向 Android 应用中添加闪屏页和启动页
开启 Flutter 的体验需要短暂地等待 Dart 的初始化。一个完整的 Flutter 应用还额外需要 Android 应用程序标准的初始化时间。
Flutter 支持在 Android 应用初始化的时候展示启动页，并且还支持在你的 Flutter 初始化时展示闪屏页。本指南展示如何在 Flutter 编写的 Android 应用中使用启动页和闪屏页。
The beginning of a Flutter experience requires a brief wait while Dart
initializes. Additionally, a full Flutter app requires standard Android app
initialization time. Flutter supports the display of a launch screen
while your Android app initializes, and also supports the display of a splash
screen while your Flutter experience initializes. This guide teaches you how to
use launch screens and splash screens in an Android app with Flutter.
备忘
使用一些策略来最小化 Flutter 初始化相关过程中的等待时间。考虑 [预热 FlutterEngine][pre-warming a FlutterEngine] 以及 [在整个应用中复用 FlutterEngine][re-using a FlutterEngine throughout your app] ，以避免大多数的等待时间。
Strategies are available to minimize wait time related to Flutter
initialization. Consider pre-warming a FlutterEngine and
re-using a FlutterEngine throughout your app to avoid most wait time.
Android 启动页
Android launch screen
当操作系统建立应用程序进程的同时，每个 Android 应用都需要初始化的时间。
Android 提供了 启动页 的概念，用于在应用初始化的过程中展示一个 Drawable。
Every Android app requires initialization time while the operating system sets
up the app’s process. Android provides the concept of a launch screen to
display a Drawable while the app is initializing.
在一个 FlutterActivity 被展示之前，Flutter 提供了展示 Android 启动页的支持。下一节将针对展示 Android 启动页的说明进行讨论。
Flutter provides support for displaying an Android launch screen before showing
a FlutterActivity. The instructions to display an Android launch screen are discussed in the next sections.
声明一个启动主题
Define a launch theme
在 styles.xml 中定义一个主题，将一个 Drawable 配置给该主题的 windowBackground，它将作为启动页被展示。
In styles.xml, define a theme whose windowBackground is set to the
Drawable that should be displayed as the launch screen.
@drawable/launch_background
备忘
默认的 Flutter 项目模版包括了对启动主题和启动背景的声明。
The default Flutter project template includes a definition of a launch theme
and a launch background.
定义一个普通主题
Define a normal theme
在 styles.xml 中定义一个普通主题，当启动页消失后，将其应用在 FlutterActivity 上。普通主题的背景仅仅展示非常短暂的时间，比如当启动页消失后、方向改变或者 Activity 恢复期间。因此对于普通主题的背景颜色，建议使用与 Flutter UI 主要背景颜色相似的纯色。
In styles.xml, define a normal theme to be applied to FlutterActivity after
the launch screen is gone. The normal theme background only shows for a very
brief moment after the splash screen disappears, and during orientation change
and Activity restoration. Therefore, it’s recommended that the normal theme
use a solid background color that looks similar to the primary background color
of the Flutter UI.
@drawable/normal_background
在 AndroidManifest.xml 中配置 FlutterActivity
Setup FlutterActivity in AndroidManifest.xml
在 AndroidManifest.xml 中，将 FlutterActivity 的主题设置为启动主题，将元数据元素添加到所需的 FlutterActivity，以指示 Flutter 在适当的时机从启动主题切换到普通主题。
In AndroidManifest.xml, set the theme of FlutterActivity to the launch
theme. Then, add a metadata element to the desired FlutterActivity to instruct
Flutter to switch from the launch theme to the normal theme at the appropriate time.
android:name=".MyActivity"
android:theme="@style/LaunchTheme"
// ...
>
android:name="io.flutter.embedding.android.NormalTheme"
android:resource="@style/NormalTheme"
/>
现在，Android 应用程序在初始化时会展示对应的启动屏。
The Android app now displays the desired launch screen while the app
initializes.
Flutter闪屏页
Flutter splash screen
应用程序中的每种 Flutter 体验都需要一些时间以初始化运行代码的 Dart isolate。这意味着用户将暂时地看到空白屏幕，直到 Flutter 渲染其第一帧为止。为了提升用户体验，Flutter 提供了在其初始化时展示一个 Android View 作为闪屏页的方式。
Each Flutter experience in an app requires a few moments to initialize the Dart
isolate that runs the code. This means a user momentarily sees a blank screen
until Flutter renders its first frame. Flutter supports an improved user
experience by displaying an Android View as a splash screen while Flutter
initializes.
针对 Flutter 的闪屏页，这里提供了两个可选项。第一个可选项是展示一个你选择的 Drawable，并在其初始化完成后淡出。另外一个可选项是提供一个自定义的闪屏页，它能够展示出任意你想要的 Android View 内容。
Flutter supports two options for a splash screen. The first option is to
display a Drawable of your choice, which fades out after the initialization
is complete. The other option is to provide a custom SplashScreen, which is
capable of displaying any Android View content that you want.
展示Drawable闪屏页
Showing a Drawable splash screen
Drawable 闪屏页可被配置在 FlutterActivity、FlutterFragment 或者 FlutterView 中。
A Drawable splash screen can be configured for a FlutterActivity,
FlutterFragment, or FlutterView.
