What the Model being the truth gives you

Because Mirror derives its representation from application fields, several behaviors fall out automatically.

Defaults#

App.initial is the default source. A field equal to its initial value is omitted from the representation by default:

initial: { filter: 'all', page: 1 }
Model:   { filter: 'all', page: 1 }
URL:     /todos

Changing only the filter yields:

/todos?filter=active

A key that should remain explicit can opt in:

keys: {
  page: { keep: true },
}

An application created without initial passes the initial Model in the mirror config instead.

Codecs#

Mirror derives a key's text representation from the field's encoded type:

string  -> text
number  -> parsed number
boolean -> true / false
other   -> JSON

Override a field when a friendlier representation is useful:

keys: {
  tags: {
    codec: TagsAsCommaSeparatedText,
  },
}

mirror.decode(keys) returns both the successfully decoded values and any issues when a caller wants to surface invalid input.

Batching and loop avoidance#

Subscriptions depend on the encoded slice, not every Model transition. One transition that changes several mirrored fields still produces at most one write, and a change that encodes to the same key set produces none.

That also prevents URL feedback loops:

popstate
  -> reduce URL into Model
  -> subscription sees same encoded keys
  -> no second navigation write

How state changes here#

Ordinary edits use Message/update/modifyFields like anywhere else; the mirror observes and represents. Restoration runs the other direction: the declared writable projection installs the URL/KV value into the Model on startup. The projection is declared up front, so restoration can only ever write the fields the application already linked.