How to access the background text-to-speech service

TextToSpeechService

public abstract class TextToSpeechService
extends Service

java.lang.Object
↳ android.content.Context
↳ android.content.ContextWrapper
↳ android.app.Service
↳ android.speech.tts.TextToSpeechService

Abstract base class for TTS engine implementations. The following methods need to be implemented:

onIsLanguageAvailable(String, String, String)
onLoadLanguage(String, String, String)
onGetLanguage()
onSynthesizeText(SynthesisRequest, SynthesisCallback)
onStop()

The first three deal primarily with language management, and are used to query the engine for it's support for a given language and indicate to it that requests in a given language are imminent.

onSynthesizeText(SynthesisRequest, SynthesisCallback) is central to the engine implementation. The implementation should synthesize text as per the request parameters and return synthesized data via the supplied callback. This class and its helpers will then consume that data, which might mean queuing it for playback or writing it to a file or similar. All calls to this method will be on a single thread, which will be different from the main thread of the service. Synthesis must be synchronous which means the engine must NOT hold on to the callback or call any methods on it after the method returns. onStop() tells the engine that it should stop all ongoing synthesis, if any. Any pending data from the current synthesis will be discarded. onGetLanguage() is not required as of JELLYBEAN_MR2 (API 18) and later, it is only called on earlier versions of Android. API Level 20 adds support for Voice objects. Voices are an abstraction that allow the TTS service to expose multiple backends for a single locale. Each one of them can have a different features set. In order to fully take advantage of voices, an engine should implement the following methods:
onGetVoices()
onIsValidVoiceName(String)
onLoadVoice(String)
onGetDefaultVoiceNameFor(String, String, String)

The first three methods are siblings of the onGetLanguage(), onIsLanguageAvailable(String, String, String) and onLoadLanguage(String, String, String) methods. The last one, onGetDefaultVoiceNameFor(String, String, String) is a link between locale and voice based methods. Since API level 21 TextToSpeech.setLanguage(Locale) is implemented by calling TextToSpeech.setVoice(Voice) with the voice returned by onGetDefaultVoiceNameFor(String, String, String). If the client uses a voice instead of a locale, SynthesisRequest will contain the requested voice name. The default implementations of Voice-related methods implement them using the pre-existing locale-based implementation.

Summary
Inherited constants
From class android.app.Service
From class android.content.Context
From interface android.content.ComponentCallbacks2
Public constructors
TextToSpeechService()
Public methods
IBinder onBind(Intent intent)

Return the communication channel to the service.
void onCreate()

Called by the system when the service is first created.
void onDestroy()

Called by the system to notify a Service that it is no longer used and is being removed.
String onGetDefaultVoiceNameFor(String lang, String country, String variant)

Return a name of the default voice for a given locale.
List onGetVoices()

Queries the service for a set of supported voices.
int onIsValidVoiceName(String voiceName)

Checks whether the engine supports a voice with a given name.
int onLoadVoice(String voiceName)

Notifies the engine that it should load a speech synthesis voice.
Protected methods
Set onGetFeaturesForLanguage(String lang, String country, String variant)

Queries the service for a set of features supported for a given language.
abstract String[] onGetLanguage()

Returns the language, country and variant currently being used by the TTS engine.
abstract int onIsLanguageAvailable(String lang, String country, String variant)

Checks whether the engine supports a given language.
abstract int onLoadLanguage(String lang, String country, String variant)

Notifies the engine that it should load a speech synthesis language.
abstract void onStop()

Notifies the service that it should stop any in-progress speech synthesis.
abstract void onSynthesizeText(SynthesisRequest request, SynthesisCallback callback)

Tells the service to synthesize speech from the given text.
Inherited methods
From class android.app.Service
From class android.content.ContextWrapper
From class android.content.Context
From class java.lang.Object
From interface android.content.ComponentCallbacks2
From interface android.content.ComponentCallbacks
Public constructors
TextToSpeechService
added in API level 14
public TextToSpeechService ()
Public methods
onBind
added in API level 14
public IBinder onBind (Intent intent)

Return the communication channel to the service. May return null if clients can not bind to the service. The returned IBinder is usually for a complex interface that has been described using aidl.

Note that unlike other application components, calls on to the IBinder interface returned here may not happen on the main thread of the process. More information about the main thread can be found in Processes and Threads.

