[Help] Extension/core questions

[emulienfou]

Hello community, I've got multiple questions regarding some core features of Loopback 4 and creating an extension.
Hope you will be able to help by putting on the right path.

Decorator

When using a decorator, can we use the MetadataInspector to get all classes who are using the decorator without having to bind the class before?
The idea here would be to:

1. Use the decorator
2. Find all classes using this decorator
3. Bind the classes from the extension's component class

Intercept action

Is it possible to intercept using maybe an interceptor/observer to intercept an action before the repository is creating/updating a model?
The idea here would be to intercept this event to be able to generate a slug (set a value to a specific property) before inserting/updating a model in the database.

Mixin

Can a mixin can be registered from an extension component without extending and wrapping the mixin in the Application instance?
By example to avoid having to do that: class MyApplication extends SeedMixin(Application) {}
The idea here would be to use the custom component inside an extension to extend/wrap the custom Application when registering the custom component.

Thanks for your help

[emulienfou]

Related to the Intercept action point:

• I saw in the loopback-datasource-juggler code, there is some documentation to register an event observer like: MyModel.observe('before save', function filterProperties(ctx, next) however there is not documentation on how can I use this if I want to apply it to all models.
• I tried to subscript to a custom observer but it seems in LB4 I only have ContextObserver available, like:

 const observer: ContextObserver = {
      // Only interested in bindings tagged with `foo`
      filter: binding => binding.tagMap.model != null,
      observe(event: ContextEventType, binding: Readonly<Binding<unknown>>) {
        console.log(binding)
      },
    };
    this.subscribe(observer)

How can I subscribe to before save in LB4 ?

Seems possible in LB3 I presume, regarding to this answer: https://stackoverflow.com/questions/42554917/implement-callbacks-in-loopback-before-save-hook

[emulienfou]

Find my answer here for this point:

• https://strongloop.com/strongblog/february-2020-milestone/
• https://loopback.io/doc/en/lb4/migration-models-operation-hooks.html