在 Flutter 里使用 Packages

Flutter 支持使用其他开发者向 Flutter 和 Dart 生态系统贡献的共享 package，这意味着你可以快速构建应用而不是一切从零开始。

Flutter supports using shared packages contributed by other developers to the Flutter and Dart ecosystems. This allows quickly building an app without having to develop everything from scratch.

现有的 package 支持许多使用场景，例如，网络请求 (http)，自定义导航/路由处理 (fluro)，集成设备 API（如 (url_launcher 和 battery，以及使用第三方平台的 SDK（如 Firebase 的 (FlutterFire)。

Existing packages enable many use cases—for example, making network requests (http), custom navigation/route handling (fluro), integration with device APIs (url_launcher and battery), and using third-party platform SDKs like Firebase (FlutterFire).

如果你正打算开发新的 package，请参阅 Flutter Packages 的开发和提交。

To write a new package, see developing packages.

如果你想添加资源、图片或字体，无论是存储在文件中还是 package 中，请参阅 添加资源和图片 这篇文档。

To add assets, images or fonts, whether stored in files or packages, see Adding assets and images.

使用 package

Using packages

下面的内容将为你描述如何使用已经发布了的 packages。

The following section describes how to use existing published packages.

搜索 package

Searching for packages

Package 会被发布到 pub.dev 网站上。

Packages are published to pub.dev.

Pub 网站上的 Flutter 页面 展示了与 Flutter 兼容的 package（即声明的依赖通常与 Flutter 兼容），并且所有已发布的 package 都支持搜索。

The Flutter landing page on pub.dev displays top packages that are compatible with Flutter (those that declare dependencies generally compatible with Flutter), and supports searching among all published packages.

Pub.dev 上的 Flutter Favorites 页面列出了一系列编写应用时可以首先考虑使用的插件和 package，关于这个项目的更多信息，请查看 Flutter Favorites 项目 页面。

The Flutter Favorites page on pub.dev lists the plugins and packages that have been identified as packages you should first consider using when writing your app. For more information on what it means to be a Flutter Favorite, see the Flutter Favorites program.

在 pub.dev 网站上同时可以过滤出适合 Android、iOS 或 Web 的插件，也可以通过复选框，过滤出组合结果（适配一个或者多个平台）。

You can also browse the packages on pub.dev by filtering on Android plugins, iOS plugins, web plugins, or any combination thereof.

将 package 依赖添加到应用

Adding a package dependency to an app

要将 package ‘css_colors’ 添加到应用：

To add the package, css_colors, to an app:

1. 添加依赖

   Depend on it

   • 打开应用文件夹下的 pubspec.yaml 文件，然后在 pubspec.yaml 下添加 css_colors:。

     Open the pubspec.yaml file located inside the app folder, and add css_colors: under dependencies.

2. 安装

   Install it

   • 在命令行中运行：flutter pub get
     

     From the terminal: Run flutter pub get
     

   或者

   OR

   • 在 Android Studio/IntelliJ 中点击 pubspec.yaml 文件顶部操作功能区的 Packages get

     From Android Studio/IntelliJ: Click Packages get in the action ribbon at the top of pubspec.yaml.

   • 在 VS Code 中点击位于 pubspec.yaml 文件顶部操作功能区右侧的 Get Packages

     From VS Code: Click Get Packages located in right side of the action ribbon at the top of pubspec.yaml.

3. 导入

   Import it

   • 在 Dart 代码中添加相关的 import 语句。

     Add a corresponding import statement in the Dart code.

4. 如果有必要，停止并重启应用

   Stop and restart the app, if necessary

   • 如果 package 内有特定平台的代码（Android 的 Java/Kotlin, iOS 的 Swift/Objective-C），代码必须内置到你的应用内。热重载和热重启只对 package 的 Dart 代码执行此操作，所以你需要完全重启应用以避免使用 package 时出现 MissingPluginException 错误。

     If the package brings platform-specific code (Kotlin/Java for Android, Swift/Objective-C for iOS), that code must be built into your app. Hot reload and hot restart only update the Dart code, so a full restart of the app might be required to avoid errors like MissingPluginException when using the package.

