Classgrdb 7.3.0GRDB

    DatabaseSnapshotPool

    A database connection that allows concurrent accesses to an unchanging database content, as it existed at the moment the snapshot was created.

    DatabaseSnapshotPool.swift:80
    final class DatabaseSnapshotPool
    Mangled symbol
    s4GRDB20DatabaseSnapshotPoolC What are these?
    FNV24 hash
    6UXZN

    Overview

    A DatabaseSnapshotPool never sees any database modification during all its lifetime. All database accesses performed from a snapshot always see the same identical database content.

    It creates a pool of up to maximumReaderCount read-only SQLite connections. All read accesses are executed in reader dispatch queues (one per read-only SQLite connection). SQLite connections are closed when the DatabasePool is deallocated.

    An SQLite database in the WAL mode is required for creating a DatabaseSnapshotPool.

    Usage

    You create a DatabaseSnapshotPool from a WAL mode database, such as databases created from a DatabasePool:

    let dbPool = try DatabasePool(path: "/path/to/database.sqlite")
let snapshot = try dbPool.makeSnapshotPool()

    When you want to control the database state seen by a snapshot, create the snapshot from a database connection, outside of a write transaction. You can for example take snapshots from a ValueObservation:

    // An observation of the 'player' table
// that notifies fresh database snapshots:
let observation = ValueObservation.tracking { db in
    // Don't fetch players now, and return a snapshot instead.
    // Register an access to the player table so that the
    // observation tracks changes to this table.
    try db.registerAccess(to: Player.all())
    return try DatabaseSnapshotPool(db)
}

// Start observing the 'player' table
let cancellable = try observation.start(in: dbPool) { error in
    // Handle error
} onChange: { (snapshot: DatabaseSnapshotPool) in
    // Handle a fresh snapshot
}

    DatabaseSnapshotPool inherits its database access methods from the DatabaseReader protocols.

    Related SQLite documentation:

    Creating a DatabaseSnapshotPool

    See also makeSnapshotPool.