在 Android 应用中添加 Flutter 页面

本指南讲述了如何在一个现有的 Android 应用中添加单个 Flutter 页面。添加到应用中的单个 Flutter 页面可以是不透明的普通页面，也可以是透明的页面。这两种页面的使用都会在本指南中提到。

This guide describes how to add a single Flutter screen to an existing Android app. A Flutter screen can be added as a normal, opaque screen, or as a see-through, translucent screen. Both options are described in this guide.

添加一个普通的 Flutter 页面

Add a normal Flutter screen

步骤 1：在 AndroidManifest.xml 中添加 FlutterActivity

Step 1: Add FlutterActivity to AndroidManifest.xml

Flutter 提供了 FlutterActivity，用于在 Android 应用内部展示一个 Flutter 的交互界面。和其他的 Activity 一样，FlutterActivity 必须在项目的 AndroidManifest.xml 文件中注册。将下边的 XML 代码添加到你的 AndroidManifest.xml 文件中的 application 标签内：

Flutter provides FlutterActivity to display a Flutter experience within an Android app. Like any other Activity, FlutterActivity must be registered in your AndroidManifest.xml. Add the following XML to your AndroidManifest.xml file under your application tag:

<activity
  android:name="io.flutter.embedding.android.FlutterActivity"
  android:theme="@style/LaunchTheme"
  android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection|fontScale|screenLayout|density|uiMode"
  android:hardwareAccelerated="true"
  android:windowSoftInputMode="adjustResize"
  />

上述代码中的 @style/LaunchTheme 可以替换为想要在你的 FlutterActivity 中使用的其他 Android 主题。主题的选择决定 Android 系统展示框架所使用的颜色，例如 Android 的导航栏，以及 Flutter UI 自身的第一次渲染前 FlutterActivity 的背景色。

The reference to @style/LaunchTheme can be replaced by any Android theme that want to apply to your FlutterActivity. The choice of theme dictates the colors applied to Android’s system chrome, like Android’s navigation bar, and to the background color of the FlutterActivity just before the Flutter UI renders itself for the first time.

步骤 2：加载 FlutterActivity

Step 2: Launch FlutterActivity

在你的清单文件中注册了 FlutterActivity 之后，根据需要，你可以在应用中的任意位置添加打开 FlutterActivity 的代码。下边的代码展示了如何在 OnClickListener 的点击事件中打开 FlutterActivity。

With FlutterActivity registered in your manifest file, add code to launch FlutterActivity from whatever point in your app that you’d like. The following example shows FlutterActivity being launched from an OnClickListener.

• Java
• Kotlin

myButton.setOnClickListener(new OnClickListener() {
  @Override
  public void onClick(View v) {
    startActivity(
      FlutterActivity.createDefaultIntent(currentActivity)
    );
  }
});

myButton.setOnClickListener {
  startActivity(
    FlutterActivity.createDefaultIntent(this)
  )
}

上述的例子假定了你的 Dart 代码入口是调用 main()，并且你的 Flutter 初始路由是 ‘/’。 Dart 代码入口不能通过 Intent 改变，但是初始路由可以通过 Intent 来修改。下面的例子讲解了如何打开一个自定义 Flutter 初始路由的 FlutterActivity。

The previous example assumes that your Dart entrypoint is called main(), and your initial Flutter route is ‘/’. The Dart entrypoint can’t be changed using Intent, but the initial route can be changed using Intent. The following example demonstrates how to launch a FlutterActivity that initially renders a custom route in Flutter.

• Java
• Kotlin

myButton.addOnClickListener(new OnClickListener() {
  @Override
  public void onClick(View v) {
    startActivity(
      FlutterActivity
        .withNewEngine()
        .initialRoute("/my_route")
        .build(currentActivity)
      );
  }
});

myButton.setOnClickListener {
  startActivity(
    FlutterActivity
      .withNewEngine()
      .initialRoute("/my_route")
      .build(this)
  )
}

可以用你想要的初始路由替换掉 "/my_route"。

Replace "/my_route" with your desired initial route.

工厂方法 withNewEngine() 可以用于配置一个 FlutterActivity，它会在内部创建一个属于自己的 FlutterEngine 实例，这会有一个明显的初始化时间。另外一种可选的做法是让 FlutterActivity 使用一个预热且缓存的 FlutterEngine，这可以最小化 Flutter 初始化的时间。这种方式接下来会讨论到。

The use of the withNewEngine() factory method configures a FlutterActivity that internally create its own FlutterEngine instance. This comes with a non-trivial initialization time. The alternative approach is to instruct FlutterActivity to use a pre-warmed, cached FlutterEngine, which minimizes Flutter’s initialization time. That approach is discussed next.

