添加 Flutter Fragment | Flutter 框架

本指南介绍了如何将 Flutter Fragment 添加到现有 Android 应用。在 Android 中，Fragment 代表着一个更大 UI 的模块化部分。Fragment 可用于呈现滑动抽屉、选项卡内容、ViewPager 中的页面，或者它可能只是单 Activity 应用中的一个普通屏幕。Flutter 提供了一个 FlutterFragment，以便开发者可以在任何使用常规 Fragment 的地方呈现 Flutter 体验。

如果 Activity 同样适用于你的应用需求，考虑使用 FlutterActivity 而不是 FlutterFragment，因为它更快、更易于使用。

FlutterFragment 允许开发者控制 Flutter 体验在 Fragment 内的以下细节：

Flutter 初始路由
要执行的 Dart 入口点
不透明与半透明背景
FlutterFragment 是否应控制其周围的 Activity
是应使用新的 FlutterEngine 还是缓存的 FlutterEngine

FlutterFragment 还附带了一些必须从其周围 Activity 转发的调用。这些调用允许 Flutter 适当地响应操作系统事件。

本指南介绍了所有类型的 FlutterFragment 及其要求。

要使用 FlutterFragment，首先要将其添加到宿主 Activity。

要将 FlutterFragment 添加到宿主 Activity，请在 Activity 的 onCreate() 中或应用可行的其他时间实例化并附着一个 FlutterFragment 实例。

Kotlin
Java

MyActivity.kt
kotlinclass 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()
    }
  }
}

MyActivity.java
javapublic 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();
        }
    }
}

上述代码足以渲染一个 Flutter UI，该 UI 从调用你的 main() Dart 入口点开始，初始 Flutter 路由为 /，并使用一个新的 FlutterEngine。然而，此代码不足以实现所有预期的 Flutter 行为。Flutter 依赖于必须从宿主 Activity 转发到 FlutterFragment 的各种操作系统信号。这些调用在以下示例中显示：

Kotlin
Java

MyActivity.kt
kotlinclass 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)
  }
}

</string?>

MyActivity.java
javapublic 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);
    }
}

随着操作系统信号转发到 Flutter，你的 FlutterFragment 将按预期工作。你现在已将 FlutterFragment 添加到现有 Android 应用中。

最简单的集成路径是使用一个新的 FlutterEngine，它会带来不小的初始化时间，导致在 Flutter 首次初始化和渲染之前出现空白 UI。大部分时间开销可以通过使用缓存的预热 FlutterEngine 来避免，这将在下文讨论。

默认情况下，FlutterFragment 会创建自己的 FlutterEngine 实例，这需要不小的预热时间。这意味着你的用户会在短时间内看到一个空白 Fragment。你 H可以通过使用现有、预热的 FlutterEngine 实例来缓解大部分预热时间。

要在 FlutterFragment 中使用预热的 FlutterEngine，请使用 withCachedEngine() 工厂方法实例化 FlutterFragment。

Kotlin
Java

MyApplication.kt
kotlin// 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)

MyActivity.java
kotlinFlutterFragment.withCachedEngine("my_engine_id").build()

MyApplication.java
java// 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);

MyActivity.java
javaFlutterFragment.withCachedEngine("my_engine_id").build();

FlutterFragment 内部了解 FlutterEngineCache 并根据传递给 withCachedEngine() 的 ID 检索预热的 FlutterEngine。

如前所述，通过提供预热的 FlutterEngine，你的应用可以尽快渲染第一个 Flutter 帧。

配置 FlutterActivity 或 FlutterFragment 并使用新的 FlutterEngine 时，可以使用初始路由的概念。但是，当使用缓存引擎时，FlutterActivity 和 FlutterFragment 不提供初始路由的概念。这是因为缓存引擎预计已经运行了 Dart 代码，这意味着配置初始路由为时已晚。

希望其缓存引擎以自定义初始路由开始的开发者可以在执行 Dart 入口点之前配置其缓存的 FlutterEngine 以使用自定义初始路由。以下示例演示了缓存引擎的初始路由用法：

Kotlin
Java

MyApplication.kt
kotlinclass 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)
  }
}

MyApplication.java
javapublic 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);
  }
}

通过设置导航通道的初始路由，关联的 FlutterEngine 会在首次执行 runApp() Dart 函数时显示所需的路由。

在首次执行 runApp() 后更改导航通道的初始路由属性无效。希望在不同的 Activity 和 Fragment 之间使用同一个 FlutterEngine 并切换这些显示之间的路由的开发者需要设置一个方法通道并明确指示其 Dart 代码更改 Navigator 路由。

