向 Android 应用中添加 Flutter Fragment

使用新的 FlutterEngine 向 Activity 中添加 FlutterFragment
使用预热的 FlutterEngine
- 缓存引擎中的初始路由
展示闪屏页
指定 Flutter 运行的初始路由
指定 Flutter 运行的入口
控制 FlutterFragment 的渲染模式
展示透明的 FlutterFragment
FlutterFragment 与其 Activity 之间的关系

Add Flutter Fragment Header

本篇指南介绍如何向一个现有的 Android 应用中添加 Flutter Fragment。在 Android 开发中，一个 Fragment 代表了一块较大的模块化 UI。 Fragment 可能被用来展示滑动抽屉、标签内容和 ViewPager 中的页面，或者在单 Activity 应用中，Fragment 可能仅代表正常的屏幕内容。 Flutter 提供了FlutterFragment，以便于开发者们可以在任何使用常规 Fragment 的地方呈现 Flutter 的内容。

如果 Activity 同样适用于您的应用需求，可以考虑使用 FlutterActivity 而非 FlutterFragment，前者更加快捷易用。

FlutterFragment 允许开发者在 Fragment 中控制以下 Flutter 的开发细节：

Flutter 初始路由
将要执行的 Dart 入口
非透明或者透明的背景
FlutterFragment 是否能控制它外层的 Activity
使用新的还是缓存的 FlutterEngine

FlutterFragment 还提供了一些回调事件，这些回调必须由它所在的 Activity 触发执行。这些回调允许 Flutter 适时地响应一些系统事件。

这篇指南介绍了 FlutterFragment 的所有使用方式和使用要求。

使用新的 `FlutterEngine` 向 `Activity` 中添加 `FlutterFragment`

使用 FlutterFragment 的第一步是将其添加进宿主 Activity。

要向宿主 Activity 中添加 FlutterFragment，需要在 Activity 的 onCreate() 或者其它合适的地方，实例化 FlutterFragment 并且与 Activity 绑定。

Java
Kotlin

public class MyActivity extends FragmentActivity {
    // Define a tag String to represent the FlutterFragment within this
    // Activity's FragmentManager. This value can be whatever you'd like.
    private static final String TAG_FLUTTER_FRAGMENT = "flutter_fragment";

    // Declare a local variable to reference the FlutterFragment so that you
    // can forward calls to it later.
    private FlutterFragment flutterFragment;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        // Inflate a layout that has a container for your FlutterFragment.
        // For this example, assume that a FrameLayout exists with an ID of
        // R.id.fragment_container.
        setContentView(R.layout.my_activity_layout);

        // Get a reference to the Activity's FragmentManager to add a new
        // FlutterFragment, or find an existing one.
        FragmentManager fragmentManager = getSupportFragmentManager();

        // Attempt to find an existing FlutterFragment,
        // in case this is not the first time that onCreate() was run.
        flutterFragment = (FlutterFragment) fragmentManager
            .findFragmentByTag(TAG_FLUTTER_FRAGMENT);

        // Create and attach a FlutterFragment if one does not exist.
        if (flutterFragment == null) {
            flutterFragment = FlutterFragment.createDefault();

            fragmentManager
                .beginTransaction()
                .add(
                    R.id.fragment_container,
                    flutterFragment,
                    TAG_FLUTTER_FRAGMENT
                )
                .commit();
        }
    }
}

class MyActivity : FragmentActivity() {
  companion object {
    // Define a tag String to represent the FlutterFragment within this
    // Activity's FragmentManager. This value can be whatever you'd like.
    private const val TAG_FLUTTER_FRAGMENT = "flutter_fragment"
  }

  // Declare a local variable to reference the FlutterFragment so that you
  // can forward calls to it later.
  private var flutterFragment: FlutterFragment? = null

  override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)

    // Inflate a layout that has a container for your FlutterFragment. For
    // this example, assume that a FrameLayout exists with an ID of
    // R.id.fragment_container.
    setContentView(R.layout.my_activity_layout)

    // Get a reference to the Activity's FragmentManager to add a new
    // FlutterFragment, or find an existing one.
    val fragmentManager: FragmentManager = supportFragmentManager

    // Attempt to find an existing FlutterFragment, in case this is not the
    // first time that onCreate() was run.
    flutterFragment = fragmentManager
      .findFragmentByTag(TAG_FLUTTER_FRAGMENT) as FlutterFragment?

