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--- title: Enable closed captions for iOS titleSuffix: An Azure Communication Services article description: This article describes how to add closed captions to your existing iOS calling app using Azure Communication Services. author: Kunaal services: azure-communication-services ms.subservice: calling ms.author: valindrae ms.date: 06/28/2025 ms.topic: include ms.service: azure-communication-services --- ## Prerequisites - Azure account with an active subscription, for details see [Create an account for free.](https://azure.microsoft.com/pricing/purchase-options/azure-account?cid=msft_learn) - Azure Communication Services resource. See [Create an Azure Communication Services resource](../../../../quickstarts/create-communication-resource.md?tabs=windows&pivots=platform-azp). Save the connection string for this resource. - An app with voice and video calling, refer to our [Voice](../../../../quickstarts/voice-video-calling/getting-started-with-calling.md) and [Video](../../../../quickstarts/voice-video-calling/get-started-with-video-calling.md) calling quickstarts. > [!NOTE] > You need to have a voice calling app using Azure Communication Services calling SDKs to access the closed captions feature that is described in this guide. ## Models | Name | Description | | --- | --- | | CaptionsCallFeature | API for captions call feature | | CommunicationCaptions | API for communication captions | | StartCaptionOptions | Closed caption options like spoken language | | CommunicationCaptionsDelegate | Delegate for communication captions | | CommunicationCaptionsReceivedEventArgs | Data object received for each communication captions received event | ## Get closed captions feature You need to get and cast the captions object to use captions specific features. ``` swift if let call = self.call { @State var captionsCallFeature = call.feature(Features.captions) captionsCallFeature.getCaptions{(value, error) in if let error = error { // failed to get captions } else { if (value?.type == CaptionsType.communicationCaptions) { // communication captions @State var communicationCaptions = value as? CommunicationCaptions } } } } ``` ## Subscribe to listeners ### Add a listener to receive captions enabled/disabled, type, spoken language, caption language status changed, and data received The event `didChangeActiveCaptionsType` triggers when the caption type changes from `CommunicationCaptions` to `TeamsCaptions` upon inviting Microsoft 365 users to ACS-only calls. ```swift extension CallObserver: CommunicationCaptionsDelegate { // listener for receive captions enabled/disabled status public func communicationCaptions(_ communicationCaptions: CommunicationCaptions, didChangeCaptionsEnabledState args: PropertyChangedEventArgs) { // communicationCaptions.isEnabled } // listener for active spoken language state change public func communicationCaptions(_ communicationCaptions: CommunicationCaptions, didChangeActiveSpokenLanguageState args: PropertyChangedEventArgs) { // communicationCaptions.activeSpokenLanguage } // listener for captions data received public func communicationCaptions(_ communicationCaptions: CommunicationCaptions, didReceiveCaptions:CommunicationCaptionsReceivedEventArgs) { // Information about the speaker. // didReceiveCaptions.speaker // The original text with no transcribed. // didReceiveCaptions.spokenText // language identifier for the speaker. // didReceiveCaptions.spokenLanguage // Timestamp denoting the time when the corresponding speech was made. // didReceiveCaptions.timestamp // CaptionsResultType is Partial if text contains partially spoken sentence. // It is set to Final once the sentence has been completely transcribed. // didReceiveCaptions.resultType } } communicationCaptions.delegate = self.callObserver extension CallObserver: CaptionsCallFeatureDelegate { // captions type changed public func captionsCallFeature(_ captionsCallFeature: CaptionsCallFeature, didChangeActiveCaptionsType args: PropertyChangedEventArgs) { // captionsCallFeature.getCaptions to get captions } } captionsCallFeature.delegate = self.callObserver ``` ## Start captions Once you set up all your listeners, you can start adding captions. ``` swift func startCaptions() { guard let communicationCaptions = communicationCaptions else { return } let startCaptionsOptions = StartCaptionsOptions() startCaptionsOptions.spokenLanguage = "en-us" communicationCaptions.startCaptions(startCaptionsOptions: startCaptionsOptions, completionHandler: { (error) in if error != nil { } }) } ``` ## Stop captions ``` swift func stopCaptions() { communicationCaptions.stopCaptions(completionHandler: { (error) in if error != nil { } }) } ``` ## Remove caption received listener ``` swift communicationCaptions?.delegate = nil ``` ## Spoken language support ### Get list of supported spoken languages Get a list of supported spoken languages that your users can select from when enabling closed captions. ``` swift // bcp 47 formatted language code let spokenLanguage : String = "en-us" for language in communicationCaptions?.supportedSpokenLanguages ?? [] { // choose required language spokenLanguage = language } ``` ### Set spoken language When the user selects the spoken language, your app can set the spoken language that it expects captions to be generated from. ``` swift func setSpokenLanguage() { guard let communicationCaptions = self.communicationCaptions else { return } communicationCaptions.set(spokenLanguage: spokenLanguage, completionHandler: { (error) in if let error = error { } }) } ``` ## Clean up See [cleaning up resources](../../../create-communication-resource.md?pivots=platform-azp&tabs=windows#clean-up-resources).