义您将需要使用的方法。
2. 创建特定于web和android依赖项的实现，以扩展此抽象类。
3. 创建一个存根，该存根提供一个方法来返回此抽象实现的实例。这只是为了让飞镖分析工具保持满意状态。
4. 在抽象类中，导入此存根文件以及特定于mobile和web的条件导入。然后在其工厂构造函数中返回特定实现的实例。如果编写正确，则将通过条件导入自动处理。

步骤1和4：

import 'key_finder_stub.dart'
    // ignore: uri_does_not_exist
    if (dart.library.io) 'package:flutter_conditional_dependencies_example/storage/shared_pref_key_finder.dart'
    // ignore: uri_does_not_exist
    if (dart.library.html) 'package:flutter_conditional_dependencies_example/storage/web_key_finder.dart';

abstract class KeyFinder {

  // some generic methods to be exposed.

  /// returns a value based on the key
  String getKeyValue(String key) {
    return "I am from the interface";
  }

  /// stores a key value pair in the respective storage.
  void setKeyValue(String key,String value) {}

  /// factory constructor to return the correct implementation.
  factory KeyFinder() => getKeyFinder();
}

步骤2.1：Web密钥查找器

import 'dart:html';

import 'package:flutter_conditional_dependencies_example/storage/key_finder_interface.dart';

Window windowLoc;

class WebKeyFinder implements KeyFinder {

  WebKeyFinder() {
    windowLoc = window;
    print("Widnow is initialized");
    // storing something initially just to make sure it works. :)
    windowLoc.localStorage["MyKey"] = "I am from web local storage";
  }

  String getKeyValue(String key) {
    return windowLoc.localStorage[key];
  }

  void setKeyValue(String key,String value) {
    windowLoc.localStorage[key] = value;
  }  
}

KeyFinder getKeyFinder() => WebKeyFinder();

步骤2.2：移动密钥查找器

import 'package:flutter_conditional_dependencies_example/storage/key_finder_interface.dart';
import 'package:shared_preferences/shared_preferences.dart';

class SharedPrefKeyFinder implements KeyFinder {
  SharedPreferences _instance;

  SharedPrefKeyFinder() {
    SharedPreferences.getInstance().then((SharedPreferences instance) {
      _instance = instance;
      // Just initializing something so that it can be fetched.
      _instance.setString("MyKey","I am from Shared Preference");
    });
  }

  String getKeyValue(String key) {
    return _instance?.getString(key) ??
        'shared preference is not yet initialized';
  }

  void setKeyValue(String key,String value) {
    _instance?.setString(key,value);
  }

}

KeyFinder getKeyFinder() => SharedPrefKeyFinder();

第3步：

import 'key_finder_interface.dart';

KeyFinder getKeyFinder() => throw UnsupportedError(
    'Cannot create a keyfinder without the packages dart:html or package:shared_preferences');

然后在您的main.dart中使用KeyFinder抽象类，就好像它是通用实现一样。这有点像适配器模式。

main.dart

import 'package:flutter/material.dart';
import 'package:flutter_conditional_dependencies_example/storage/key_finder_interface.dart';

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    KeyFinder keyFinder = KeyFinder();
    return MaterialApp(
      title: 'Flutter Demo',theme: ThemeData(
        primarySwatch: Colors.blue,),home: SafeArea(
        child: KeyValueWidget(
          keyFinder: keyFinder,);
  }
}

class KeyValueWidget extends StatefulWidget {
  final KeyFinder keyFinder;

  KeyValueWidget({this.keyFinder});
  @override
  _KeyValueWidgetState createState() => _KeyValueWidgetState();
}

class _KeyValueWidgetState extends State<KeyValueWidget> {
  String key = "MyKey";
  TextEditingController _keyTextController = TextEditingController();
  TextEditingController _valueTextController = TextEditingController();
  @override
  Widget build(BuildContext context) {
    return Material(
      child: Container(
        width: 200.0,child: Column(
          children: <Widget>[
            Expanded(
              child: Text(
                '$key / ${widget.keyFinder.getKeyValue(key)}',style: TextStyle(fontSize: 20.0,fontWeight: FontWeight.bold),Expanded(
              child: TextFormField(
                decoration: InputDecoration(
                  labelText: "Key",border: OutlineInputBorder(),controller: _keyTextController,Expanded(
              child: TextFormField(
                decoration: InputDecoration(
                  labelText: "Value",controller: _valueTextController,RaisedButton(
              child: Text('Save new Key/Value Pair'),onPressed: () {
                widget.keyFinder.setKeyValue(
                  _keyTextController.text,_valueTextController.text,);
                setState(() {
                  key = _keyTextController.text;
                });
              },)
          ],);
  }
}

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