WidgetsFlutterBinding.ensureInitialized() 有什么作用

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await Firebase.initializeApp();
  runApp(MyApp());
}

WidgetFlutterBinding是用来与Flutter 引擎交互的

Firebase.initializeApp()需要调用本机代码来初始化Firebase，并且由于插件需要使用平台通道来调用本机代码(这是异步完成的)，因此您必须调用ensureInitialized()来确保您拥有WidgetsBinding的实例

WidgetsFlutterBinding 的源码, 一个简单的单例

/// A concrete binding for applications based on the Widgets framework.
///
/// This is the glue that binds the framework to the Flutter engine.
///
/// When using the widgets framework, this binding, or one that
/// implements the same interfaces, must be used. The following
/// mixins are used to implement this binding:
///
/// * [GestureBinding], which implements the basics of hit testing.
/// * [SchedulerBinding], which introduces the concepts of frames.
/// * [ServicesBinding], which provides access to the plugin subsystem.
/// * [PaintingBinding], which enables decoding images.
/// * [SemanticsBinding], which supports accessibility.
/// * [RendererBinding], which handles the render tree.
/// * [WidgetsBinding], which handles the widget tree.
class WidgetsFlutterBinding extends BindingBase with GestureBinding, SchedulerBinding, ServicesBinding, PaintingBinding, SemanticsBinding, RendererBinding, WidgetsBinding {
  /// Returns an instance of the binding that implements
  /// [WidgetsBinding]. If no binding has yet been initialized, the
  /// [WidgetsFlutterBinding] class is used to create and initialize
  /// one.
  ///
  /// You only need to call this method if you need the binding to be
  /// initialized before calling [runApp].
  ///
  /// In the `flutter_test` framework, [testWidgets] initializes the
  /// binding instance to a [TestWidgetsFlutterBinding], not a
  /// [WidgetsFlutterBinding]. See
  /// [TestWidgetsFlutterBinding.ensureInitialized].
  static WidgetsBinding ensureInitialized() {
    if (WidgetsBinding._instance == null) {
      WidgetsFlutterBinding();
    }
    return WidgetsBinding.instance;
  }
}