如何在Flutter / Dart中导入特定于平台的依赖项？ （结合使用Android / iOS的Web）

问题描述

我在适用于iOS和Android的Flutter应用程序中使用 shared_preferences 。在网络上，我使用的是 http：dart 依赖项（ window.localStorage ）本身。由于Flutter for Web已合并到Flutter存储库中，因此我想创建一个跨平台解决方案。

这意味着我需要导入两个单独的API。似乎在Dart中还没有很好的支持，但这是我所做的：

  import'package：some_project / stub /preference_utils_stub.dart'
 if（dart.library.html）'dart：html'
 if（dart.library.io）'package：shared_preferences / shared_preferences.dart'; 
 
  

在我的 preference_utils_stub.dart 文件，我实现了在编译时需要可见的所有类/变量：

  Window window; 
 
类SharedPreferences {
 static Future< SharedPreferences> get getInstance async {} 
 setString（String key，String value）{} 
 getString（String key）{} 
} 
 
 class Window {
 Map< String，String> localStorage; 
} 
 
  

这可以消除编译前的所有错误。现在，我实现了一些用于检查应用程序是否正在使用Web的方法：

  static Future< String> getString（String key）async {
 if（kIsWeb）{
 return window.localStorage [key]; 
} 
 SharedPreferences首选项=等待SharedPreferences.getInstance; 
 return preferences.getString（key）; 
} 
  

但是，这会带来很多错误：

  lib / utils / preference_utils.dart：13：7：错误：找不到Getter：
'window'。 
 window.localStorage [key] =值； 
 ^^^^^^ lib / utils / preference_utils.dart：15：39：错误：类型为‘Future< SharedPreferences>的值无法将Function（）分配给 SharedPreferences类型的
变量。 
-未来来自 dart：async。 
-'SharedPreferences'来自'package：shared_preferences / shared_preferences.dart'
（'../../flutter/.pub-cache/hosted/pub.dartlang.org/shared_preferences-0.5。 4 + 3 / lib / shared_preferences.dart'）。 
 SharedPreferences首选项=等待SharedPreferences.getInstance; 
 ^ lib / utils / preference_utils.dart：22：14：错误：找不到Getter：
窗口。 
返回window.localStorage [key]; 
  

以此类推。如何在没有这些错误的情况下根据平台使用不同的方法/类？请注意，我以这种方式使用了更多的依赖关系，而不仅仅是首选项。谢谢！

解决方案

这是我处理您问题的方法。这基于


移动

I am using shared_preferences in my Flutter application for iOS and Android. On the web I am using the http:dart dependency (window.localStorage) itself. Since Flutter for web was merged into the Flutter repo, I want to create a cross platform solution.

This means I need to import two seperate API's. This seems not to be very good supported in Dart yet, but this is what I did:

import 'package:some_project/stub/preference_utils_stub.dart'
    if (dart.library.html) 'dart:html'
    if (dart.library.io) 'package:shared_preferences/shared_preferences.dart';

In my preference_utils_stub.dart file, I implemented all classes/variables which need to be visible during compile time:

Window window;

class SharedPreferences {
  static Future<SharedPreferences> get getInstance async {}
  setString(String key, String value) {}
  getString(String key) {}
}

class Window {
  Map<String, String> localStorage;
}

This gets rid of all errors before compilation. Now I implemented some method which checks if the application is using the web or not:

static Future<String> getString(String key) async {
    if (kIsWeb) {
       return window.localStorage[key];
    }
    SharedPreferences preferences = await SharedPreferences.getInstance;
    return preferences.getString(key);
}

However, this gives loads of errors:

lib/utils/preference_utils.dart:13:7: Error: Getter not found:
'window'.
      window.localStorage[key] = value;
      ^^^^^^ lib/utils/preference_utils.dart:15:39: Error: A value of type 'Future<SharedPreferences> Function()' can't be assigned to a
variable of type 'SharedPreferences'.
 - 'Future' is from 'dart:async'.
 - 'SharedPreferences' is from 'package:shared_preferences/shared_preferences.dart'
('../../flutter/.pub-cache/hosted/pub.dartlang.org/shared_preferences-0.5.4+3/lib/shared_preferences.dart').
      SharedPreferences preferences = await SharedPreferences.getInstance;
                                      ^ lib/utils/preference_utils.dart:22:14: Error: Getter not found:
'window'.
      return window.localStorage[key];

And so on. How can one use different methods/classes depending on the platform without these errors? Note that I am using more dependencies this way, not just preferences. Thanks!

解决方案

Here is my approach to your issue. This is based on the implementations from http package as in here.

The core idea is as follows.

1. Create an abstract class to define the methods you will need to use.
2. Create implementations specific to web and android dependencies which extends this abstract class.
3. Create a stub which exposes a method to return the instance of this abstract implementation. This is only to keep the dart analysis tool happy.
4. In the abstract class import this stub file along with the conditional imports specific for mobile and web. Then in its factory constructor return the instance of the specific implementation. This will be handled automatically by conditional import if written correctly.

Step-1 and 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();
}

Step-2.1: Web Key finder

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();

Step-2.2: Mobile Key finder

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();

Step-3:

import 'key_finder_interface.dart';

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

Then in your main.dart use the KeyFinder abstract class as if its a generic implementation. This is somewhat like an adapter pattern.

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",
                  border: OutlineInputBorder(),
                ),
                controller: _valueTextController,
              ),
            ),
            RaisedButton(
              child: Text('Save new Key/Value Pair'),
              onPressed: () {
                widget.keyFinder.setKeyValue(
                  _keyTextController.text,
                  _valueTextController.text,
                );
                setState(() {
                  key = _keyTextController.text;
                });
              },
            )
          ],
        ),
      ),
    );
  }
}

some screen shots

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