Expo Docs MCP Server

--- title: Module API Reference description: An API reference of Expo modules API. sidebar_title: Module API --- The native modules API is an abstraction layer on top of [JSI](https://reactnative.dev/architecture/glossary#javascript-interfaces-jsi) and other low-level primitives that React Native is built upon. It is built with modern languages (Swift and Kotlin) and provides an easy-to-use and convenient API that is consistent across platforms where possible. ## Definition components As you might have noticed in the snippets on the [Get Started](/modules/get-started) page, each module class must implement the `definition` function. The module definition consists of the DSL components that describe the module's functionality and behavior. 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. ```swift Swift / Kotlin Name("MyModuleName") ``` Sets constant properties on the module. Can take a dictionary or a closure that returns a dictionary. <CodeBlocksTable> ```swift // Created from the dictionary Constants([ "PI": Double.pi ]) // or returned by the closure Constants { return [ "PI": Double.pi ] } ``` ```kotlin // Passed as arguments Constants( "PI" to kotlin.math.PI ) // or returned by the closure Constants { return@Constants mapOf( "PI" to kotlin.math.PI ) } ``` </CodeBlocksTable> 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](#argument-types) section for more details on what types can be used in the function body. <CodeBlocksTable> ```swift Function("mySyncFunction") { (message: String) in return message } ``` ```kotlin Function("mySyncFunction") { message: String -> return@Function message } ``` </CodeBlocksTable> ```js JavaScript // Assume that we have named the module "MyModule" const MyModule = requireNativeModule('MyModule'); function getMessage() { return MyModule.mySyncFunction('bar'); } ``` 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](#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 <CodeBlocksTable> ```swift AsyncFunction("myAsyncFunction") { (message: String) in return message } // or AsyncFunction("myAsyncFunction") { (message: String, promise: Promise) in promise.resolve(message) } ``` ```kotlin 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) } ``` </CodeBlocksTable> ```js JavaScript // 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. <CodeBlocksTable> ```swift AsyncFunction("myAsyncFunction") { (message: String) in return message }.runOnQueue(.main) ``` ```kotlin AsyncFunction("myAsyncFunction") { message: String -> return@AsyncFunction message }.runOnQueue(Queues.MAIN) ``` </CodeBlocksTable> --- #### Kotlin coroutines `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](https://kotlinlang.org/docs/coroutines-overview.html). `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. ```kotlin Kotlin 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 } ``` Defines event names that the module can send to JavaScript. > **Note:** This component can be used inside of the [`View`](#view) block to define callback names. See [`View callbacks`](#view-callbacks) <CodeBlocksTable> ```swift Events("onCameraReady", "onPictureSaved", "onBarCodeScanned") ``` ```kotlin Events("onCameraReady", "onPictureSaved", "onBarCodeScanned") ``` </CodeBlocksTable> See [Sending events](#sending-events) to learn how to send events from the native code to JavaScript/TypeScript. Defines a new property directly on the JavaScript object that represents a native module. It is the same as calling [`Object.defineProperty`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/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. <CodeBlocksTable> ```swift Property("foo") { return "bar" } ``` ```kotlin Property("foo") { return@Property "bar" } ``` </CodeBlocksTable> 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. <CodeBlocksTable> ```swift Property("foo") .get { return "bar" } .set { (newValue: String) in // do something with new value } ``` ```kotlin Property("foo") .get { return@get "bar" } .set { newValue: String -> // do something with new value } ``` </CodeBlocksTable> ```js JavaScript // 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'; ``` Enables the module to be used as a native view. Definition components that are accepted as part of the view definition: [`Prop`](#prop), [`Events`](#events), [`GroupView`](#groupview) and [`AsyncFunction`](#asyncfunction). [`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`](#expoview), on iOS it's optional. See [`Extending ExpoView`](#extending--expoview). - **definition**: `() -> ViewDefinition` — A builder of the view definition. <CodeBlocksTable> ```swift View(UITextView.self) { Prop("text") { /* @hide ... */ /* @end */ } AsyncFunction("focus") { (view: UITextView) in view.becomeFirstResponder() } } ``` ```kotlin View(TextView::class) { Prop("text") { /* @hide ... */ /* @end */ } AsyncFunction("focus") { view: TextView -> view.requestFocus() } } ``` </CodeBlocksTable> > **Info** Support for rendering SwiftUI views is planned. For now, you can use [`UIHostingController`](https://developer.apple.com/documentation/swiftui/uihostingcontroller) and add its content view to your UIKit view. 