Protocols

A type that supplies the values of some external resource, one at a time.

Overview

The most common way to iterate over the elements of a cursor is to use a while loop:

let cursor = ...
while let element = try cursor.next() {
    ...
}

Relationship with standard Sequence and IteratorProtocol

Cursors share traits with lazy sequences and iterators from the Swift standard library. Differences are:

• Cursor types are classes, and have a lifetime.
• Cursor iteration may throw errors.
• A cursor can not be repeated.

The protocol comes with default implementations for many operations similar to those defined by Swift’s Sequence protocol: contains, dropFirst, dropLast, drop(while:), enumerated, filter, first, flatMap, forEach, joined, joined(separator:), max, max(by:), min, min(by:), map, prefix, prefix(while:), reduce, reduce(into:), suffix.

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public protocol Cursor : AnyObject

The protocol for custom SQLite aggregates.

For example:

struct MySum : DatabaseAggregate {
    var sum: Int = 0

    mutating func step(_ dbValues: [DatabaseValue]) {
        if let int = Int.fromDatabaseValue(dbValues[0]) {
            sum += int
        }
    }

    func finalize() -> DatabaseValueConvertible? {
        return sum
    }
}

let dbQueue = DatabaseQueue()
let fn = DatabaseFunction("mysum", argumentCount: 1, aggregate: MySum.self)
dbQueue.add(function: fn)
try dbQueue.write { db in
    try db.execute("CREATE TABLE test(i)")
    try db.execute("INSERT INTO test(i) VALUES (1)")
    try db.execute("INSERT INTO test(i) VALUES (2)")
    try Int.fetchOne(db, "SELECT mysum(i) FROM test")! // 3
}

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public protocol DatabaseAggregate

The protocol for all types that can fetch values from a database.

It is adopted by DatabaseQueue and DatabasePool.

The protocol comes with isolation guarantees that describe the behavior of adopting types in a multithreaded application.

Types that adopt the protocol can provide in practice stronger guarantees. For example, DatabaseQueue provides a stronger isolation level than DatabasePool.

Warning: Isolation guarantees stand as long as there is no external connection to the database. Should you have to cope with external connections, protect yourself with transactions, and be ready to setup a busy handler.

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public protocol DatabaseReader : AnyObject

Undocumented

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public protocol DatabaseRegionConvertible

Types that adopt DatabaseValueConvertible can be initialized from database values.

The protocol comes with built-in methods that allow to fetch cursors, arrays, or single values:

try String.fetchCursor(db, "SELECT name FROM ...", arguments:...) // Cursor of String
try String.fetchAll(db, "SELECT name FROM ...", arguments:...)    // [String]
try String.fetchOne(db, "SELECT name FROM ...", arguments:...)    // String?

let statement = try db.makeSelectStatement("SELECT name FROM ...")
try String.fetchCursor(statement, arguments:...) // Cursor of String
try String.fetchAll(statement, arguments:...)    // [String]
try String.fetchOne(statement, arguments:...)    // String?

DatabaseValueConvertible is adopted by Bool, Int, String, etc.

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public protocol DatabaseValueConvertible : SQLExpressible

The protocol for all requests that run from a single select statement, and tell how fetched rows should be interpreted.

struct Player: FetchableRecord { ... }
let request: ... // Some FetchRequest that fetches Player
try request.fetchCursor(db) // Cursor of Player
try request.fetchAll(db)    // [Player]
try request.fetchOne(db)    // Player?

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public protocol FetchRequest : DatabaseRegionConvertible

The protocol for all types that can update a database.

It is adopted by DatabaseQueue and DatabasePool.

The protocol comes with isolation guarantees that describe the behavior of adopting types in a multithreaded application.

Types that adopt the protocol can in practice provide stronger guarantees. For example, DatabaseQueue provides a stronger isolation level than DatabasePool.

Warning: Isolation guarantees stand as long as there is no external connection to the database. Should you have to cope with external connections, protect yourself with transactions, and be ready to setup a busy handler.

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public protocol DatabaseWriter : DatabaseReader

RowAdapter is a protocol that helps two incompatible row interfaces working together.

GRDB ships with four concrete types that adopt the RowAdapter protocol:

• ColumnMapping: renames row columns
• SuffixRowAdapter: hides the first columns of a row
• RangeRowAdapter: only exposes a range of columns
• ScopeAdapter: groups several adapters together to define named scopes

To use a row adapter, provide it to any method that fetches:

let adapter = SuffixRowAdapter(fromIndex: 2)
let sql = "SELECT 1 AS foo, 2 AS bar, 3 AS baz"

// [baz:3]
try Row.fetchOne(db, sql, adapter: adapter)

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public protocol RowAdapter

The StatementColumnConvertible protocol grants access to the low-level C interface that extracts values from query results: https://www.sqlite.org/c3ref/column_blob.html. It can bring performance improvements.

