Module API Reference

Name

Sets the name of the module that JavaScript code will use to refer to the module. Takes a string as an argument. This can be inferred from the module's class name, but it's recommended to set it explicitly for clarity.

Name("MyModuleName")

Constants

Sets constant properties on the module. Can take a dictionary or a closure that returns a dictionary.

// Created from the dictionary
Constants([
  "PI": Double.pi
])

// or returned by the closure
Constants {
  return [
    "PI": Double.pi
  ]
}

// Passed as arguments
Constants(
  "PI" to kotlin.math.PI
)

// or returned by the closure
Constants {
  return@Constants mapOf(
    "PI" to kotlin.math.PI
  )
}

Function

Defines a native synchronous function that will be exported to JavaScript. Synchronous means that when the function is executed in JavaScript, its native code is run on the same thread and blocks further execution of the script until the native function returns.

Arguments

• name: String — Name of the function that you'll call from JavaScript.
• body: (args...) -> ReturnType — The closure to run when the function is called.

The function can receive up to 8 arguments. This is due to the limitations of generics in both Swift and Kotlin because this component must be implemented separately for each arity.

See the Argument types section for more details on what types can be used in the function body.

Function("mySyncFunction") { (message: String) in
  return message
}

Function("mySyncFunction") { message: String ->
  return@Function message
}

import { requireNativeModule } from 'expo-modules-core';

// Assume that we have named the module "MyModule"
const MyModule = requireNativeModule('MyModule');

function getMessage() {
  return MyModule.mySyncFunction('bar');
}

AsyncFunction

Defines a JavaScript function that always returns a Promise and whose native code is by default dispatched on a different thread than the JavaScript runtime runs on.

Arguments

• name: String — Name of the function that you'll call from JavaScript.
• body: (args...) -> ReturnType — The closure to run when the function is called.

If the type of the last argument is Promise, the function will wait for the promise to be resolved or rejected before the response is passed back to JavaScript. Otherwise, the function is immediately resolved with the returned value or rejected if it throws an exception. The function can receive up to 8 arguments (including the promise).

See the Argument types section for more details on what types can be used in the function body.

It is recommended to use AsyncFunction over Function when it:

• does I/O bound tasks such as sending network requests or interacting with the file system
• needs to be run on a different thread, for example, the main UI thread for UI-related tasks
• is an extensive or long-lasting operation that would block the JavaScript thread which in turn would reduce the responsiveness of the application

AsyncFunction("myAsyncFunction") { (message: String) in
  return message
}

// or

AsyncFunction("myAsyncFunction") { (message: String, promise: Promise) in
  promise.resolve(message)
}

AsyncFunction("myAsyncFunction") { message: String ->
  return@AsyncFunction message
}

// or

// Make sure to import `Promise` class from `expo.modules.kotlin` instead of `expo.modules.core`.
AsyncFunction("myAsyncFunction") { message: String, promise: Promise ->
  promise.resolve(message)
}

import { requireNativeModule } from 'expo-modules-core';

// Assume that we have named the module "MyModule"
const MyModule = requireNativeModule('MyModule');

async function getMessageAsync() {
  return await MyModule.myAsyncFunction('bar');
}

It is possible to change the native queue of AsyncFunction by calling the .runOnQueue function on the result of that component.

AsyncFunction("myAsyncFunction") { (message: String) in
  return message
}.runOnQueue(.main)

AsyncFunction("myAsyncFunction") { message: String ->
  return@AsyncFunction message
}.runOnQueue(Queues.MAIN)

Kotlin coroutines

Android

AsyncFunction can receive a suspendable body on Android. However, it has to be passed in the infix notation after the Coroutine block. You can read more about suspendable functions and coroutines on coroutine overview.

AsyncFunction with a suspendable body can't receive Promise as an argument. It uses a suspension mechanism to execute asynchronous calls. The function is immediately resolved with the returned value of the provided suspendable block or rejected if it throws an exception. The function can receive up to 8 arguments.

By default, suspend functions are dispatched on the module's coroutine scope. Moreover, every other suspendable function called from the body block is run within the same scope. This scope's lifecycle is bound to the module's lifecycle - all unfinished suspend functions will be canceled when the module is deallocated.

