MissingPluginException for video_player_win when using desktop_multi_window on Windows

[nnovation]

Flutter 3.35.4
Tools • Dart 3.9.2 • DevTools 2.48.0
OS: Windows 10 / 11
desktop_multi_window: ^0.2.1
video_player_win: ^3.2.0
video_player: ^2.9.2

Video playback works in the main window but fails in the secondary one.
It seems the plugin is not registered in the new Flutter engine created by desktop_multi_window.

When opening a second window with desktop_multi_window, using video_player_win causes this error:

[ERROR:flutter/runtime/dart_vm_initializer.cc(40)] Unhandled Exception: MissingPluginException(No implementation found for method clearAll on channel video_player_win)
#0 MethodChannel._invokeMethod (package:flutter/src/services/platform_channel.dart:365:7)
platform_channel.dart:365

[log] controller.initialize() error occurs: MissingPluginException(No implementation found for method openVideo on channel video_player_win)

[stuartmorgan-g]

To provide some context (expanding on my comments closing the linked flutter/flutter issue): this error is happening because no handler is registered for the channel video_player_win on the engine instance running the Dart code for the secondary window.

The reason for this is that plugins are per-engine, and this plugin is (indirectly) creating new engine instances when it makes new windows. In a standard Flutter application, the app template registers all the project's plugins with the template-created engine, and that registration step is where the plugin in instantiated for the engine—and in most plugins, where the plugin registers any channel listeners with that engine. No comparable step is happening for the engines created by this plugin, so none of the project's plugins have the native code registered with the new engine instances.

The standard way for plugins that create engines (e.g., plugins that run background engines on iOS) to manage this is to provide a native method that plugin clients can call from their native application code that provides a callback taking an engine argument, which the client can implement to register plugins (usually by calling the same method the app template calls, so in the case of Windows, the RegisterPlugins method linked above).

[boyan01]

the same as #438