Navigator 和 Hero Controller Scope 中更嚴格的斷言

當框架檢測到一個 Hero Controller Scope 中註冊了多個 Navigator 時，它會丟擲一個斷言錯誤。

Hero Controller Scope 為其 Widget 子樹託管一個 Hero Controller。Hero Controller 一次只能支援一個 Navigator。以前，並沒有斷言來保證這一點。

如果程式碼在此更改後開始丟擲斷言錯誤，這意味著該程式碼在此更改之前就已經存在問題。同一個 Hero Controller Scope 下可能註冊了多個 Navigator，並且當它們的路由發生變化時，它們無法觸發 Hero 動畫。此更改只是暴露了這個問題。

一個開始丟擲異常的示例應用程式。

dartimport 'package:flutter/material.dart';

void main() {
  runApp(
    MaterialApp(
      builder: (BuildContext context, Widget child) {
        // Builds two parallel navigators. This throws
        // error because both of navigators are under the same
        // hero controller scope created by MaterialApp.
        return Stack(
          children: <Widget>[
            Navigator(
              onGenerateRoute: (RouteSettings settings) {
                return MaterialPageRoute<void>(
                  settings: settings,
                  builder: (BuildContext context) {
                    return const Text('first Navigator');
                  }
                );
              },
            ),
            Navigator(
              onGenerateRoute: (RouteSettings settings) {
                return MaterialPageRoute<void>(
                  settings: settings,
                  builder: (BuildContext context) {
                    return const Text('Second Navigator');
                  }
                );
              },
            ),
          ],
        );
      }
    )
  );
}

您可以透過引入自己的 Hero Controller Scope 來修復此應用程式。

dartimport 'package:flutter/material.dart';

void main() {
  runApp(
    MaterialApp(
      builder: (BuildContext context, Widget child) {
        // Builds two parallel navigators.
        return Stack(
          children: <Widget>[
            HeroControllerScope(
              controller: MaterialApp.createMaterialHeroController(),
              child: Navigator(
                onGenerateRoute: (RouteSettings settings) {
                  return MaterialPageRoute<void>(
                    settings: settings,
                    builder: (BuildContext context) {
                      return const Text('first Navigator');
                    }
                  );
                },
              ),
            ),
            HeroControllerScope(
              controller: MaterialApp.createMaterialHeroController(),
              child: Navigator(
                onGenerateRoute: (RouteSettings settings) {
                  return MaterialPageRoute<void>(
                    settings: settings,
                    builder: (BuildContext context) {
                      return const Text('second Navigator');
                    }
                  );
                },
              ),
            ),
          ],
        );
      }
    )
  );
}