[Android][add_to_app] Limited Support for Sharing the Same Engine Across Multiple FlutterViews

[0xZOne]

Flutter allows users to reuse the same engine with multiple FlutterActivity/FlutterFragment (or FlutterView) through the way of attaching/detaching.

This feature is very useful in the add_to_app scenario, where business logic can share data across different pages(FlutterView) by reusing the same engine.

However, Flutter's support for reusing the same engine with multiple FlutterViews is not very good.

• It does not support users to customize the timing of attaching FlutterActivity/FlutterFragment to the engine.
• Attaching a new Flutter to an engine that has already been attached by another FlutterView will cause issues.

flutter doctor -v

[✓] Flutter (Channel main, 3.12.0-15.0.pre.59, on macOS 13.1 22C65 darwin-x64, locale zh-Hans-CN)
    • Flutter version 3.12.0-15.0.pre.59 on channel main at /Users/zero/fvm/versions/main
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision 5345221701 (3 hours ago), 2023-07-09 21:01:34 -0400
    • Engine revision 09c6ce4243
    • Dart version 3.1.0 (build 3.1.0-285.0.dev)
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[✓] Android toolchain - develop for Android devices (Android SDK version 33.0.2)
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    • Platform android-33, build-tools 33.0.2
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[✓] Android Studio (version 2022.1)
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    • Java version OpenJDK Runtime Environment (build 11.0.15+0-b2043.56-8887301)

[✓] VS Code (version 1.79.2)
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    • Flutter extension version 3.68.0

[✓] Connected device (3 available)
    • zero的iPhone (mobile) • 50131fb4dcef9d96405ec953e272bd5c24468370 • ios            • iOS 16.2 20C65
    • macOS (desktop)      • macos                                    • darwin-x64     • macOS 13.1 22C65 darwin-x64
    • Chrome (web)         • chrome                                   • web-javascript • Google Chrome 114.0.5735.198

[✓] Network resources
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• No issues found!