Reuse the plugin system without depending on the Lumino Widgets

[jtpio]

Problem

The Lumino plugin system is simple and powerful. It could also be used for other purposes that don't necessarily provide UI elements to the interface.

However the Lumino Application currently requires a T extends Widget = Widget as the generic type for a shell:

lumino/packages/application/src/index.ts

Line 138 in 613dc9d

export class Application<T extends Widget = Widget> {

Proposed Solution

Make it possible to use the plugin and dependency injection system without a shell.

Additional context

This was discussed with @echarles during JupyterCon 2023.

For reference JupyterLite uses a Lumino application without a shell by providing never as the generic type: https://github.com/jupyterlite/jupyterlite/blob/f98c5c0f90f25466aea6beeb4f2bd2ce8549aa68/packages/server/src/app.ts#L17

export class JupyterLiteServer extends Application<never>

[jtpio]

@echarles if you have more examples or use cases feel free to add them here, thanks!

[echarles]

Thx @jtpio for logging this. I can share for now https://github.com/datalayer-examples/jupyterlab-extensions-example/blob/main/test/lumino-inject.spec.ts which aims to showcase generic dependency injection based on Lumino. I will feed more there and document what could be done if we would abstract away from Widget.

[fcollonval]

We could provide an raw ABCApplication than encapsulated only the plugin logic without command registry, context menu and shell.
Or simply template without restrictions the context menu and the shell.