Episode 310 - Question on usage of @Dependency inside init of FetchAllKey

[grennis]

In episode 310 in the FetchAllKey init, (see here) I'm curious why the database is declared as a dependency in the init and then copied to a member variable.

Why not just declare @Dependency(\.defaultDatabase) var database as a member variable and then nothing is needed in init?

[mbrandonw]

Hi @grennis, an important aspect of a SharedKey (and SharedReaderKey) conformance is that it provides a Hashable ID. And we use the object ID of the database connection as part of the ID.

So, if we decided to resolve the database connection each time the ID was computed (which is what would happen if we used @Dependency(…) as an instance variable), then the ID could change depending on the context:

let key = FetchAllKey(…)
key.id // one database connection
withDependencies {
  $0.defaultDatabase = DatabaseQueue()
} operation: {
  key.id // a different database connection
}

And that would mean that the hashValue for the key can change without anything in the key changing. We go into detail why that is bad in our "Back to basics: Equatable & Hashable" series.

So, the fix is to resolve and capture the database connection at the moment of create the SharedKey instead of re-resolving each time it is used.

[grennis]

Very interesting thanks for explanation @mbrandonw !