对于这些步骤，Pub 上任何 package 页面的 Installing tab 选项卡都是一个很方便的参考。

The Installing tab, available on any package page on pub.dev, is a handy reference for these steps.

完整示例，参阅下面的 css_colors 示例 。

For a complete example, see the css_colors example below.

冲突解决

Conflict resolution

假设你想在应用中使用 some_package 和 other_package，并且它们依赖于不同版本的 url_launcher。于是我们便有了潜在的冲突。避免这种情况的最好方法是 package 的作者在指定依赖项时使用 版本范围 而非特定版本。

Suppose you want to use some_package and another_package in an app, and both of these depend on url_launcher, but in different versions. That causes a potential conflict. The best way to avoid this is for package authors to use version ranges rather than specific versions when specifying dependencies.

dependencies:
  url_launcher: ^5.4.0    # Good, any version >= 5.4.0 but < 6.0.0
  image_picker: '5.4.3'   # Not so good, only version 5.4.3 works.

如果 some_package 声明了以上依赖，并且 another_package 声明了一个兼容的 url_launcher 依赖项，如 '5.4.6' 或 ^5.5.0， pub 能够自动解决冲突问题。 Gradle modules 和 CocoaPods 也是用类似的方式解决平台依赖的。

If some_package declares the dependencies above and another_package declares a compatible url_launcher dependency like '5.4.6' or ^5.5.0, pub resolves the issue automatically. Platform-specific dependencies on Gradle modules and/or CocoaPods are solved in a similar way.

即使 some_package 和 another_package 声明了不兼容的 url_launcher 版本，它们实际上仍可能以兼容的方式使用 url_launcher。在这种情况下，可在 pubspec.yaml 文件中添加一个依赖覆盖声明来强制使用特定版本，从而处理冲突。

Even if some_package and another_package declare incompatible versions for url_launcher, they might actually use url_launcher in compatible ways. In this situation, the conflict can be resolved by adding a dependency override declaration to the app’s pubspec.yaml file, forcing the use of a particular version.

为了强制使用版本为 5.4.0 的 url_launcher，你可以对应用的 pubspec.yaml 文件做如下更改：

For example, to force the use of url_launcher version 5.4.0, make the following changes to the app’s pubspec.yaml file:

dependencies:
  some_package:
  another_package:
dependency_overrides:
  url_launcher: '5.4.0'

如果依赖冲突项不是 package 自身，而是如 guava 这样特定于 Android 的库，那么依赖的覆盖声明必须添加到 Gradle 的构建逻辑中。

如果依赖冲突项不是 package 自身，而是如 guava 这样特定于 Android 的库，那么依赖的覆盖声明必须添加到 Gradle 的构建逻辑中。

If the conflicting dependency is not itself a package, but an Android-specific library like guava, the dependency override declaration must be added to Gradle build logic instead.

为了强制使用版本为 28.0 的 guava，你可以对 android/build.gradle 文件做如下更改：

To force the use of guava version 28.0, make the following changes to the app’s android/build.gradle file:

configurations.all {
    resolutionStrategy {
        force 'com.google.guava:guava:28.0-android'
    }
}

CocoaPods 目前尚不提供依赖项覆盖功能。

CocoaPods does not currently offer dependency override functionality.

开发新的 package

Developing new packages

如果某个 package 不适用于你的特定需求，你可以 开发新的自定义 package。

If no package exists for your specific use case, you can write a custom package.

管理 package 的依赖和版本

Managing package dependencies and versions

为了使版本冲突的风险最小化，请在 pubspec.yaml 文件中指定一个版本范围。

To minimize the risk of version collisions, specify a version range in the pubspec.yaml file.

Package 版本

Package versions

所有 package 都有一个版本号，在它们的 pubspec.yaml 文件中指定。当前的 package 版本会在其名称旁边显示当前版本号。（例如，参阅 url_launcher package）以及所有先前版本的列表： url_launcher 版本列表。

All packages have a version number, specified in the package’s pubspec.yaml file. The current version of a package is displayed next to its name (for example, see the url_launcher package), as well as a list of all prior versions (see url_launcher versions).