即使使用预热的 FlutterEngine，Flutter 内容的初始显示仍需要一些等待时间。为了帮助改善此短暂等待期间的用户体验，Flutter 支持显示启动屏幕（也称为“闪屏”），直到 Flutter 渲染其第一个帧。有关如何显示启动屏幕的说明，请参阅启动屏幕指南。

一个 Android 应用可能包含许多独立的 Flutter 体验，在不同的 FlutterFragment 中运行，使用不同的 FlutterEngine。在这些场景中，每个 Flutter 体验通常会以不同的初始路由（除 / 之外的路由）开始。为了方便这一点，FlutterFragment 的 Builder 允许你指定所需的初始路由，如下所示：

Kotlin
Java

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

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

类似于不同的初始路由，不同的 FlutterFragment 可能希望执行不同的 Dart 入口点。在典型的 Flutter 应用中，只有一个 Dart 入口点：main()，但你可以定义其他入口点。

FlutterFragment 支持指定给定 Flutter 体验要执行的所需 Dart 入口点。要指定入口点，请构建 FlutterFragment，如下所示：

Kotlin
Java

MyActivity.kt
kotlinval flutterFragment = FlutterFragment.withNewEngine()
    .dartEntrypoint("mySpecialEntrypoint")
    .build()

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

FlutterFragment 配置会导致执行一个名为 mySpecialEntrypoint() 的 Dart 入口点。请注意，dartEntrypoint 字符串名称中不包含括号 ()。

FlutterFragment 可以使用 SurfaceView 来渲染其 Flutter 内容，也可以使用 TextureView。默认是 SurfaceView，它的性能明显优于 TextureView。然而，SurfaceView 不能穿插在 Android View 层次结构的中间。SurfaceView 必须是层次结构中最底层的 View 或最顶层的 View。此外，在 Android N 之前的 Android 版本上，SurfaceView 无法动画化，因为它们的布局和渲染与 View 层次结构的其他部分不同步。如果你的应用需要这些用例中的任何一个，那么你需要使用 TextureView 而不是 SurfaceView。通过使用 texture RenderMode 构建 FlutterFragment 来选择 TextureView：

Kotlin
Java

MyActivity.kt
kotlin// 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()

MyActivity.java
java// 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();

使用所示配置，生成的 FlutterFragment 将其 UI 渲染到 TextureView。

默认情况下，FlutterFragment 使用 SurfaceView 以不透明背景渲染。（请参阅“控制 FlutterFragment 的渲染模式”。）对于任何未由 Flutter 绘制的像素，该背景是黑色的。出于性能原因，使用不透明背景渲染是首选渲染模式。Flutter 在 Android 上透明渲染会对性能产生负面影响。然而，有许多设计要求 Flutter 体验中存在透明像素，这些像素会透过显示底层的 Android UI。因此，Flutter 在 FlutterFragment 中支持半透明。

要为 FlutterFragment 启用透明度，请使用以下配置构建它：

Kotlin
Java

MyActivity.kt
kotlin// 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()

MyActivity.java
java// 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();

有些应用选择使用 Fragment 作为完整的 Android 屏幕。在这些应用中，Fragment 控制系统 UI（如 Android 的状态栏、导航栏和方向）是合理的。

在其他应用中，Fragment 仅用于表示 UI 的一部分。FlutterFragment 可以用于实现抽屉内部、视频播放器或单个卡片。在这些情况下，FlutterFragment 影响 Android 的系统 UI 将是不合适的，因为同一 Window 中还有其他 UI 部分。

FlutterFragment 带有一个概念，有助于区分 FlutterFragment 应该能够控制其宿主 Activity 的情况，以及 FlutterFragment 应该只影响自身行为的情况。为了防止 FlutterFragment 将其 Activity 暴露给 Flutter 插件，并防止 Flutter 控制 Activity 的系统 UI，请在 FlutterFragment 的 Builder 中使用 shouldAttachEngineToActivity() 方法，如下所示：

Kotlin
Java

MyActivity.kt
kotlin// 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()

MyActivity.java
java// 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();

将 false 传递给 shouldAttachEngineToActivity() Builder 方法可以防止 Flutter 与周围的 Activity 交互。默认值为 true，这允许 Flutter 和 Flutter 插件与周围的 Activity 交互。

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将 Flutter Fragment 添加到 Android 应用

将 `FlutterFragment` 与新的 `FlutterEngine` 一起添加到 `Activity`

使用预热的 `FlutterEngine`

缓存引擎的初始路由

显示启动屏幕

使用指定的初始路由运行 Flutter

从指定的入口点运行 Flutter

控制 `FlutterFragment` 的渲染模式

显示一个具有透明度的 `FlutterFragment`

`FlutterFragment` 与其 `Activity` 的关系

将 FlutterFragment 与新的 FlutterEngine 一起添加到 Activity

使用预热的 FlutterEngine