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Test and Debug - Swift SDK

On this page

  • Testing
  • Test Using a Default Realm
  • Injecting Realm Instances
  • Simplify Testing with Class Projections
  • Test Targets
  • Debugging
  • Debug Using Realm Studio
  • LLDB
  • Troubleshooting
  • Resolve Build Issues
  • Issues Opening Realm Before Loading the UI
  • No Properties are Defined for Model
  • Bad Alloc/Not Enough Memory Available
  • Swift Package Target Cannot be Built Dynamically

The easiest way to use and test Realm-backed applications is to use the default realm. To avoid overriding application data or leaking state between tests, set the default realm to a new file for each test.

// A base class which each of your Realm-using tests should inherit from rather
// than directly from XCTestCase
class TestCaseBase: XCTestCase {
override func setUp() {
// Use an in-memory Realm identified by the name of the current test.
// This ensures that each test can't accidentally access or modify the data
// from other tests or the application itself, and because they're in-memory,
// there's nothing that needs to be cleaned up.
Realm.Configuration.defaultConfiguration.inMemoryIdentifier =

Another way to test Realm-related code is to have all the methods you'd like to test accept a realm instance as an argument. This enables you to pass in different realms when running the app and when testing it.

For example, suppose your app has a method to GET a user profile from a JSON API. You want to test that the local profile is properly created:

// Application Code
func updateUserFromServer() {
let url = URL(string: "")
URLSession.shared.dataTask(with: url!) { data, _, _ in
let realm = try! Realm()
createOrUpdateUser(in: realm, with: data!)
public func createOrUpdateUser(in realm: Realm, with data: Data) {
let object = try! JSONSerialization.jsonObject(with: data) as? [String: String]
try! realm.write {
realm.create(User.self, value: object, update: .modified)
// Test Code
let realmPath = URL(fileURLWithPath: "...")
func testThatUserIsUpdatedFromServer() {
let config = Realm.Configuration(fileURL: realmPath)
let testRealm = try! Realm(configuration: config)
let jsonData = "{\"email\": \"\"}".data(using: .utf8)!
// In our test, we're passing in the testRealm. This is where we'd
// pass in our "real" realm in the application code above.
createOrUpdateUser(in: testRealm, with: jsonData)
XCTAssertEqual(testRealm.objects(User.self).first!.email, "",
"User was not properly updated from server.")

New in version 10.21.0.

If you want to work with a subset of an object's properties for testing, you can create a class projection. A class projection is a model abstraction where you can pass through, rename, or exclude realm object properties. While this feature simplifies view model implementation, it also simplifies testing with Realm.


This example uses the object models and the class projection from the Define and Use Class Projections page.

In this example, we create a realm object using the full object model. Then, we view retrieve the object as a class projection, working with only a subset of its properties.

With this class projection, we don't need to access or account for properties that we don't need to test.

func testWithProjection() {
let realm = try! Realm()
// Create a Realm object, populate it with values
let jasonBourne = Person(value: ["firstName": "Jason",
"lastName": "Bourne",
"address": [
"city": "Zurich",
"country": "Switzerland"]])
try! realm.write {
// Retrieve all class projections of the given type `PersonProjection`
// and filter for the first class projection where the `firstName` property
// value is "Jason"
let person = realm.objects(PersonProjection.self).first(where: { $0.firstName == "Jason" })!
// Verify that we have the correct PersonProjection
XCTAssert(person.firstName == "Jason")
// See that `homeCity` exists as a projection property
// Although it is not on the object model
XCTAssert(person.homeCity == "Zurich")
// Change a value on the class projection
try! realm.write {
person.firstName = "David"
// Verify that the projected property's value has changed
XCTAssert(person.firstName == "David")

Don't link the Realm framework directly to your test target. This can cause your tests to fail with an exception message "Object type 'YourObject' is not managed by the Realm." Unlinking Realm from your test target should resolve this issue.