AsyncFunction("suspendFunction") Coroutine { message: String ->
  // You can execute other suspendable functions here.
  // For example, you can use `kotlinx.coroutines.delay` to delay resolving the underlying promise.
  delay(5000)
  return@Coroutine message
}

Events

Defines event names that the module can send to JavaScript.

Note: This component can be used inside of the View block to define callback names. See View callbacks

Events("onCameraReady", "onPictureSaved", "onBarCodeScanned")

Events("onCameraReady", "onPictureSaved", "onBarCodeScanned")

See Sending events to learn how to send events from the native code to JavaScript/TypeScript.

Property

Defines a new property directly on the JavaScript object that represents a native module. It is the same as calling Object.defineProperty on the module object.

To declare a read-only property, you can use a shorthanded syntax that requires two arguments:

• name: String — Name of the property that you'll use from JavaScript.
• getter: () -> PropertyType — The closure to run when the getter for a property was called.

Property("foo") {
  return "bar"
}

Property("foo") {
  return@Property "bar"
}

In the case of the mutable property, both the getter and the setter closure are needed (using the syntax below is also possible to declare a property with only a setter):

• name: String — Name of the property that you'll use from JavaScript.
• getter: () -> PropertyType — The closure to run when the getter for a property was called.
• setter: (newValue: PropertyType) -> void — The closure to run when the setter for a property was called.

Property("foo")
  .get { return "bar" }
  .set { (newValue: String) in
    // do something with new value
  }

Property("foo")
  .get { return@get "bar" }
  .set { newValue: String ->
    // do something with new value
  }

import { requireNativeModule } from 'expo-modules-core';

// Assume that we have named the module "MyModule"
const MyModule = requireNativeModule('MyModule');

// Obtain the property value
MyModule.foo;

// Set a new value
MyModule.foo = 'foobar';

View

Enables the module to be used as a native view. Definition components that are accepted as part of the view definition: Prop, Events, GroupView and AsyncFunction.

AsyncFunction in the view definition is added to the React ref of the React component representing the native view. Such async functions automatically receive an instance of the native view as the first argument and run on the UI thread by default.

Arguments

• viewType — The class of the native view that will be rendered. Note: On Android, the provided class must inherit from the ExpoView, on iOS it's optional. See Extending ExpoView.
• definition: () -> ViewDefinition — A builder of the view definition.

View(UITextView.self) {
  Prop("text") { ... }

  AsyncFunction("focus") { (view: UITextView) in
    view.becomeFirstResponder()
  }
}

View(TextView::class) {
  Prop("text") { ... }

  AsyncFunction("focus") { view: TextView ->
    view.requestFocus()
  }
}

Support for rendering SwiftUI views is planned. For now, you can use UIHostingController and add its content view to your UIKit view.

Prop

Defines a setter for the view prop of given name.

Arguments

• name: String — Name of view prop that you want to define a setter.
• setter: (view: ViewType, value: ValueType) -> () — Closure that is invoked when the view rerenders.

This property can only be used within a ViewManager closure.

Prop("background") { (view: UIView, color: UIColor) in
  view.backgroundColor = color
}

Prop("background") { view: View, @ColorInt color: Int ->
  view.setBackgroundColor(color)
}

Note Props of function type (callbacks) are not supported yet.

OnCreate

Defines module's lifecycle listener that is called right after module initialization. If you need to set up something when the module gets initialized, use this instead of module's class initializer.

OnDestroy

Defines module's lifecycle listener that is called when the module is about to be deallocated. Use it instead of module's class destructor.

OnStartObserving

Defines the function that is invoked when the first event listener is added.

OnStopObserving

Defines the function that is invoked when all event listeners are removed.

OnAppContextDestroys

Defines module's lifecycle listener that is called when the app context owning the module is about to be deallocated.

OnAppEntersForeground

Defines the listener that is called when the app is about to enter the foreground mode.

Note This function is not available on Android — you may want to use OnActivityEntersForeground instead.

OnAppEntersBackground

Defines the listener that is called when the app enters the background mode.

Note This function is not available on Android — you may want to use OnActivityEntersBackground instead.

