Supported Types - Swift SDK

Collection Types

Realm has several types to represent groups of objects, which we call collections. A collection is an object that contains zero or more instances of one Realm type. Realm collections are homogenous: all objects in a collection are of the same type.

You can filter and sort any collection using Realm's query engine. Collections are live, so they always reflect the current state of the realm instance on the current thread. You can also listen for changes in the collection by subscribing to collection notifications.

All collection types conform to the RealmCollection protocol. This protocol inherits from CollectionType, so you can use a Realm collection as you would any other standard library collections.

Using the RealmCollection protocol, you can write generic code that can operate on any Realm collection:

func operateOn<C: RealmCollection>(collection: C) {
    // Collection could be either Results or List
    print("operating on collection containing \(collection.count) objects")
}

Results and Sectioned Results

The Swift SDK Results collection is a class representing objects retrieved from queries. A [Results collection represents the lazily-evaluated results of a query operation. Results are immutable: you cannot add or remove elements to or from the results collection. Results have an associated query that determines their contents.

The Swift SDK also provides SectionedResults, a type-safe collection which holds ResultsSection as its elements. Each ResultSection is a results collection that contains only objects that belong to a given section key.

For example, an app that includes a contact list might use SectionedResults to display a list of contacts divided into sections, where each section contains all the contacts whose first name starts with the given letter. The ResultsSection whose key is "L" would contain "Larry", "Liam", and "Lisa".

Seealso: Reads

Collections as Properties

The Swift SDK also offers several collection types you can use as properties in your data model:

1. List, a class representing to-many relationships in models.
2. LinkingObjects, a class representing inverse relationships in models.
3. MutableSet, a class representing a to-many relationship.
4. Map, a class representing an associative array of key-value pairs with unique keys.
5. AnyRealmCollection, a type-erased class that can forward calls to a concrete Realm collection like Results, List or LinkingObjects.

Collections are Live

Like live objects, Realm collections are usually live:

• Live results collections always reflect the current results of the associated query.
• Live lists always reflect the current state of the relationship on the realm instance.

There are two cases when a collection is not live:

• The collection is unmanaged. For example, a List property of a Realm object that has not been added to a realm yet or that has been copied from a realm is not live.
• The collection is frozen.

Combined with collection notifications, live collections enable clean, reactive code. For example, suppose your view displays the results of a query. You can keep a reference to the results collection in your view class, then read the results collection as needed without having to refresh it or validate that it is up-to-date.

Important: Since results update themselves automatically, do not store the positional index of an object in the collection or the count of objects in a collection. The stored index or count value could be outdated by the time you use it.

Supported Property Types

You can use the following types to define your object model properties.

Property Cheat Sheet

Swift

Version changed: 10.10.0 @Persisted property declaration syntax

CGFloat properties are discouraged, as the type is not platform independent.

To use Key-Value Coding with a user-defined object in the @Persisted syntax, add the @objc attribute: @Persisted @objc var myObject: MyClass?

Setting Default Values

With the @Persisted property declaration syntax, you may see a performance impact when setting default values for:

• List
• MutableSet
• Dictionary
• Decimal128
• UUID
• ObjectId

@Persisted var listProperty: List<Int> and @Persisted var listProperty = List<Int>() are both valid, and are functionally equivalent. However, the second declaration will result in poorer performance.

This is because the List is created when the parent object is created, rather than lazily as needed. For most types, this is a difference so small you can't measure it. For the types listed here, you may see a performance impact when using the second declaration style.

Objective-C

Additionally:

• Integral types int, NSInteger, long, long long

CGFloat properties are discouraged, as the type is not platform independent.

Swift Pre 10.10.0

Version changed: 10.8.0 RealmProperty replaces RealmOptional

Additionally:

• EmbeddedObject-derived types
• Enum

You can use RealmProperty <T?> to represent integers, doubles, and other types as optional.

CGFloat properties are discouraged, as the type is not platform independent.