Compile your model class files in your application or framework targets; don't add them to your unit test targets. Otherwise, those classes are duplicated when testing, which can lead to difficult-to-debug issues.

Expose all the code that you need for testing to your unit test targets. Use the public access modifier or @testable.

Since you're using Realm as a dynamic framework, you'll need to make sure your unit test target can find Realm. Add the parent path to RealmSwift.framework to your unit test's "Framework Search Paths".

Realm Studio enables you to open and edit local realms. It supports Mac, Windows and Linux.

Debugging apps using Realm's Swift API must be done through the LLDB console.

Although the LLDB script allows inspecting the contents of your realm variables in Xcode's UI, this doesn't yet work for Swift. Those variables will show incorrect data. Instead, use LLDB's po command to inspect the contents of data stored in a realm.

Some developers experience build issues after installing the Realm Swift SDK via CocoaPods or Carthage. Common causes of these issues include:

  • Installation issues:

    • Initial install failed

    • Using an unsupported version of the dependency manager

  • Build tool issues:

    • Build tools have stale caches

    • Updating build tool versions

  • Making changes to your project setup, such as:

    • Adding a new target

    • Sharing dependencies across targets

A fix that often clears these issues is to delete derived data and clean the Xcode build folder.

You may open a realm and immediately see crashes with error messages related to properties being optional or required. Issues with your object model can cause these types of crashes. These errors occur after you open a realm, but before you get to the UI.

Realm has a "schema discovery" phase when a realm opens on the device. At this time, Realm examines the schema for any objects that it manages. You can specify that a given realm should manage only a subset of objects in your application.

If you see errors related to properties during schema discovery, these are likely due to schema issues and not issues with data from a specific object. For example, you may see schema discovery errors if you define a to-one relationship as required instead of optional.

To debug these crashes, check the schema you've defined.

You can tell these are schema discovery issues because they occur before the UI loads. This means that no UI element is attempting to incorrectly use a property, and there aren't any objects in memory that could have bad data. If you get errors related to properties after the UI loads, this is probably not due to invalid schema. Instead, those errors are likely a result of incorrect, wrongly-typed or missing data.

The Realm Swift SDK uses the Swift language reflection feature to determine the properties in your model at runtime. If you get a crash similar to the following, confirm that your project has not disabled reflection metadata:

Terminating app due to uncaught exception 'RLMException', reason: 'No properties are defined for 'ObjectName'.

If you set SWIFT_REFLECTION_METADATA_LEVEL = none, Realm cannot discover children of types, such as properties and enums. Reflection is enabled by default if your project does not specifically set a level for this setting.

In iOS or iPad devices with little available memory, or where you have a memory-intensive application that uses multiple realms or many notifications, you may encounter the following error:

libc++abi: terminating due to an uncaught exception of type std::bad_alloc: std::bad_alloc

This error typically indicates that a resource cannot be allocated because not enough memory is available.

If you are building for iOS 15+ or iPad 15+, you can add the Extended Virtual Addressing Entitlement to resolve this issue.

Add these keys to your Property List, and set the values to true:


Changed in version 10.49.3.

Swift SDK v10.49.3 changed the details for installing the package with Swift Package Manager (SPM). When you update from an older version of the package to v10.49.3 or newer, you may get a build error similar to:

Swift package target `Realm` is linked as a static library by `TargetName`
and `Realm`, but cannot be built dynamically because there is a package
product with the same name.

To resolve this error, unlink either the Realm or the RealmSwift package from your build target. You can do this in Xcode by following these steps:

  1. In your project Targets, select your build target.

  2. Go to the Build Phases tab.

  3. Expand the Link Binary With Libraries element.

  4. Select either Realm or RealmSwift, and click the Remove items (-) button to remove the unneeded binary.

    • If you use Swift or Swift and Objective-C APIs, keep RealmSwift.

    • If you use only Objective-C APIs, keep Realm.

Now your target should build without this error.

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