在FlutterActivity中配置
In a FlutterActivity
在 FlutterActivity 中将一个 Drawable 作为 Flutter 闪屏页进行展示，请将以下元数据添加到 AndroidManifest.xml 中所关联的 FlutterActivity 中。
To display a Drawable as a Flutter splash screen in a FlutterActivity, add
the following metadata to the associated FlutterActivity in
AndroidManifest.xml.
android:name="io.flutter.embedding.android.SplashScreenDrawable"
android:resource="@drawable/my_splash"
/>
为了将闪屏页展示出与启动页相同的效果，在 io.flutter.embedding.android.SplashScreenDrawable meta-data
中引用相同的 @drawable/launch_background。
To display a splash screen with the same visual as a launch screen,
reference the same @drawable/launch_background in the
io.flutter.embedding.android.SplashScreenDrawable meta-data.
在FlutterFragment中配置
In a FlutterFragment
在 FlutterFragment 中将一个 Drawable 作为 Flutter 闪屏页进行展示，创建一个 FlutterFragment
的子类并重写其 provideSplashScreen()。
To display a Drawable as a Flutter splash screen in a FlutterFragment,
make FlutterFragment a subclass and override provideSplashScreen().
public class MyFlutterFragment extends FlutterFragment {
@Override
protected SplashScreen provideSplashScreen() {
// Load the splash Drawable.
Drawable splash = getResources().getDrawable(R.drawable.my_splash);
// Construct a DrawableSplashScreen with the loaded splash Drawable and
// return it.
return new DrawableSplashScreen(splash);
}
}
创建一个自定义的闪屏页
Creating a custom SplashScreen
闪屏页是一个非常好推广品牌的时机。因此，很多团队实现了独特且高度定制化的闪屏体验。为方便起见，Flutter 允许你将任意的 Android View 作为显示屏进行展示，甚至可以控制 Flutter 第一帧渲染完毕之后如何将 View 过渡到 Flutter。
Splash screens are a great branding opportunity. Because of that, many teams
implement unique, highly customized splash experiences. To facilitate this,
Flutter allows you to display an arbitrary Android View as a splash
screen, and even allows you to control how that View transitions to
Flutter after Flutter renders its first frame.
实现一个自定义闪屏View
Implement a custom splash View
首先，定义一个将被作为闪屏页展示的自定义 View。
First, define the custom View that should be displayed as the splash screen.
该 View 可以展示任意内容，从简单的纯色到一个动画，由于可选项太多，因此不提供示例。
This View could display anything, from a simple solid color to an animation. An example isn’t provided because there are too many options.
实现SplashScreen接口
Implement the SplashScreen interface
为已声明的自定义 View 实现 SplashScreen 接口。
With a custom View defined, implement the SplashScreen interface.
本指南展示了两种实现 SplashScreen 的方式。首先，下面是一个 SplashScreen 的示例，它既没有视觉状态，也没有过渡动画。
This guide shows two approaches to a SplashScreen implementation. First, the following
is an example of a SplashScreen that has no visual state and no transition
animation.
public class SimpleSplashScreen implements SplashScreen {
@Override
@Nullable
public View createSplashView(
@NonNull Context context,
@Nullable Bundle savedInstanceState
) {
// Return a new MySplashView without saving a reference, because it
// has no state that needs to be tracked or controlled.
return new MySplashView(context);
}
@Override
public void transitionToFlutter(@NonNull Runnable onTransitionComplete) {
// Immediately invoke onTransitionComplete because this SplashScreen
// doesn't display a transition animation.
//
// Every SplashScreen *MUST* invoke onTransitionComplete at some point
// for the splash system to work correctly.
onTransitionComplete.run();
}
}
第二个示例更为复杂。在此示例中，自定义 SplashScreen 保留了对其自定义 View 的引用，并指示自定义 View 的过渡效果，将 onTransitionComplete 回调传递给自定义 View 进行调用。
The second example is a bit more sophisticated. In this example, the custom
SplashScreen keeps a reference to its custom View and instructs the custom
View to transition away, passing the onTransitionComplete callback to the
custom View to invoke.
public class SplashScreenWithTransition implements SplashScreen {
private MySplashView mySplashView;
@Override
@Nullable
public View createSplashView(
@NonNull Context context,
@Nullable Bundle savedInstanceState
) {
// A reference to the MySplashView is retained so that it can be told
// to transition away at the appropriate time.
mySplashView = new MySplashView(context);
return mySplashView;
}
@Override
public void transitionToFlutter(@NonNull Runnable onTransitionComplete) {
// Instruct MySplashView to animate away in whatever manner it wants.
// The onTransitionComplete Runnable is passed to the MySplashView to be
// invoked when the transition animation is complete.
mySplashView.animateAway(onTransitionComplete);
}
}
对于自定义的闪屏页而言，天空才是极限。实际上，你可以创建一个展示天空动画的闪屏页！享受这个灵活的启动系统所带来的乐趣，并在社区中分享你的创作！
With custom splash screens, the sky is the limit. In fact, you could create a
splash screen that shows an animated sky! Have fun with this flexible splash
system, and share your creations with the community!