Unique Identifiers

Version added: 10.8.0 UUID type

ObjectId is a 12-byte unique value. UUID is a 16-byte globally-unique value. You can index both types, and use either as a primary key.

Note: When declaring default values for @Persisted UUID or ObjectId property attributes, both of these syntax types are valid:

• @Persisted var value: UUID
• @Persisted var value = UUID()

However, the second will result in poorer performance. This is because the latter creates a new identifier that is never used any time an object is read from the realm, while the former only creates them when needed.

@Persisted var id: ObjectId has equivalent behavior to @objc dynamic var _id = ObjectId.generate(). They both make random ObjectIds.

@Persisted var _id = ObjectId() has equivalent behavior to @objc dynamic var _id = ObjectId(). They both make zero-initialized ObjectIds.

Size Limitations

Data and string properties cannot hold more than 16MB. To store larger amounts of data, either:

• Break the data into 16MB chunks, or
• Store data directly on the file system and store paths to the files in the realm.

Realm throws a runtime exception if your app attempts to store more than 16MB in a single property.

To avoid size limitations and a performance impact, it is best not to store large blobs, such as image and video files, directly in a realm. Instead, save the file to a file store and keep only the location of the file and any relevant metadata in the realm.

AnyRealmCollection

To store a collection as a property or variable without needing to know the concrete collection type, Swift's type system requires a type-erased wrapper like AnyRealmCollection:

class ViewController {
//    let collection: RealmCollection
//                    ^
//                    error: protocol 'RealmCollection' can only be used
//                    as a generic constraint because it has Self or
//                    associated type requirements
//
//    init<C: RealmCollection>(collection: C) where C.ElementType == MyModel {
//        self.collection = collection
//    }

    let collection: AnyRealmCollection<MyModel>

    init<C: RealmCollection & _ObjcBridgeable>(collection: C) where C.ElementType == MyModel {
        self.collection = AnyRealmCollection(collection)
    }
}

Mutable Set

Version added: 10.8.0

A MutableSet collection represents a to-many relationship containing distinct values. A MutableSet supports the following types (and their optional versions):

• Bool
• Data
• Date
• Decimal128
• Double
• Float
• Int
• Int8
• Int16
• Int32
• Int64
• Object
• ObjectId
• String
• UUID

Like Swift's Set, MutableSet is a generic type that is parameterized on the type it stores. Unlike native Swift collections, Realm mutable sets are reference types, as opposed to value types (structs).

You can only call the MutableSets mutation methods during a write transaction. As a result, MutableSets are immutable if you open the managing realm as a read-only realm.

You can filter and sort a MutableSet with the same predicates as Results. Like other Realm collections, you can register a change listener on a MutableSet.

For example, a Dog class model might contain a MutableSet for citiesVisited:

class Dog: Object {
    @Persisted var name = ""
    @Persisted var currentCity = ""
    @Persisted var citiesVisited: MutableSet<String>
}

Note: When declaring default values for @Persisted MutableSet property attributes, both of these syntax types is valid:

• @Persisted var value: MutableSet<String>
• @Persisted var value = MutableSet<String>()

However, the second will result in significantly worse performance. This is because the MutableSet is created when the parent object is created, rather than lazily as needed.

Map/Dictionary

Version added: 10.8.0

The Map is an associative array that contains key-value pairs with unique keys.

Like Swift's Dictionary, Map is a generic type that is parameterized on its key and value types. Unlike native Swift collections, Realm Maps are reference types (classes), as opposed to value types (structs).

You can declare a Map as a property of an object:

class Dog: Object {
    @Persisted var name = ""
    @Persisted var currentCity = ""

    // Map of city name -> favorite park in that city
    @Persisted var favoriteParksByCity: Map<String, String>
}

Realm disallows the use of . or $ characters in map keys. You can use percent encoding and decoding to store a map key that contains one of these disallowed characters.

