feat: Decorater Hook

[matthyk]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the feature has not already been requested

🚀 Feature Proposal

Hooks called onDecorate, onDecorateRequest and onDecorateReply, which is triggered each time decorate, decorateRequest, decorateReply is called.

Motivation

Very interesting ideas could be implemented with this feature. For example:

• Plugin that automatically generate a types.d.ts file that provides types for all decorators, which is useful even for js projects

Example

import Fastify from 'fastify'

const app = Fastify()

app.addHook('onDecorate', function(instance, name, fn) {
   console.log(`Fastify instance got decorated with a property named ${name} of type ${typeof fn}`)
})

app.decorate('test', () => 'Test')

[mcollina]

Thanks for reporting! Would you like to send a Pull Request to address this issue? Remember to add unit tests.

[climba03003]

Gathering information for diagnostics is the only usage I can think of.

Gathering for types generation is not realistic since you need to at least start the server before it can generate the types.

[metcoder95]

I like the idea but have concerns about its implications, like when and how to run it.
Usually, the number of decorations is bigger than the number of hooks per app.

Every time a decoration happens, we will need to run the hooks at least once (although we do that already with onRoute, I doubt the cardinality will be the same with decorate and its variants; I imagine it will be bigger).

[matthyk]

https://github.com/Eomm/fastify-overview is a good example of how such a hook could be used.