当使用简写形式 plugin1: 将 package 添加到 pubspec.yaml 时，表明 plugin1 package 的任何版本都可以被使用。为了确保在更新 package 的时候你的应用不会崩溃，我们建议使用以下格式之一来指定版本范围：

When a package is added to pubspec.yaml, the shorthand form plugin1: means that any version of the plugin1 package can be used. To ensure that the app doesn’t break when a package is updated, specify a version range using one of the following formats:

• 范围限制：指定一个最小和最大的版本号，例如：

  Range constraints: Specify a minimum and maximum version. For example:

  dependencies:
    url_launcher: '>=5.4.0 <6.0.0'
  

• 使用 caret语法 的范围约束与常规的范围约束类似：

  Range constraints with caret syntax are similar to regular range constraints:

  dependencies:
    collection: '^5.4.0'
  

了解更详细的信息，参阅 Pub 版本管理指南。

For additional details, see the package versioning guide.

更新 package 依赖

Updating package dependencies

当你添加一个 package 后首次运行 flutter pub get（IntelliJ 或 Android Studio 中的 Packages Get）， Flutter 将会保存在 pubspec.lock lockfile 中找到的具体 package 版本。这将确保当你或者团队中其他开发者运行 flutter pub get 后能得到相同版本的 package。

When running flutter pub get (Packages get in IntelliJ or Android Studio) for the first time after adding a package, Flutter saves the concrete package version found in the pubspec.lock lockfile. This ensures that you get the same version again if you, or another developer on your team, run flutter pub get.

如果你想升级到 package 的最新版本，比如使用 package 的最新特性，请运行 flutter packages upgrade （IntelliJ 或 Android Studio 的 Upgrade dependencies 功能）。这将检索你在 pubspec.yaml 文件中指定的版本约束所允许的最高可用版本。请注意，flutter upgrade 与 flutter update-packages 是两个不同的命令，但它们都会更新 Flutter。

To upgrade to a new version of the package, for example to use new features in that package, run flutter pub upgrade (Upgrade dependencies in IntelliJ or Android Studio) to retrieve the highest available version of the package that is allowed by the version constraint specified in pubspec.yaml. Note that this is a different command from flutter upgrade or flutter update-packages, which both update Flutter itself.

依赖未发布的 package

Dependencies on unpublished packages

即使未在 Pub site 上发布，也可以使用 package。对于不用于公开发布的私有插件，或者尚未准备好发布的 package，可以使用其他依赖选项。

Packages can be used even when not published on pub.dev. For private plugins, or for packages not ready for publishing, additional dependency options are available:

Path 依赖
Flutter 应用可以通过文件系统 path: 依赖而依赖于插件。路径可以是相对的，也可以是绝对的。例如，要依赖位于应用相邻目录中的插件 plugin1，可以使用以下语法：

Path dependency
A Flutter app can depend on a plugin via a file system path: dependency. The path can be either relative or absolute. Relative paths are evaluated relative to the directory containing pubspec.yaml. For example, to depend on a plugin plugin1 located in a directory next to the app, use the following syntax:

  dependencies:
    plugin1:
      path: ../plugin1/

Git 依赖
你也可以依赖存储在 Git 仓库中的 package，如果 package 位于仓库的根目录，可以使用以下语法：

Git dependency
You can also depend on a package stored in a Git repository. If the package is located at the root of the repo, use the following syntax:

  dependencies:
    plugin1:
      git:
        url: git://github.com/flutter/plugin1.git

**Git 依赖于文件夹中的 package **
默认情况下，pub 会默认假定 package 位于 Git 仓库的根目录。如果不是这种情况，你可以使用 path 参数指定位置，例如：

Git dependency on a package in a folder
Pub assumes the package is located in the root of the Git repository. If that is not the case, specify the location with the path argument. For example:

  dependencies:
    package1:
      git:
        url: git://github.com/flutter/packages.git
        path: packages/package1

最后，你可以使用 ref 参数将依赖固定到 git 特定的 commit、branch 或者 tag。更多详细信息，请参阅 Package dependencies。

Finally, use the ref argument to pin the dependency to a specific git commit, branch, or tag. For more details, see Package dependencies.