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. Defines module's lifecycle listener that is called when the module is about to be deallocated. Use it instead of module's class destructor. Defines the function that is invoked when the first event listener is added. Defines the function that is invoked when all event listeners are removed. Defines module's lifecycle listener that is called when the app context owning the module is about to be deallocated. 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`](#onactivityentersforeground) instead. 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`](#onactivityentersbackground) instead. 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`](#onactivityentersforeground) instead. 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`](#onappentersforeground) instead. 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`](#onappentersbackground) instead. 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`](#onappentersbackground) instead. Defines the activity lifecycle listener that is called when the activity launched with `startActivityForResult` returns a result. #### Arguments - **activity** — The Android activity that received the result. - **payload** — An object containing data about the activity result. - **requestCode**: `Int` — The request code originally supplied to `startActivityForResult`, used to identify the source of the result. - **resultCode**: `Int` — The result code returned by the child activity (for example, `Activity.RESULT_OK` or `Activity.RESULT_CANCELED`). - **data** — An optional intent that carries the result data returned from the launched activity. Can be `null`. ```kotlin Kotlin AsyncFunction('someFunc') { /* @hide ... */ /* @end */ activity.startActivityForResult(someIntent, SOME_REQUEST_CODE) } OnActivityResult { activity, payload -> /* @hide ... */ /* @end */ } ``` ## View definition components The view definition consists of the DSL components that describe the view's functionality and behavior. Those components can only be used within a [`View`](#view) closure. Sets the name of the view that JavaScript code will use to refer to the view. Takes a string as an argument. This can be inferred from the view's class name, but it's recommended to set it explicitly for clarity. ```swift Swift / Kotlin Name("MyViewName") ``` Defines a setter for the view prop of given name. #### Arguments - **name**: `String` — Name of view prop that you want to define a setter. - **defaultValue**: `ValueType` — Optional default value used when the setter is called with `null`. - **setter**: `(view: ViewType, value: ValueType) -> ()` — Closure that is invoked when the view rerenders. This property can only be used within a [`View`](#view) closure. <CodeBlocksTable> ```swift Prop("background") { (view: UIView, color: UIColor) in view.backgroundColor = color } ``` ```kotlin Prop("background") { view: View, @ColorInt color: Int -> view.setBackgroundColor(color) } ``` </CodeBlocksTable> Prop definition with default value. <CodeBlocksTable> ```swift Prop("background", UIColor.black) { (view: UIView, color: UIColor) in view.backgroundColor = color } ``` ```kotlin Prop("background", Color.BLACK) { view: View, @ColorInt color: Int -> view.setBackgroundColor(color) } ``` </CodeBlocksTable> > **Note:** Props of function type (callbacks) are not supported yet. Defines the view lifecycle method that is called when the view finished updating all props. ```swift View(MyView.self) { OnViewDidUpdateProps { view: MyView in /* @hide ... */ /* @end */ } } ``` ```kotlin View(MyView::class) { OnViewDidUpdateProps { view: MyView -> /* @hide ... */ /* @end */ } } ``` Similarly to the [`AsyncFunction`](#asyncfunction) inside the module definition, you can define functions attached to the view ref to allow direct modification of the native view. View async functions will always be dispatched on the main queue and can receive the view instance as the first argument. <CodeBlocksTable> ```swift View(MyView.self) { AsyncFunction("myAsyncFunction") { (view: MyView, message: String) in view.displayMessage(message) } } ``` ```kotlin