Support middleware for hooking into get- and onSnapshot requests

[IjzerenHein]

Adds a middleware for intercepting, logging and providing custom connection logic for get,onSnapshot, updateDoc and deleteDoc requests. The following methods have been added for registering middleware:

experimental_registerOnDocSnapshotMiddleware
experimental_registerOnDocsSnapshotMiddleware
experimental_registerGetDocMiddleware (handles getDoc, getDocFromCache & getDocFromServer)
experimental_registerGetDocsMiddleware (handles getDocs, getDocsFromCache & getDocsFromServer)
experimental_registerDeleteDocMiddleware
experimental_registerUpdateDocMiddleware

Conceptually I decided to use separater middleware handlers to support code isolation and tree shaking; and to be consistent with the paradigm of exporting separate functions in firestore.

Example snapshot middleware:

// 1) Install log handler (will be executed first)
experimental_registerOnDocSnapshotMiddleware((context, next) => {
  console.log('onDocSnapshot', context.reference.path)
  const unsubscribe = next(context)
  return () => {
    console.log('onDocSnapshot unsubscribed', context.reference.path)
    unsubscribe()
  }
})

// 2) Install a custom snapshot listener
experimental_registerOnDocSnapshotMiddleware((context, next) => {
  // forbid any request to __internal
  if (context.reference.path.startsWith('__internal')) {
    context.onError(new Error('Not allowed'))
    return () => {}
  }
  return next(context)
})

Example getDoc middleware:

// 1) Install log handler (will be executed first)
experimental_registerGetDocsMiddleware((context, next) => {
  console.log('getDocs...')
  const result = await next(context)
  console.log('getDocs DONE')
  return result
})

experimental_registerGetDocsMiddleware(async (context, next) => {
  // for the sake of demonstration, delay the request
  await new Promise(resolve => setTimeout(resolve, 5000))
  return next(context)
})

[IjzerenHein]

@0x80 ik heb de middleware PR ge-update en experimental_ aan de functies toegevoegd. Ik heb besloten de middleware te laten zoals ik hem oorspronkelijk gebouwd heb. Dit maakt het namelijk het eenvoudigst om de logica die ik voor ogen heb te schrijven in de app. Zou je dit kunnen reviewen?

[0x80]

@IjzerenHein Looks good, but since we have no way to test this I will first do a pre-release from this branch, so we can start verifying it

[IjzerenHein]

@IjzerenHein Looks good, but since we have no way to test this I will first do a pre-release from this branch, so we can start verifying it

sounds good 👍

[IjzerenHein]

@0x80 ik heb twee fixes doorgevoerd, zou jij nog eens een nieuwe next versie kunnen publishen op npm?