// Percent encode . or $ characters to use them in map keys
let mapKey = "New York.Brooklyn"
let encodedMapKey = "New York%2EBrooklyn"

Note: When declaring default values for @Persisted Map property attributes, both of these syntax types is valid:

• @Persisted var value: Map<String, String>
• @Persisted var value = Map<String, String>()

However, the second will result in significantly worse performance. This is because the Map is created when the parent object is created, rather than lazily as needed.

AnyRealmValue

Version changed: 10.51.0 AnyRealmValue properties can hold lists or maps of mixed data.

Version added: 10.8.0

AnyRealmValue is a Realm property type that can hold different data types. Supported AnyRealmValue data types include:

• Int
• Float
• Double
• Decimal128
• ObjectID
• UUID
• Bool
• Date
• Data
• String
• List
• Map
• Object

AnyRealmValue cannot hold a MutableSet or embedded object.

This mixed data type is indexable, but you can't use it as a primary key. Because null is a permitted value, you can't declare an AnyRealmValue as optional.

class Dog: Object {
    @Persisted var name = ""
    @Persisted var currentCity = ""

    @Persisted var companion: AnyRealmValue
}

Collections as Mixed

In version 10.51.0 and later, a AnyRealmValue data type can contain collections (a list or map, but not a set) of AnyRealmValue elements. You can use mixed collections to model unstructured or variable data. For more information, refer to Define Unstructured Data.

• You can nest mixed collections up to 100 levels.
• You can query mixed collection properties and register a listener for changes, as you would a normal collection.
• You can find and update individual mixed collection elements
• You cannot store sets or embedded objects in mixed collections.

To use mixed collections in your app, define the AnyRealmValue type property in your data model. Then, you can create the list or map collections like any other mixed data value.

Geospatial Data

Version added: 10.47.0

Geospatial data, or "geodata", specifies points and geometric objects on the Earth's surface.

If you want to persist geospatial data, it must conform to the GeoJSON spec.

To persist geospatial data with the Swift SDK, create a GeoJSON-compatible embedded class that you can use in your data model.

Your custom embedded object must contain the two fields required by the GeoJSON spec:

• A field of type String property that maps to a type property with the value of "Point": @Persisted var type: String = "Point"
• A field of type List<Double> that maps to a coordinates property containing a latitude/longitude pair: @Persisted private var coordinates: List<Double>

class CustomGeoPoint: EmbeddedObject {
    @Persisted private var type: String = "Point"
    @Persisted private var coordinates: List<Double>

    public var latitude: Double { return coordinates[1] }
    public var longitude: Double { return coordinates[0] }

    convenience init(_ latitude: Double, _ longitude: Double) {
        self.init()
        // Longitude comes first in the coordinates array of a GeoJson document
        coordinates.append(objectsIn: [longitude, latitude])
    }
}

Map Unsupported Types to Supported Types

Version added: 10.20.0

You can use Type Projection to persist unsupported types as supported types in Realm. This enables you to work with Swift types that Realm does not support, but store them as types that Realm does support. You could store a URL as a String, for example, but read it from Realm and use it in your application as though it were a URL.

Declare Type Projections

To use type projection with Realm:

1. Use one of Realm's custom type protocols to map an unsupported data type to a type that Realm supports
2. Use the projected types as @Persisted properties in the Realm object model

Conform to the Type Projection Protocol

You can map an unsupported data type to a type that Realm supports using one of the Realm type projection protocols.

The Swift SDK provides two type projection protocols:

• CustomPersistable
• FailableCustomPersistable

Use CustomPersistable when there is no chance the conversion can fail.

Use FailableCustomPersistable when it is possible for the conversion to fail.