    // Create and attach a FlutterFragment if one does not exist.
    if (flutterFragment == null) {
      var newFlutterFragment = FlutterFragment.createDefault()
      flutterFragment = newFlutterFragment
      fragmentManager
        .beginTransaction()
        .add(
          R.id.fragment_container,
          newFlutterFragment,
          TAG_FLUTTER_FRAGMENT
        )
        .commit()
    }
  }
}

上面的代码会以 main() 为 Dart 入口函数， / 为初始路由，并使用新的 FlutterEngine，能够正确渲染出 Flutter UI。但是，这些代码还无法使 Flutter 如预期一样完全正常地工作。 Flutter 依赖操作系统的各种信号，这些信号必须通过宿主 Activity 发送到 FlutterFragment 中。下面的示例展示了这些系统回调：

Java
Kotlin

public class MyActivity extends FragmentActivity {
    @Override
    public void onPostResume() {
        super.onPostResume();
        flutterFragment.onPostResume();
    }

    @Override
    protected void onNewIntent(@NonNull Intent intent) {
        flutterFragment.onNewIntent(intent);
    }

    @Override
    public void onBackPressed() {
        flutterFragment.onBackPressed();
    }

    @Override
    public void onRequestPermissionsResult(
        int requestCode,
        @NonNull String[] permissions,
        @NonNull int[] grantResults
    ) {
        flutterFragment.onRequestPermissionsResult(
            requestCode,
            permissions,
            grantResults
        );
    }

    @Override
    public void onActivityResult(
        int requestCode,
        int resultCode,
        @Nullable Intent data
    ) {
        super.onActivityResult(requestCode, resultCode, data);
        flutterFragment.onActivityResult(
            requestCode,
            resultCode,
            data
        );
    }

    @Override
    public void onUserLeaveHint() {
        flutterFragment.onUserLeaveHint();
    }

    @Override
    public void onTrimMemory(int level) {
        super.onTrimMemory(level);
        flutterFragment.onTrimMemory(level);
    }
}

class MyActivity : FragmentActivity() {
  override fun onPostResume() {
    super.onPostResume()
    flutterFragment!!.onPostResume()
  }

  override fun onNewIntent(@NonNull intent: Intent) {
    flutterFragment!!.onNewIntent(intent)
  }

  override fun onBackPressed() {
    flutterFragment!!.onBackPressed()
  }

  override fun onRequestPermissionsResult(
    requestCode: Int,
    permissions: Array<String?>,
    grantResults: IntArray
  ) {
    flutterFragment!!.onRequestPermissionsResult(
      requestCode,
      permissions,
      grantResults
    )
  }

  override fun onActivityResult(
    requestCode: Int,
    resultCode: Int,
    data: Intent?
  ) {
    super.onActivityResult(requestCode, resultCode, data)
    flutterFragment!!.onActivityResult(
      requestCode,
      resultCode,
      data
    )
  }

  override fun onUserLeaveHint() {
    flutterFragment!!.onUserLeaveHint()
  }

  override fun onTrimMemory(level: Int) {
    super.onTrimMemory(level)
    flutterFragment!!.onTrimMemory(level)
  }
}

随着 OS 信号传递到 Flutter，您的 FlutterFragment 可以如预期正常工作。现在可以尝试将 FlutterFragment 添加进您的 Android 应用了。

使用新的 FlutterEngine 是最简单的集成方式，但是会存在一段明显的初始化时间，此时，在 Flutter 初始化和首次渲染完成之前会出现短暂的白屏。使用缓存、预热的 FlutterEngine 则可以避免上述的大部分耗时，下面我们将讨论这些内容。

使用预热的 `FlutterEngine`

默认情况下，FlutterFragment 会创建它自己的 FlutterEngine 实例，同时也需要不少的启动时间。这就意味着您的用户会看到短暂的白屏。通过使用已存在的、预热的 FlutterEngine 就可以大幅度减少启动的耗时。

要在 FlutterFragment 中使用预热 FlutterEngine，可以使用工厂方法 withCachedEngine() 实例化 FlutterFragment。