OnAppBecomesActive

Defines the listener that is called when the app becomes active again (after OnAppEntersForeground).

Note This function is not available on Android — you may want to use OnActivityEntersForeground instead.

OnActivityEntersForeground

Defines the activity lifecycle listener that is called right after the activity is resumed.

Note This function is not available on iOS — you may want to use OnAppEntersForeground instead.

OnActivityEntersBackground

Defines the activity lifecycle listener that is called right after the activity is paused.

Note This function is not available on iOS — you may want to use OnAppEntersBackground instead.

OnActivityDestroys

Defines the activity lifecycle listener that is called when the activity owning the JavaScript context is about to be destroyed.

Note This function is not available on iOS — you may want to use OnAppEntersBackground instead.

GroupView

Enables the view to be used as a view group. Definition components that are accepted as part of the group view definition: AddChildView, GetChildCount, GetChildViewAt, RemoveChildView, RemoveChildViewAt.

Arguments

• viewType — The class of the native view. Note that the provided class must inherit from the Android ViewGroup.
• definition: () -> ViewGroupDefinition — A builder of the view group definition.

This property can only be used within a View closure.

GroupView<ViewGroup> {
  AddChildView { parent, child, index -> ... }
}

AddChildView

Defines action that adds a child view to the view group.

Arguments

• action: (parent: ParentType, child: ChildType, index: Int) -> () — An action that adds a child view to the view group.

This property can only be used within a GroupView closure.

AddChildView { parent, child: View, index ->
  parent.addView(child, index)
}

GetChildCount

Defines action the retrieves the number of child views in the view group.

Arguments

• action: (parent: ParentType) -> Int — A function that returns number of child views.

This property can only be used within a GroupView closure.

GetChildCount { parent ->
  return@GetChildCount parent.childCount
}

GetChildViewAt

Defines action that retrieves a child view at a specific index from the view group.

Arguments

• action: (parent: ParentType, index: Int) -> ChildType — A function that retrieves a child view at a specific index from the view group.

This property can only be used within a GroupView closure.

GetChildViewAt { parent, index ->
  parent.getChildAt(index)
}

RemoveChildView

Defines action that removes a specific child view from the view group.

Arguments

• action: (parent: ParentType, child: ChildType) -> () — A function that remove a specific child view from the view group.

This property can only be used within a GroupView closure.

RemoveChildView { parent, child: View ->
  parent.removeView(child)
}

RemoveChildViewAt

Defines action that removes a child view at a specific index from the view group.

Arguments

• action: (parent: ParentType, child: ChildType) -> () — A function that removes a child view at a specific index from the view group.

This property can only be used within a GroupView closure.

RemoveChildViewAt { parent, index ->
  parent.removeViewAt(child)
}

Primitives

All functions and view prop setters accept all common primitive types in Swift and Kotlin as the arguments. This includes arrays, dictionaries/maps and optionals of these primitive types.

Convertibles

Convertibles are native types that can be initialized from certain specific kinds of data received from JavaScript. Such types are allowed to be used as an argument type in Function's body. For example, when the CGPoint type is used as a function argument type, its instance can be created from an array of two numbers (x, y) or a JavaScript object with numeric x and y properties.

Some common iOS types from CoreGraphics and UIKit system frameworks are already made convertible.

Similarly, some common Android types from packages like java.io, java.net, or android.graphics are also made convertible.

Records

Record is a convertible type and an equivalent of the dictionary (Swift) or map (Kotlin), but represented as a struct where each field can have its type and provide a default value. It is a better way to represent a JavaScript object with the native type safety.

struct FileReadOptions: Record {
  @Field
  var encoding: String = "utf8"

  @Field
  var position: Int = 0

  @Field
  var length: Int?
}

// Now this record can be used as an argument of the functions or the view prop setters.
Function("readFile") { (path: String, options: FileReadOptions) -> String in
  // Read the file using given `options`
}

class FileReadOptions : Record {
  @Field
  val encoding: String = "utf8"

  @Field
  val position: Int = 0

  @Field
  val length: Int? = null
}

// Now this record can be used as an argument of the functions or the view prop setters.
Function("readFile") { path: String, options: FileReadOptions ->
  // Read the file using given `options`
}