// Extend a type as a CustomPersistable if if is impossible for
// conversion between the mapped type and the persisted type to fail.
extension CLLocationCoordinate2D: CustomPersistable {
    // Define the storage object that is persisted to the database.
    // The `PersistedType` must be a type that Realm supports.
    // In this example, the PersistedType is an embedded object.
    public typealias PersistedType = Location
    // Construct an instance of the mapped type from the persisted type.
    // When reading from the database, this converts the persisted type to the mapped type.
    public init(persistedValue: PersistedType) {
        self.init(latitude: persistedValue.latitude, longitude: persistedValue.longitude)
    }
    // Construct an instance of the persisted type from the mapped type.
    // When writing to the database, this converts the mapped type to a persistable type.
    public var persistableValue: PersistedType {
        Location(value: [self.latitude, self.longitude])
    }
}

// Extend a type as a FailableCustomPersistable if it is possible for
// conversion between the mapped type and the persisted type to fail.
// This returns nil on read if the underlying column contains nil or
// something that can't be converted to the specified type.
extension URL: FailableCustomPersistable {
    // Define the storage object that is persisted to the database.
    // The `PersistedType` must be a type that Realm supports.
    public typealias PersistedType = String
    // Construct an instance of the mapped type from the persisted type.
    // When reading from the database, this converts the persisted type to the mapped type.
    // This must be a failable initializer when the conversion may fail.
    public init?(persistedValue: String) { self.init(string: persistedValue) }
    // Construct an instance of the persisted type from the mapped type.
    // When writing to the database, this converts the mapped type to a persistable type.
    public var persistableValue: String { self.absoluteString }
}

Seealso: These are protocols modeled after Swift's built-in RawRepresentable.

Supported PersistedTypes

The PersistedType can use any of the primitive types that the Swift SDK supports. It can also be an Embedded Object.

PersistedType cannot be an optional or a collection. However you can use the mapped type as an optional or collection property in your object model.

extension URL: FailableCustomPersistable {
   // The `PersistedType` cannot be an optional, so this is not a valid
   // conformance to the FailableCustomPersistable protocol.
   public typealias PersistedType = String?
   ...
}

class Club: Object {
   @Persisted var id: ObjectId
   @Persisted var name: String
   // Although the `PersistedType` cannot be optional, you can use the
   // custom-mapped type as an optional in your object model.
   @Persisted var url: URL?
}

Use Type Projection in the Model

A type that conforms to one of the type projection protocols can be used with the @Persisted property declaration syntax introduced in Swift SDK version 10.10.0. It does not work with the @objc dynamic syntax.

You can use projected types for:

• Top-level types
• Optional versions of the type
• The types for a collection

When using a FailableCustomPersistable as a property, define it as an optional property. When it is optional, the FailableCustomPersistable protocol maps invalid values to nil. When it is a required property, it is force-unwrapped. If you have a value that can't be converted to the projected type, reading that property throws an unwrapped fail exception.

class Club: Object {
    @Persisted var id: ObjectId
    @Persisted var name: String
    // Since we declared the URL as a FailableCustomPersistable,
    // it must be optional.
    @Persisted var url: URL?
    // Here, the `location` property maps to an embedded object.
    // We can declare the property as required.
    // If the underlying field contains nil, this becomes
    // a default-constructed instance of CLLocationCoordinate
    // with field values of `0`.
    @Persisted var location: CLLocationCoordinate2D
}

public class Location: EmbeddedObject {
    @Persisted var latitude: Double
    @Persisted var longitude: Double
}

When your model contains projected types, you can create the object with values using the persisted type, or by assigning to the field properties of an initialized object using the projected types.

// Initialize objects and assign values
let club = Club(value: ["name": "American Kennel Club", "url": "https://akc.org"])
let club2 = Club()
club2.name = "Continental Kennel Club"
// When assigning the value to a type-projected property, type safety
// checks for the mapped type - not the persisted type.
club2.url = URL(string: "https://ckcusa.com/")!
club2.location = CLLocationCoordinate2D(latitude: 40.7509, longitude: 73.9777)

Type Projection in the Schema

When you declare your type as conforming to a type projection protocol, you specify the type that should be persisted in realm. For example, if you map a custom type URL to a persisted type of String, a URL property appears as a String in the schema, and dynamic access to the property acts on strings.

The schema does not directly represent mapped types. Changing a property from its persisted type to its mapped type, or vice versa, does not require a migration.