步骤 3：（可选）使用缓存的 FlutterEngine

Step 3: (Optional) Use a cached FlutterEngine

每一个 FlutterActivity 默认会创建它自己的 FlutterEngine。每一个 FlutterEngine 会有一个明显的预热时间。这意味着加载一个标准的 FlutterActivity 时，在你的 Flutter 交互页面可见之前会有一个短暂的延迟。想要最小化这个延迟时间，你可以在抵达你的 FlutterActivity 之前，初始化一个 FlutterEngine，然后使用这个已经预热好的 FlutterEngine。

Every FlutterActivity creates its own FlutterEngine by default. Each FlutterEngine has a non-trivial warm-up time. This means that launching a standard FlutterActivity comes with a brief delay before your Flutter experience becomes visible. To minimize this delay, you can warm up a FlutterEngine before arriving at your FlutterActivity, and then you can use your pre-warmed FlutterEngine instead.

要预热一个 FlutterEngine，可以在你的应用中找一个合理的地方实例化一个 FlutterEngine。下面的这个例子是在 Application 类中预先初始化一个 FlutterEngine：

To pre-warm a FlutterEngine, find a reasonable location in your app to instantiate a FlutterEngine. The following example arbitrarily pre-warms a FlutterEngine in the Application class:

• Java
• Kotlin

public class MyApplication extends Application {
  public FlutterEngine flutterEngine;
  
  @Override
  public void onCreate() {
    super.onCreate();
    // Instantiate a FlutterEngine.
    flutterEngine = new FlutterEngine(this);

    // Start executing Dart code to pre-warm the FlutterEngine.
    flutterEngine.getDartExecutor().executeDartEntrypoint(
      DartEntrypoint.createDefault()
    );

    // Cache the FlutterEngine to be used by FlutterActivity.
    FlutterEngineCache
      .getInstance()
      .put("my_engine_id", flutterEngine);
  }
}

class MyApplication : Application() {
  lateinit var flutterEngine : FlutterEngine

  override fun onCreate() {
    super.onCreate()

    // Instantiate a FlutterEngine.
    flutterEngine = FlutterEngine(this)

    // Start executing Dart code to pre-warm the FlutterEngine.
    flutterEngine.dartExecutor.executeDartEntrypoint(
      DartExecutor.DartEntrypoint.createDefault()
    )

    // Cache the FlutterEngine to be used by FlutterActivity.
    FlutterEngineCache
      .getInstance()
      .put("my_engine_id", flutterEngine)
  }
}

传给 FlutterEngineCache 的 ID 可以是你想要的任何名称。确保 FlutterActivity 或 FlutterFragment 在使用缓存的 FlutterEngine 时，传递了同样的 ID。基于缓存的 FlutterEngine 来使用 FlutterActivity 会在后续讨论到。

The ID passed to the FlutterEngineCache can be whatever you want. Make sure that you pass the same ID to any FlutterActivity or FlutterFragment that should use the cached FlutterEngine. Using FlutterActivity with a cached FlutterEngine is discussed next.

要使用预热且缓存的 FlutterEngine 时，让你的 FlutterActivity 从缓存中获取 FlutterEngine，而不是创建一个新的。可以使用 FlutterActivity 的 withCachedEngine() 方法来实现：

With a pre-warmed, cached FlutterEngine, you now need to instruct your FlutterActivity to use the cached FlutterEngine instead of creating a new one. To accomplish this, use FlutterActivity’s withCachedEngine() builder:

• Java
• Kotlin

myButton.addOnClickListener(new OnClickListener() {
  @Override
  public void onClick(View v) {
    startActivity(
      FlutterActivity
        .withCachedEngine("my_engine_id")
        .build(currentActivity)
      );
  }
});

myButton.setOnClickListener {
  startActivity(
    FlutterActivity
      .withCachedEngine("my_engine_id")
      .build(this)
  )
}

当使用 withCachedEngine() 方法时，传递你缓存对应 FlutterEngine 时用的相同 ID。

When using the withCachedEngine() factory method, pass the same ID that you used when caching the desired FlutterEngine.

现在，当你加载 FlutterActivity 时，在展示 Flutter 内容前的延迟会明显降低。

Now, when you launch FlutterActivity, there is significantly less delay in the display of Flutter content.

Initial route with a cached engine

当配置一个使用新 FlutterEngine 的 FlutterActivity 或者 FlutterFragment 时，会使用到初始路由的概念。但是，使用缓存中的 Flutter 引擎时， FlutterActivity 或者 FlutterFragment 则没有涉及初始路由的概念。这是因为被缓存的引擎理论上已经执行了 Dart 代码，在这时配置初始路由已经太迟了。

The concept of an initial route is available when configuring a FlutterActivity or a FlutterFragment with a new FlutterEngine. However, FlutterActivity and FlutterFragment don’t offer the concept of an initial route when using a cached engine. This is because a cached engine is expected to already be running Dart code, which means it’s too late to configure the initial route.

