如何在 Flutter/Dart 中导入特定于平台的依赖项? (结合 Web 与 Android/iOS)
How to import platform specific dependency in Flutter/Dart? (Combine Web with Android/iOS)
我在 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 repo,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。这似乎在 Dart 中还没有得到很好的支持,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
文件中,我实现了所有需要在编译期间可见的 classes/variables:
Window window;
class SharedPreferences {
static Future<SharedPreferences> get getInstance async {}
setString(String key, String value) {}
getString(String key) {}
}
class Window {
Map<String, String> localStorage;
}
这会在编译前消除所有错误。现在我实现了一些方法来检查应用程序是否正在使用网络:
static Future<String> getString(String key) async {
if (kIsWeb) {
return window.localStorage[key];
}
SharedPreferences preferences = await SharedPreferences.getInstance;
return preferences.getString(key);
}
但是,这会产生大量错误:
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];
等等。如何根据平台使用不同的 methods/classes 而不出现这些错误?请注意,我以这种方式使用了更多依赖项,而不仅仅是首选项。谢谢!
这是我解决您的问题的方法。这是基于 http
包的实现,如 here.
核心思想如下
- 创建一个抽象 class 来定义您需要使用的方法。
- 创建特定于
web
和 android
依赖项的实现,扩展此抽象 class。
- 创建一个向 return 这个抽象实现的实例公开方法的存根。这只是为了让 dart 分析工具开心。
- 在摘要 class 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return实例中具体实现。如果编写正确,这将由条件导入自动处理。
第 1 步和第 4 步:
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:手机钥匙查找器
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 class 就好像它是一个通用实现一样。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",
border: OutlineInputBorder(),
),
controller: _valueTextController,
),
),
RaisedButton(
child: Text('Save new Key/Value Pair'),
onPressed: () {
widget.keyFinder.setKeyValue(
_keyTextController.text,
_valueTextController.text,
);
setState(() {
key = _keyTextController.text;
});
},
)
],
),
),
);
}
}
一些屏幕截图
网络
手机
你可以只使用包 universal_html
我在 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 repo,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。这似乎在 Dart 中还没有得到很好的支持,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
文件中,我实现了所有需要在编译期间可见的 classes/variables:
Window window;
class SharedPreferences {
static Future<SharedPreferences> get getInstance async {}
setString(String key, String value) {}
getString(String key) {}
}
class Window {
Map<String, String> localStorage;
}
这会在编译前消除所有错误。现在我实现了一些方法来检查应用程序是否正在使用网络:
static Future<String> getString(String key) async {
if (kIsWeb) {
return window.localStorage[key];
}
SharedPreferences preferences = await SharedPreferences.getInstance;
return preferences.getString(key);
}
但是,这会产生大量错误:
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];
等等。如何根据平台使用不同的 methods/classes 而不出现这些错误?请注意,我以这种方式使用了更多依赖项,而不仅仅是首选项。谢谢!
这是我解决您的问题的方法。这是基于 http
包的实现,如 here.
核心思想如下
- 创建一个抽象 class 来定义您需要使用的方法。
- 创建特定于
web
和android
依赖项的实现,扩展此抽象 class。 - 创建一个向 return 这个抽象实现的实例公开方法的存根。这只是为了让 dart 分析工具开心。
- 在摘要 class 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return实例中具体实现。如果编写正确,这将由条件导入自动处理。
第 1 步和第 4 步:
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:手机钥匙查找器
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 class 就好像它是一个通用实现一样。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",
border: OutlineInputBorder(),
),
controller: _valueTextController,
),
),
RaisedButton(
child: Text('Save new Key/Value Pair'),
onPressed: () {
widget.keyFinder.setKeyValue(
_keyTextController.text,
_valueTextController.text,
);
setState(() {
key = _keyTextController.text;
});
},
)
],
),
),
);
}
}
一些屏幕截图
网络
手机
你可以只使用包 universal_html