Enums

With enums, we can go even further with the above example (with FileReadOptions record) and limit supported encodings to "utf8" and "base64". To use an enum as an argument or record field, it must represent a primitive value (for example, String, Int) and conform to Enumerable.

enum FileEncoding: String, Enumerable {
  case utf8
  case base64
}

struct FileReadOptions: Record {
  @Field
  var encoding: FileEncoding = .utf8
  // ...
}

// Note: the constructor must have an argument called value.
enum class FileEncoding(val value: String) : Enumerable {
  utf8("utf8"),
  base64("base64")
}

class FileReadOptions : Record {
  @Field
  val encoding: FileEncoding = FileEncoding.utf8
  // ...
}

Eithers

There are some use cases where you want to pass various types for a single function argument. This is where Either types might come in handy. They act as a container for a value of one of a couple of types.

Function("foo") { (bar: Either<String, Int>) in
  if let bar: String = bar.get() {
    // `bar` is a String
  }
  if let bar: Int = bar.get() {
    // `bar` is an Int
  }
}

Function("foo") { bar: Either<String, Int> ->
  bar.get(String::class).let {
    // `it` is a String
  }
  bar.get(Int::class).let {
    // `it` is an Int
  }
}

The implementation for three Either types is currently provided out of the box, allowing you to use up to four different subtypes.

• Either<FirstType, SecondType> — A container for one of two types.
• EitherOfThree<FirstType, SecondType, ThirdType> — A container for one of three types.
• EitherOfFour<FirstType, SecondType, ThirdType, FourthType> — A container for one of four types.

JavaScript values

It's also possible to use a JavaScriptValue type which is a holder for any value that can be represented in JavaScript. This type is useful when you want to mutate the given argument or when you want to omit type validations and conversions. Note that using JavaScript-specific types is restricted to synchronous functions as all reads and writes in the JavaScript runtime must happen on the JavaScript thread. Any access to these values from different threads will result in a crash.

In addition to the raw value, the JavaScriptObject type can be used to allow only object types and JavaScriptFunction<ReturnType> for callbacks.

Function("mutateMe") { (value: JavaScriptValue) in
  if value.isObject() {
    let jsObject = value.getObject()
    jsObject.setProperty("expo", value: "modules")
  }
}

// or

Function("mutateMe") { (jsObject: JavaScriptObject) in
  jsObject.setProperty("expo", value: "modules")
}

Function("mutateMe") { value: JavaScriptValue ->
  if (value.isObject()) {
    val jsObject = value.getObject()
    jsObject.setProperty("expo", "modules")
  }
}

// or

Function("mutateMe") { jsObject: JavaScriptObject ->
  jsObject.setProperty("expo", "modules")
}

Module

A base class for a native module.

Properties

Methods

AppContext

The app context is an interface to a single Expo app.

Properties

ExpoView

A base class that should be used by all exported views.

On iOS, ExpoView extends the RCTView which handles some styles (for example, borders) and accessibility.

Properties

Extending ExpoView

To export your view using the View component, your custom class must inherit from the ExpoView. By doing that you will get access to the AppContext object. It's the only way of communicating with other modules and the JavaScript runtime. Also, you can't change constructor parameters, because provided view will be initialized by expo-modules-core.

class LinearGradientView: ExpoView {}

public class LinearGradientModule: Module {
  public func definition() -> ModuleDefinition {
    View(LinearGradientView.self) {
      // ...
    }
  }
}

class LinearGradientView(
  context: Context,
  appContext: AppContext,
) : ExpoView(context, appContext)

class LinearGradientModule : Module() {
  override fun definition() = ModuleDefinition {
    View(LinearGradientView::class) {
      // ...
    }
  }
}

Sending events

While JavaScript/TypeScript to Native communication is mostly covered by native functions, you might also want to let the JavaScript/TypeScript code know about certain system events, for example, when the clipboard content changes.