开发者如果想要让缓存中的引擎从自定义的初始路由开始运行，那么可以执行 Dart 入口前，为缓存的 FlutterEngine 配置自定义的初始路由。如下面这个例子：

Developers that would like their cached engine to begin with a custom initial route can configure their cached FlutterEngine to use a custom initial route just before executing the Dart entrypoint. The following example demonstrates the use of an initial route with a cached engine:

• Java
• Kotlin

public class MyApplication extends Application {
  @Override
  public void onCreate() {
    super.onCreate();
    // Instantiate a FlutterEngine.
    flutterEngine = new FlutterEngine(this);
    // Configure an initial route.
    flutterEngine.getNavigationChannel().setInitialRoute("your/route/here");
    // Start executing Dart code to pre-warm the FlutterEngine.
    flutterEngine.getDartExecutor().executeDartEntrypoint(
      DartEntrypoint.createDefault()
    );
    // Cache the FlutterEngine to be used by FlutterActivity or FlutterFragment.
    FlutterEngineCache
      .getInstance()
      .put("my_engine_id", flutterEngine);
  }
}

class MyApplication : Application() {
  lateinit var flutterEngine : FlutterEngine
  override fun onCreate() {
    super.onCreate()
    // Instantiate a FlutterEngine.
    flutterEngine = FlutterEngine(this)
    // Configure an initial route.
    flutterEngine.navigationChannel.setInitialRoute("your/route/here");
    // Start executing Dart code to pre-warm the FlutterEngine.
    flutterEngine.dartExecutor.executeDartEntrypoint(
      DartExecutor.DartEntrypoint.createDefault()
    )
    // Cache the FlutterEngine to be used by FlutterActivity or FlutterFragment.
    FlutterEngineCache
      .getInstance()
      .put("my_engine_id", flutterEngine)
  }
}

通过设置导航通道中的初始路由，会让关联的 FlutterEngine 在 runApp() 方法首次执行后，展示已配置的路由页面。

By setting the initial route of the navigation channel, the associated FlutterEngine displays the desired route upon initial execution of the runApp() Dart function.

在 runApp() 的首次执行之后，修改导航通道中的初始路由属性是不会生效的。想要在不同的 Activity 和 Fragment 之间使用同一个 FlutterEngine，并且在其展示时切换不同的路由，开发者需要设置一个方法通道，来显式地通知他们的 Dart 代码切换 Navigator 路由。

Changing the initial route property of the navigation channel after the initial execution of runApp() has no effect. Developers who would like to use the same FlutterEngine between different Activitys and Fragments and switch the route between those displays need to setup a method channel and explicitly instruct their Dart code to change Navigator routes.

Add a translucent Flutter screen

大部分的全屏 Flutter 交互页面是不透明的。但是，一些应用可能会发布一个类似模态框的 Flutter 页面，例如，一个对话框或者底部工作表。 Flutter 默认支持透明的 FlutterActivity。

Most full-screen Flutter experiences are opaque. However, some apps would like to deploy a Flutter screen that looks like a modal, for example, a dialog or bottom sheet. Flutter supports translucent FlutterActivitys out of the box.

要将你的 FlutterActivity 设置为透明，在创建和加载 FlutterActivity 的常规步骤中做如下的变更。

To make your FlutterActivity translucent, make the following changes to the regular process of creating and launching a FlutterActivity.

步骤 1：使用透明的主题

Step 1: Use a theme with translucency

Android 需要一个特殊的主题属性来让 Activity 以一个透明的背景渲染。使用如下属性来创建或者修改一个 Android 主题：

Android requires a special theme property for Activitys that render with a translucent background. Create or update an Android theme with the following property:

<style name="MyTheme" parent="@style/MyParentTheme">
  <item name="android:windowIsTranslucent">true</item>
</style>

然后，将透明主题应用到你的 FlutterActivity。

Then, apply the translucent theme to your FlutterActivity.

<activity
  android:name="io.flutter.embedding.android.FlutterActivity"
  android:theme="@style/MyTheme"
  android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection|fontScale|screenLayout|density|uiMode"
  android:hardwareAccelerated="true"
  android:windowSoftInputMode="adjustResize"
  />

现在你的 FlutterActivity 已经支持透明化。下一步，你需要在打开 FlutterActivity 时显式启用透明模式。

Your FlutterActivity now supports translucency. Next, you need to launch your FlutterActivity with explicit transparency support.

步骤 2：启动透明的 FlutterActivity

Step 2: Start FlutterActivity with transparency

要打开透明背景的 FlutterActivity，需要把对应的 BackgroundMode 传递给 IntentBuilder：

To launch your FlutterActivity with a transparent background, pass the appropriate BackgroundMode to the IntentBuilder:

• Java
• Kotlin

// Using a new FlutterEngine.
startActivity(
  FlutterActivity
    .withNewEngine()
    .backgroundMode(FlutterActivityLaunchConfigs.BackgroundMode.transparent)
    .build(context)
);

// Using a cached FlutterEngine.
startActivity(
  FlutterActivity
    .withCachedEngine("my_engine_id")
    .backgroundMode(FlutterActivityLaunchConfigs.BackgroundMode.transparent)
    .build(context)
);

// Using a new FlutterEngine.
startActivity(
  FlutterActivity
    .withNewEngine()
    .backgroundMode(FlutterActivityLaunchConfigs.BackgroundMode.transparent)
    .build(this)
);

// Using a cached FlutterEngine.
startActivity(
  FlutterActivity
    .withCachedEngine("my_engine_id")
    .backgroundMode(FlutterActivityLaunchConfigs.BackgroundMode.transparent)
    .build(this)
);

现在你的 FlutterAcivity 的背景已经是透明的了。

You now have a FlutterActivity with a transparent background.