例子

Examples

下面的示例将介绍使用 packages 的一些必要步骤。

The following examples walk through the necessary steps for using packages.

例子：使用 CSS Colors package

Example: Using the CSS Colors package

css_colors package 为 CSS 颜色定义颜色常量，允许你在 Flutter 框架中任何需要 Color 类型的地方使用它们。

The css_colors package defines color constants for CSS colors, so use the constants wherever the Flutter framework expects the Color type.

要使用这个 package：

To use this package:

1. 创建一个名为 cssdemo 的新项目

   Create a new project called cssdemo.

2. 打开 pubspec.yaml，并添加依赖 css-colors：

   Open pubspec.yaml, and add the css-colors dependency:

   dependencies:
     flutter:
       sdk: flutter
   

   替换为：

   with:

   dependencies:
     flutter:
       sdk: flutter
     css_colors: ^1.0.0
   

3. 在命令行中运行 flutter packages get，或者点击 Intellij 中的 Packages get

   Run flutter pub get in the terminal, or click Packages get in IntelliJ or Android Studio.

4. 打开 lib/main.dart 并将其全部内容替换为：

   Open lib/main.dart and replace its full contents with:

    import 'package:css_colors/css_colors.dart';
    import 'package:flutter/material.dart';
   
    void main() {
      runApp(MyApp());
    }
   
    class MyApp extends StatelessWidget {
      @override
      Widget build(BuildContext context) {
        return MaterialApp(
          home: DemoPage(),
        );
      }
    }
   
    class DemoPage extends StatelessWidget {
      @override
      Widget build(BuildContext context) {
        return Scaffold(body: Container(color: CSSColors.orange));
      }
    }
   

5. 运行应用。当你点击 ‘Show Flutter homepage’ 时，你将看到手机默认浏览器打开并出现 Flutter 主页。

   Run the app. The app’s background should now be orange.

例子：使用 url_launcher package 来打开浏览器

Example: Using the url_launcher package to launch the browser

url_launcher 插件可以让你在移动平台上打开默认浏览器以显示给定的 URL。它演示了 package 如何也可能包含特定于平台的代码我们将这一类包含各平台不同代码的 package 称为 插件 package 或者 插件。

The url_launcher plugin package enables opening the default browser on the mobile platform to display a given URL, and is supported on Android, iOS, web, and macos. This package is a special Dart package called a plugin package (or plugin), which includes platform-specific code.

要使用这个插件：

To use this plugin:

1. 新建一个名为 lauchdemo 的新项目；

   Create a new project called launchdemo.

2. 打开 pubspec.yaml，然后添加 url_launcher 的依赖：

   Open pubspec.yaml, and add the url_launcher dependency:

   dependencies:
     flutter:
       sdk: flutter
     url_launcher: ^5.4.0
   

3. 在命令行中运行 flutter packages get，或者点击 Intellij 或 Android Studio 中的 Packages get；

   Run flutter pub get in the terminal, or click Packages get in IntelliJ or Android Studio.

4. 打开 lib/main.dart 并将其全部内容替换为：

   Open lib/main.dart and replace its full contents with the following:

    import 'package:flutter/material.dart';
    import 'package:url_launcher/url_launcher.dart';
   
    void main() {
      runApp(MyApp());
    }
   
    class MyApp extends StatelessWidget {
      @override
      Widget build(BuildContext context) {
        return MaterialApp(
          home: DemoPage(),
        );
      }
    }
   
    class DemoPage extends StatelessWidget {
      launchURL() {
        launch('https://flutter.dev');
      }
   
      @override
      Widget build(BuildContext context) {
        return Scaffold(
          body: Center(
            child: ElevatedButton(
              onPressed: launchURL,
              child: Text('Show Flutter homepage'),
            ),
          ),
        );
      }
    }
   

5. 运行应用（如果你的应用在添加插件之前已经运行，请停止并重启应用）。当你点击 Show Flutter homepage 时，你将看到手机默认浏览器打开并出现 Flutter 主页。

   Run the app (or stop and restart it, if it was already running before adding the plugin). Click Show Flutter homepage. You should see the default browser open on the device, displaying the homepage for flutter.dev.