Java
Kotlin

// Somewhere in your app, before your FlutterFragment is needed,
// like in the Application class ...
// Instantiate a FlutterEngine.
FlutterEngine flutterEngine = new FlutterEngine(context);

// Start executing Dart code in the FlutterEngine.
flutterEngine.getDartExecutor().executeDartEntrypoint(
    DartEntrypoint.createDefault()
);

// Cache the pre-warmed FlutterEngine to be used later by FlutterFragment.
FlutterEngineCache
  .getInstance()
  .put("my_engine_id", flutterEngine);

FlutterFragment.withCachedEngine("my_engine_id").build();

// Somewhere in your app, before your FlutterFragment is needed,
// like in the Application class ...
// Instantiate a FlutterEngine.
val flutterEngine = FlutterEngine(context)

// Start executing Dart code in the FlutterEngine.
flutterEngine.getDartExecutor().executeDartEntrypoint(
    DartEntrypoint.createDefault()
)

// Cache the pre-warmed FlutterEngine to be used later by FlutterFragment.
FlutterEngineCache
  .getInstance()
  .put("my_engine_id", flutterEngine)

FlutterFragment.withCachedEngine("my_engine_id").build()

FlutterFragment 内部可访问 FlutterEngineCache，并且可以根据传递给 withCachedEngine() 的 ID 获取预热的 FlutterEngine。

如上所示，通过提供预热的 FlutterEngine，您的应用将以最快速度渲染出第一帧。

缓存引擎中的初始路由

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。

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：

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。

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。

展示闪屏页

即使使用了预热的 FlutterEngine，第一次展示 Flutter 的内容仍然需要一些时间。为了更进一步提升用户体验，Flutter 支持在第一帧渲染完成之前展示闪屏页。关于如何展示闪屏页的详细说明，请参阅这篇闪屏页指南。

指定 Flutter 运行的初始路由

一个 Android 应用中可能包含很多独立的 Flutter 界面，这些界面显示在不同的 FlutterFragment 上，每个 FlutterFragment 的 FlutterEngine 也是独立的。在这些情况下，每个 Flutter 界面通过不同的初始路由（除 / 以外的路由）启动是很正常的。为此，FlutterFragment 的 Builder 允许指定一个您希望的初始路由，如下所示：

Java
Kotlin

// With a new FlutterEngine.
FlutterFragment flutterFragment = FlutterFragment.withNewEngine()
    .initialRoute("myInitialRoute/")
    .build();

// With a new FlutterEngine.
val flutterFragment = FlutterFragment.withNewEngine()
    .initialRoute("myInitialRoute/")
    .build()

指定 Flutter 运行的入口

和变化的初始路由类似，不同的 FlutterFragment 可能需要执行不同的 Dart 代码入口。正常的 Flutter 应用中，只会有一个 main() 入口，但是您也可以定义不同的入口。

FlutterFragment 支持指定需要的 Dart 入口以运行对应的 Flutter 界面。下面的代码展示了如何在构建 FlutterFragment 时指定一个入口。

Java
Kotlin

FlutterFragment flutterFragment = FlutterFragment.withNewEngine()
    .dartEntrypoint("mySpecialEntrypoint")
    .build();

val flutterFragment = FlutterFragment.withNewEngine()
    .dartEntrypoint("mySpecialEntrypoint")
    .build()

这里，FlutterFragment 的配置会将 Dart 入口的执行函数设置为 mySpecialEntrypoint()。需要注意的是，括号 () 不包含在 dartEntrypoint 的 String 类型的参数中。

控制 `FlutterFragment` 的渲染模式

FlutterFragment 可以选择使用 SurfaceView 或者 TextureView 来渲染其内容。默认配置的 SurfaceView 在性能上明显好于 TextureView。然而，SurfaceView 无法插入到 Android 的 View 层级之中。 SurfaceView 在视图层级中必须是最底层的 View 或者最顶层的 View。此外，在 Android N 之前，SurfaceView 无法用于制作动画，因为它们的布局和渲染无法和视图层级中的其它 View 同步。如果上述这些用例之一在您的应用需求之中，您需要使用 TextureView 替换 SurfaceView。要选择 TextureView，可以在构建 FlutterFragment 时指定 RenderMode 为 texture：

Java
Kotlin

// With a new FlutterEngine.
FlutterFragment flutterFragment = FlutterFragment.withNewEngine()
    .renderMode(FlutterView.RenderMode.texture)
    .build();