To use it, have a value type adopt both StatementColumnConvertible and DatabaseValueConvertible. GRDB will then automatically apply the optimization whenever direct access to SQLite is possible:

let rows = Row.fetchCursor(db, "SELECT ...")
while let row = try rows.next() {
    let int: Int = row[0]                 // there
}
let ints = Int.fetchAll(db, "SELECT ...") // there
struct Player {
    init(row: Row) {
        name = row["name"]                // there
        score = row["score"]              // there
    }
}

StatementColumnConvertible is already adopted by all Swift integer types, Float, Double, String, and Bool.

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public protocol StatementColumnConvertible

A transaction observer is notified of all changes and transactions committed or rollbacked on a database.

Adopting types must be a class.

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public protocol TransactionObserver : AnyObject

The protocol for custom FTS5 tokenizers.

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The protocol for FTS5 tokenizers

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The protocol for custom FTS5 tokenizers that wrap another tokenizer.

Types that adopt FTS5WrapperTokenizer don’t have to implement the low-level FTS5Tokenizer.tokenize(context:flags:pText:nText:tokenCallback:).

Instead, they process regular Swift strings.

Here is the implementation for a trivial tokenizer that wraps the built-in ascii tokenizer without any custom processing:

class TrivialAsciiTokenizer : FTS5WrapperTokenizer {
    static let name = "trivial"
    let wrappedTokenizer: FTS5Tokenizer

    init(db: Database, arguments: [String]) throws {
        wrappedTokenizer = try db.makeTokenizer(.ascii())
    }

    func accept(token: String, flags: FTS5TokenFlags, for tokenization: FTS5Tokenization, tokenCallback: FTS5WrapperTokenCallback) throws {
        try tokenCallback(token, flags)
    }
}

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Experimental

The base protocol for all associations that define a connection between two record types.

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public protocol Association : DerivableRequest

Types that adopt TableRecord declare a particular relationship with a database table.

Types that adopt both TableRecord and FetchableRecord are granted with built-in methods that allow to fetch instances identified by key:

try Player.fetchOne(db, key: 123)  // Player?
try Citizenship.fetchOne(db, key: ["citizenId": 12, "countryId": 45]) // Citizenship?

TableRecord is adopted by Record.

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public protocol TableRecord

Adopt the ColumnExpression protocol when you define a column type.

You can, for example, define a String-based column enum:

enum Columns: String, ColumnExpression {
    case id, name, score
}
let arthur = try Player.filter(Columns.name == "Arthur").fetchOne(db)

You can also define a genuine column type:

struct MyColumn: ColumnExpression {
    var name: String
    var sqlType: String
}
let nameColumn = MyColumn(name: "name", sqlType: "VARCHAR")
let arthur = try Player.filter(nameColumn == "Arthur").fetchOne(db)

See https://github.com/groue/GRDB.swift#the-query-interface

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public protocol ColumnExpression : SQLExpression

Types that adopt MutablePersistableRecord can be inserted, updated, and deleted.

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public protocol MutablePersistableRecord : TableRecord

The protocol for SQLite virtual table modules. It lets you define a DSL for the Database.create(virtualTable:using:) method:

let module = ...
try db.create(virtualTable: "item", using: module) { t in
    ...
}

GRDB ships with three concrete classes that implement this protocol: FTS3, FTS4 and FTS5.

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public protocol VirtualTableModule

Types that adopt FetchableRecord can be initialized from a database Row.

let row = try Row.fetchOne(db, "SELECT ...")!
let player = Player(row)

The protocol comes with built-in methods that allow to fetch cursors, arrays, or single records:

try Player.fetchCursor(db, "SELECT ...", arguments:...) // Cursor of Player
try Player.fetchAll(db, "SELECT ...", arguments:...)    // [Player]
try Player.fetchOne(db, "SELECT ...", arguments:...)    // Player?

let statement = try db.makeSelectStatement("SELECT ...")
try Player.fetchCursor(statement, arguments:...) // Cursor of Player
try Player.fetchAll(statement, arguments:...)    // [Player]
try Player.fetchOne(statement, arguments:...)    // Player?

FetchableRecord is adopted by Record.

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public protocol FetchableRecord

Types that adopt PersistableRecord can be inserted, updated, and deleted.

This protocol is intented for types that don’t have an INTEGER PRIMARY KEY.

Unlike MutablePersistableRecord, the insert() and save() methods are not mutating methods.

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public protocol PersistableRecord : MutablePersistableRecord

Experimental

The ValueReducer protocol supports ValueObservation.

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public protocol ValueReducer