To do this, in the module definition, you need to provide the event names that the module can send using the Events definition component. After that, you can use the sendEvent(eventName, payload) function on the module instance to send the actual event with some payload. For example, a minimal clipboard implementation that sends native events may look like this:

let CLIPBOARD_CHANGED_EVENT_NAME = "onClipboardChanged"

public class ClipboardModule: Module {
  public func definition() -> ModuleDefinition {
    Events(CLIPBOARD_CHANGED_EVENT_NAME)

    OnStartObserving {
      NotificationCenter.default.addObserver(
        self,
        selector: #selector(self.clipboardChangedListener),
        name: UIPasteboard.changedNotification,
        object: nil
      )
    }

    OnStopObserving {
      NotificationCenter.default.removeObserver(
        self,
        name: UIPasteboard.changedNotification,
        object: nil
      )
    }
  }

  @objc
  private func clipboardChangedListener() {
    sendEvent(CLIPBOARD_CHANGED_EVENT_NAME, [
      "contentTypes": availableContentTypes()
    ])
  }
}

const val CLIPBOARD_CHANGED_EVENT_NAME = "onClipboardChanged"

class ClipboardModule : Module() {
  override fun definition() = ModuleDefinition {
    Events(CLIPBOARD_CHANGED_EVENT_NAME)

    OnStartObserving {
      clipboardManager?.addPrimaryClipChangedListener(listener)
    }

    OnStopObserving {
      clipboardManager?.removePrimaryClipChangedListener(listener)
    }
  }

  private val clipboardManager: ClipboardManager?
    get() = appContext.reactContext?.getSystemService(Context.CLIPBOARD_SERVICE) as? ClipboardManager

  private val listener = ClipboardManager.OnPrimaryClipChangedListener {
    clipboardManager?.primaryClipDescription?.let { clip ->
      this@ClipboardModule.sendEvent(
        CLIPBOARD_CHANGED_EVENT_NAME,
        bundleOf(
          "contentTypes" to availableContentTypes(clip)
        )
      )
    }
  }
}

To subscribe to these events in JavaScript/TypeScript, you need to wrap the native module with EventEmitter class as shown:

import { requireNativeModule, EventEmitter, Subscription } from 'expo-modules-core';

const ClipboardModule = requireNativeModule('Clipboard');
const emitter = new EventEmitter(ClipboardModule);

export function addClipboardListener(listener: (event) => void): Subscription {
  return emitter.addListener('onClipboardChanged', listener);
}

View callbacks

Some events are connected to a certain view. For example, the touch event should be sent only to the underlying JavaScript view which was pressed. In that case, you can't use sendEvent described in Sending events. The expo-modules-core introduces a view callbacks mechanism to handle view-bound events.

To use it, in the view definition, you need to provide the event names that the view can send using the Events definition component. After that, you need to declare a property of type EventDispatcher in your view class. The name of the declared property has to be the same as the name exported in the Events component. Later, you can call it as a function and pass a payload of type [String: Any?] on iOS and Map<String, Any?> on Android.

Note: On Android, it's possible to specify the payload type. In case of types that don't convert into objects, the payload will be encapsulated and stored under the payload key: {payload: <provided value>}.

class CameraViewModule: Module {
  public func definition() -> ModuleDefinition {
    View(CameraView.self) {
      Events(
        "onCameraReady"
      )

      // ...
    }
  }
}

class CameraView: ExpoView {
  let onCameraReady = EventDispatcher()

  func callOnCameraReady() {
    onCameraReady([
      "message": "Camera was mounted"
    ]);
  }
}

class CameraViewModule : Module() {
  override fun definition() = ModuleDefinition {
    View(ExpoCameraView::class) {
      Events(
        "onCameraReady"
      )

      // ...
    }
  }
}

class CameraView(
  context: Context,
  appContext: AppContext
) : ExpoView(context, appContext) {
  val onCameraReady by EventDispatcher()

  fun callOnCameraReady() {
    onCameraReady(mapOf(
      "message" to "Camera was mounted"
    ));
  }
}

To subscribe to these events in JavaScript/TypeScript, you need to pass a function to the native view as shown:

import { requireNativeViewManager } from 'expo-modules-core';

const CameraView = requireNativeViewManager('CameraView');

export default function MainView() {
  const onCameraReady = event => {
    console.log(event.nativeEvent);
  };

  return <CameraView onCameraReady={onCameraReady} />;
}

Provided payload is available under the nativeEvent key.