// With a cached FlutterEngine.
FlutterFragment flutterFragment = FlutterFragment.withCachedEngine("my_engine_id")
    .renderMode(FlutterView.RenderMode.texture)
    .build();

// With a new FlutterEngine.
val flutterFragment = FlutterFragment.withNewEngine()
    .renderMode(FlutterView.RenderMode.texture)
    .build()

// With a cached FlutterEngine.
val flutterFragment = FlutterFragment.withCachedEngine("my_engine_id")
    .renderMode(FlutterView.RenderMode.texture)
    .build()

使用上面展示的代码配置，FlutterFragment 可以将它的 UI 渲染为 TextureView。

展示透明的 `FlutterFragment`

默认情况下，FlutterFragment 使用 SurfaceView 渲染且背景不透明。（参考「控制 FlutterFragment 的渲染模式」）任何未经 Flutter 绘制的像素在背景中都是黑色的。出于性能方面的考虑，我们优先选择使用不透明的背景进行渲染。渲染透明的 Flutter 界面在 Android 平台上会产生性能方面的负面影响。但是许多设计都需要 Flutter 界面中包含透明的像素以显示底层的 Android UI。因此，Flutter 支持 FlutterFragment 半透明。

要启动一个透明的 FlutterFragment，可以使用以下方式进行构建：

Java
Kotlin

// Using a new FlutterEngine.
FlutterFragment flutterFragment = FlutterFragment.withNewEngine()
    .transparencyMode(FlutterView.TransparencyMode.transparent)
    .build();

// Using a cached FlutterEngine.
FlutterFragment flutterFragment = FlutterFragment.withCachedEngine("my_engine_id")
    .transparencyMode(FlutterView.TransparencyMode.transparent)
    .build();

// Using a new FlutterEngine.
val flutterFragment = FlutterFragment.withNewEngine()
    .transparencyMode(FlutterView.TransparencyMode.transparent)
    .build()

// Using a cached FlutterEngine.
val flutterFragment = FlutterFragment.withCachedEngine("my_engine_id")
    .transparencyMode(FlutterView.TransparencyMode.transparent)
    .build()

`FlutterFragment` 与其 `Activity` 之间的关系

一些应用选择使用 Fragment 作为整个 Android 屏幕内容。在这些应用里，Fragment 可能会需要控制一些系统属性，例如 Android 的状态栏、导航栏以及屏幕方向。

Fullscreen Flutter

在其它应用中，Fragment 通常只是整个 UI 的一部分。 FlutterFragment 可能用于实现抽屉、视频播放器或卡片的内容。在这些情况下，FlutterFragment 就不应当影响 Android 的系统属性，因为同一个 Window 中还有其它 UI 组件。

Flutter as Partial UI

FlutterFragment 自身包含一种特性，可以用于决定 FlutterFragment 是否应该控制宿主 Activity，或者只影响自身行为。要预防 FlutterFragment 将其 Activity 暴露给 Flutter 插件，以免 Flutter 控制 Activity 的系统 UI，可以使用 FlutterFragment 的 Builder 中的 shouldAttachEngineToActivity() 方法。如下所示：

Java
Kotlin

// Using a new FlutterEngine.
FlutterFragment flutterFragment = FlutterFragment.withNewEngine()
    .shouldAttachEngineToActivity(false)
    .build();

// Using a cached FlutterEngine.
FlutterFragment flutterFragment = FlutterFragment.withCachedEngine("my_engine_id")
    .shouldAttachEngineToActivity(false)
    .build();

// Using a new FlutterEngine.
val flutterFragment = FlutterFragment.withNewEngine()
    .shouldAttachEngineToActivity(false)
    .build()

// Using a cached FlutterEngine.
val flutterFragment = FlutterFragment.withCachedEngine("my_engine_id")
    .shouldAttachEngineToActivity(false)
    .build()

传递 false 给 Builder 的 shouldAttachEngineToActivity() 方法，可防止 Flutter 与所属的 Activity 交互。默认值为 true，此时允许 Flutter 和 Flutter 插件与 Activity 交互。

使用新的 FlutterEngine 向 Activity 中添加 FlutterFragment

使用预热的 FlutterEngine