feat(boot): add helpers to create a booter for component applications

[raymondfeng]

Use case:

• Create a LoopBack application
• Add controllers, services, datasources
• Now use this application as a component with the main applicaiton
• The main application needs to have artifacts from the component application

Checklist

👉 Read and sign the CLA (Contributor License Agreement) 👈

• npm test passes on your machine
• New tests added or existing tests modified to cover all changes
• Code conforms with the style guide
• API Documentation in code was updated
• Documentation in /docs/site was updated
• Affected artifact templates in packages/cli were updated
• Affected example projects in examples/* were updated

👉 Check out how to submit a PR 👈

[jannyHou]

👍

[jannyHou]

A question for !this.mainApp.contains(binding.key):
what if the sub app wants to override the main app's bindings?

[raymondfeng]

There are certain bindings from the sub-app should not override the main app. We should carefully review such bindings - for example, CoreBindings.*, RestBindings.*. Maybe one way to solve it is to mark such bindings locked. If so, we can update this code to only skip locked bindings in the main app.

[jannyHou]

We should carefully review such bindings - for example, CoreBindings.*, RestBindings.*

True, agreed. I came up with this question due to the chat happened on public slack this morning, I think that community user tries to contribute custom artifacts(like models, in his case) from the component.

Maybe one way to solve it is to mark such bindings locked. If so, we can update this code to only skip locked bindings in the main app.

I feel it's a case by case preference, maybe we can provide a default filter to skip those core bindings. And if user really wants to override, they provide custom filters :)

(don't have to address it in this PR)

[raymondfeng]

I improved the binding filtering as follows:

• Bindings that exist in the component application before boot are skipped.
• Locked bindings in the main application will not be overridden.
• Other bindings are selected by the binding filter. By default, all bindings are picked.

[bhupesh-sf]

Documentation for sub application binding in main application doesn't contain any exmaple. That will be helpful

[bajtos]

@raymondfeng I find these instructions quite involved. Would the following approach work & make sense too?

class MyApp extends BootMixin(Application) {
  constructor(options: ApplicationOptions) {
    super(options);

    this.add(
      createComponentApplicationBooterBinding(new SubApp()),
    );
  }
}

I think it would be great to add a helper method to BootMixin to wrap those two calls in a single API, but I find it difficult to find a good method name. How about addApplicationBooter?

Intended usage:

class MyApp extends BootMixin(Application) {
  constructor(options: ApplicationOptions) {
    super(options);

    this.addApplicationBooter(new SubApp());
  }
}

WDYT?