View(MyView::class) { AsyncFunction("myAsyncFunction") { view: MyView, message: String -> view.displayMessage(message); } } ``` </CodeBlocksTable> ```js JavaScript const MyNativeView = requireNativeViewManager('MyView'); function MyComponent() { const ref = React.useRef(null); React.useEffect(() => { ref.current?.myAsyncFunction(); }, [ref]); return ; } ``` Enables the view to be used as a view group. Definition components that are accepted as part of the group view definition: [`AddChildView`](#addchildview), [`GetChildCount`](#getchildcount), [`GetChildViewAt`](#getchildviewat), [`RemoveChildView`](#removechildview), [`RemoveChildViewAt`](#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`](#view) closure. ```kotlin Kotlin GroupView<ViewGroup> { AddChildView { parent, child, index -> /* @hide ... */ /* @end */} } ``` 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`](#groupview) closure. ```kotlin Kotlin AddChildView { parent, child: View, index -> parent.addView(child, index) } ``` 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`](#groupview) closure. ```kotlin Kotlin GetChildCount { parent -> return@GetChildCount parent.childCount } ``` 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`](#groupview) closure. ```kotlin Kotlin GetChildViewAt { parent, index -> parent.getChildAt(index) } ``` 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`](#groupview) closure. ```kotlin Kotlin RemoveChildView { parent, child: View -> parent.removeView(child) } ``` 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`](#groupview) closure. ```kotlin Kotlin RemoveChildViewAt { parent, index -> parent.removeViewAt(child) } ``` ## Argument types Fundamentally, only primitive and serializable data can be passed back and forth between the runtimes. However, usually native modules need to receive custom data structures — more sophisticated than just the dictionary/map where the values are of unknown (`Any`) type and so each value has to be validated and cast on its own. The Expo Modules API provides protocols to make it more convenient to work with data objects, to provide automatic validation, and finally, to ensure native type-safety on each object member. 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. | Language | Supported primitive types | | -------- | ------------------------------------------------------------------------------------------------------------------------------ | | Swift | `Bool`, `Int`, `Int8`, `Int16`, `Int32`, `Int64`, `UInt`, `UInt8`, `UInt16`, `UInt32`, `UInt64`, `Float32`, `Double`, `String` | | Kotlin | `Boolean`, `Int`, `Long`, `Float`, `Double`, `String`, `Pair` | _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. The built-in Convertibles are documented [further below](#built-in-convertibles). You can define additional Convertibles by making native Swift types conform to the `Convertible` protocol: <PaddedAPIBox header="Convertible" platforms={["ios"]}> `Convertible` is a Swift protocol with one static method: ### Example ```swift Swift import ExpoModulesCore extension CMTime: @retroactive Convertible { public static func convert(from value: Any?, appContext: AppContext) throws -> CMTime { if let seconds = value as? Double { return CMTime(seconds: seconds, preferredTimescale: .max) } throw Conversions.ConvertingException<CMTime>(value) } } ``` > **Info** Support for defining Convertibles with Kotlin is planned to be available by SDK 53. --- ## Built-in Convertibles Some common iOS types from the `CoreGraphics` and `UIKit` system frameworks are already made convertible. | Native iOS Type | TypeScript | | ----------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | `URL` | `string` with a URL. When a scheme is not provided, it's assumed to be a file URL. | | `CGFloat` | `number` | | `CGPoint` | `{ x: number, y: number }` or `number[]` with _x_ and _y_ coords | | `CGSize` | `{ width: number, height: number }` or `number[]` with _width_ and _height_ | | `CGVector` | `{ dx: number, dy: number }` or `number[]` with _dx_ and _dy_ vector differentials | | `CGRect` | `{ x: number, y: number, width: number, height: number }` or `number[]` with _x_, _y_, _width_ and _height_ values | | `CGColor`<br/>`UIColor` | Color hex strings (`#RRGGBB`, `#RRGGBBAA`, `#RGB`, `#RGBA`), named colors following the [CSS3/SVG specification](https://www.w3.org/TR/css-color-3/#svg-color) or `"transparent"` | | `Data` | `Uint8Array` | --- Similarly, some common Android types from packages like `java.io`, `java.net`, or `android.graphics` are also made convertible. > **Note:** On Android, primitive arrays should be used whenever possible. | Native Android Type | TypeScript | | ----------------------------------------------------------------------------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | `java.net.URL` | `string` with a URL. Note that the scheme has to be provided (URL should not contain any unencoded `%` character) | | `android.net.Uri`<br/>`java.net.URI` | `string` with a URI. Note that the scheme has to be provided (URI should not contain any unencoded `%` character) | | `java.io.File`<br/>`java.nio.file.Path` (is only available on Android API 26) | `string` with a path to the file | | `android.graphics.Color` | Color hex strings (`#RRGGBB`, `#RRGGBBAA`, `#RGB`, `#RGBA`), named colors following the [CSS3/SVG specification](https://www.w3.org/TR/css-color-3/#svg-color) or `"transparent"` | | `kotlin.Pair` | Array with two values, where the first one is of type _A_ and the second is of type _B_ | | `kotlin.ByteArray` | `Uint8Array` | | `kotlin.BooleanArray` | `boolean[]` | | `kotlin.IntArray`<br/>`kotlin.FloatArray`<br/>`kotlin.LongArray`<br/>`kotlin.DoubleArray` | `number[]` | | `kotlin.time.Duration` | `number` represents a duration in seconds | </PaddedAPIBox> <PaddedAPIBox header="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. <CodeBlocksTable> ```swift 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` } ``` ```kotlin 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` } ``` </CodeBlocksTable> </PaddedAPIBox> <PaddedAPIBox header="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`. <CodeBlocksTable> ```swift enum FileEncoding: String, Enumerable { case utf8 case base64 } struct FileReadOptions: Record { @Field var encoding: FileEncoding = .utf8 /* @hide ... */ /* @end */ } ``` ```kotlin // 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 /* @hide ... */ /* @end */ } ``` </CodeBlocksTable> </PaddedAPIBox> <PaddedAPIBox header="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. <CodeBlocksTable> ```swift 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 } } ``` ```kotlin Function("foo") { bar: Either<String, Int> -> bar.get(String::class).let { // `it` is a String } bar.get(Int::class).let { // `it` is an Int } } ``` </CodeBlocksTable> 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. </PaddedAPIBox> <PaddedAPIBox header="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. <CodeBlocksTable> ```swift 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") } ``` ```kotlin Function("mutateMe") { value: JavaScriptValue -> if (value.isObject()) { val jsObject = value.getObject() jsObject.setProperty("expo", "modules") } } // or Function("mutateMe") { jsObject: JavaScriptObject -> jsObject.setProperty("expo", "modules") } ``` </CodeBlocksTable> </PaddedAPIBox> ## Native classes <PaddedAPIBox header="Module"> A base class for a native module. #### Properties #### Methods <APIMethod name="sendEvent" comment="Sends an event with a given name and a payload to JavaScript. See [`Sending events`](#sending-events)" returnTypeName="void" parameters={[ { name: 'eventName', comment: 'The name of the JavaScript event', typeName: 'string', }, { name: 'payload', comment: 'The event payload', typeName: 'Android: Map<String, Any?> | Bundle\niOS: [String: Any?]', }, ]} /> </PaddedAPIBox> <PaddedAPIBox header="AppContext"> The app context is an interface to a single Expo app. #### Properties </PaddedAPIBox> <PaddedAPIBox header="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`](#view) component, your custom class must inherit from the `ExpoView`. By doing that you will get access to the [`AppContext`](#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`. <CodeBlocksTable> ```swift class LinearGradientView: ExpoView {} public class LinearGradientModule: Module { public func definition() -> ModuleDefinition { View(LinearGradientView.self) { /* @hide ... */ /* @end */ } } } ``` ```kotlin class LinearGradientView( context: Context, appContext: AppContext, ) : ExpoView(context, appContext) class LinearGradientModule : Module() { override fun definition() = ModuleDefinition { View(LinearGradientView::class) { /* @hide ... */ /* @end */ } } } ``` </CodeBlocksTable> </PaddedAPIBox> ## Guides <PaddedAPIBox> ### 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](#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: <CodeBlocksTable> ```swift 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() ]) } } ``` ```kotlin 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) ) ) } } } ``` </CodeBlocksTable> To subscribe to these events in JavaScript/TypeScript, use [`addListener`](/versions/latest/sdk/expo/#addlistenereventname-listener) on the module object returned by `requireNativeModule`. Modules are extending the built-in [`EventEmitter`](/versions/latest/sdk/expo/#eventemitter) class. Alternatively, you can use [`useEvent`](/versions/latest/sdk/expo/#useeventeventemitter-eventname-initialvalue) or [`useEventListener`](/versions/latest/sdk/expo/#useeventlistenereventemitter-eventname-listener) hooks. ```ts TypeScript type ClipboardChangeEvent = { contentTypes: string[]; }; type ClipboardModuleEvents = { onClipboardChanged(event: ClipboardChangeEvent): void; }; declare class ClipboardModule extends NativeModule<ClipboardModuleEvents> {} const Clipboard = requireNativeModule<ClipboardModule>('Clipboard'); Clipboard.addListener('onClipboardChanged', (event: ClipboardChangeEvent) => { alert('Clipboard has changed'); }); ``` </PaddedAPIBox> <PaddedAPIBox> ### 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`](#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](#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>}`. <CodeBlocksTable> ```swift class CameraViewModule: Module { public func definition() -> ModuleDefinition { View(CameraView.self) { Events( "onCameraReady" ) /* @hide ... */ /* @end */ } } } class CameraView: ExpoView { let onCameraReady = EventDispatcher() func callOnCameraReady() { onCameraReady([ "message": "Camera was mounted" ]); } } ``` ```kotlin class CameraViewModule : Module() { override fun definition() = ModuleDefinition { View(ExpoCameraView::class) { Events( "onCameraReady" ) /* @hide ... */ /* @end */ } } } class CameraView( context: Context, appContext: AppContext ) : ExpoView(context, appContext) { val onCameraReady by EventDispatcher() fun callOnCameraReady() { onCameraReady(mapOf( "message" to "Camera was mounted" )); } } ``` </CodeBlocksTable> To subscribe to these events in JavaScript/TypeScript, you need to pass a function to the native view as shown: ```tsx TypeScript const CameraView = requireNativeViewManager('CameraView'); const onCameraReady = event => { console.log(event.nativeEvent); }; return ; } ``` Provided payload is available under the `nativeEvent` key. </PaddedAPIBox> ## Examples <CodeBlocksTable> ```swift public class MyModule: Module { public func definition() -> ModuleDefinition { Name("MyFirstExpoModule") Function("hello") { (name: String) in return "Hello \(name)!" } } } ``` ```kotlin class MyModule : Module() { override fun definition() = ModuleDefinition { Name("MyFirstExpoModule") Function("hello") { name: String -> return "Hello $name!" } } } ``` </CodeBlocksTable> For more examples from real modules, you can refer to Expo modules that already use this API on GitHub: <FileTree files={[ [ 'expo-battery', <A href="https://github.com/expo/expo/tree/main/packages/expo-battery/ios">Swift, ], [ 'expo-cellular', <A href="https://github.com/expo/expo/tree/main/packages/expo-cellular/android/src/main/java/expo/modules/cellular"> Kotlin </A> , <A href="https://github.com/expo/expo/tree/main/packages/expo-cellular/ios">Swift</A> , ], [ 'expo-clipboard', <A href="https://github.com/expo/expo/tree/main/packages/expo-clipboard/android/src/main/java/expo/modules/clipboard"> Kotlin </A> , <A href="https://github.com/expo/expo/tree/main/packages/expo-clipboard/ios">Swift</A> , ], [ 'expo-crypto', <A href="https://github.com/expo/expo/tree/main/packages/expo-crypto/android/src/main/java/expo/modules/crypto"> Kotlin </A> , <A href="https://github.com/expo/expo/tree/main/packages/expo-crypto/ios">Swift</A> , ], [ 'expo-device', Swift, ], [ 'expo-haptics', Swift, ], [ 'expo-image-manipulator', Swift , ], [ 'expo-image-picker', <A href="https://github.com/expo/expo/tree/main/packages/expo-image-picker/android/src/main/java/expo/modules/imagepicker"> Kotlin </A> , <A href="https://github.com/expo/expo/tree/main/packages/expo-image-picker/ios">Swift</A> , ], [ 'expo-linear-gradient', <A href="https://github.com/expo/expo/tree/main/packages/expo-linear-gradient/android/src/main/java/expo/modules/lineargradient"> Kotlin </A> ,{' '} <A href="https://github.com/expo/expo/tree/main/packages/expo-linear-gradient/ios">Swift</A> , ], [ 'expo-localization', <A href="https://github.com/expo/expo/tree/main/packages/expo-localization/android/src/main/java/expo/modules/localization"> Kotlin </A> , <A href="https://github.com/expo/expo/tree/main/packages/expo-localization/ios">Swift</A> , ], [ 'expo-store-review', Swift, ], [ 'expo-system-ui', Swift , ], [ 'expo-video-thumbnails', Swift, ], [ 'expo-web-browser', <A href="https://github.com/expo/expo/tree/main/packages/expo-web-browser/android/src/main/java/expo/modules/webbrowser"> Kotlin </A> , <A href="https://github.com/expo/expo/tree/main/packages/expo-web-browser/ios">Swift</A> , ], ]} />