Parameters

intent Intent: The Intent that was used to bind to this service, as given to Context.bindService. Note that any extras that were included with the Intent at that point will not be seen here.
Returns

IBinder Return an IBinder through which clients can call on to the service.
onCreate
added in API level 14
public void onCreate ()

Called by the system when the service is first created. Do not call this method directly.
onDestroy
added in API level 14
public void onDestroy ()

Called by the system to notify a Service that it is no longer used and is being removed. The service should clean up any resources it holds (threads, registered receivers, etc) at this point. Upon return, there will be no more calls in to this Service object and it is effectively dead. Do not call this method directly.

onGetDefaultVoiceNameFor
added in API level 21
public String onGetDefaultVoiceNameFor (String lang, String country, String variant)

Return a name of the default voice for a given locale. This method provides a mapping between locales and available voices. This method is used in TextToSpeech.setLanguage(Locale), which calls this method and then calls TextToSpeech.setVoice(Voice) with the voice returned by this method. Also, it's used by TextToSpeech.getDefaultVoice() to find a default voice for the default locale.

Parameters
lang String: ISO-3 language code.
country String: ISO-3 country code. May be empty or null.
variant String: Language variant. May be empty or null.
Returns

String A name of the default voice for a given locale.
onGetVoices
added in API level 21
public List onGetVoices ()

Queries the service for a set of supported voices. Can be called on multiple threads. The default implementation tries to enumerate all available locales, pass them to onIsLanguageAvailable(String, String, String) and create Voice instances (using the locale's BCP-47 language tag as the voice name) for the ones that are supported. Note, that this implementation is suitable only for engines that don't have multiple voices for a single locale. Also, this implementation won't work with Locales not listed in the set returned by the Locale.getAvailableLocales() method.

Returns
List A list of voices supported.
onIsValidVoiceName
added in API level 21

public int onIsValidVoiceName (String voiceName)

Checks whether the engine supports a voice with a given name. Can be called on multiple threads. The default implementation treats the voice name as a language tag, creating a Locale from the voice name, and passes it to onIsLanguageAvailable(String, String, String).

Parameters

voiceName String: Name of the voice.

Returns

int TextToSpeech.ERROR or TextToSpeech.SUCCESS.

onLoadVoice

added in API level 21

public int onLoadVoice (String voiceName)

Notifies the engine that it should load a speech synthesis voice. There is no guarantee that this method is always called before the voice is used for synthesis. It is merely a hint to the engine that it will probably get some synthesis requests for this voice at some point in the future. Will be called only on synthesis thread. The default implementation creates a Locale from the voice name (by interpreting the name as a BCP-47 tag for the locale), and passes it to onLoadLanguage(String, String, String).

Parameters

voiceName String: Name of the voice.
Returns

int TextToSpeech.ERROR or TextToSpeech.SUCCESS.
Protected methods
onGetFeaturesForLanguage
added in API level 15
protected Set onGetFeaturesForLanguage (String lang, String country, String variant)
Queries the service for a set of features supported for a given language. Can be called on multiple threads.

Parameters

lang String: ISO-3 language code.
country String: ISO-3 country code. May be empty or null.
variant String: Language variant. May be empty or null.
Returns
Set A list of features supported for the given language.
onGetLanguage
added in API level 14
protected abstract String[] onGetLanguage ()

Returns the language, country and variant currently being used by the TTS engine. This method will be called only on Android 4.2 and before (API <= 17). In later versions this method is not called by the Android TTS framework. Can be called on multiple threads.

Returns
String[] A 3-element array, containing language (ISO 3-letter code), country (ISO 3-letter code) and variant used by the engine. The country and variant may be "". If country is empty, then variant must be empty too.
See also:

Locale.getISO3Language()
Locale.getISO3Country()
Locale.getVariant()
onIsLanguageAvailable
added in API level 14
protected abstract int onIsLanguageAvailable (String lang, String country, String variant)

Checks whether the engine supports a given language. Can be called on multiple threads. Its return values HAVE to be consistent with onLoadLanguage.

Parameters

lang String: ISO-3 language code.
country String: ISO-3 country code. May be empty or null.
variant String: Language variant. May be empty or null.
Returns

int Code indicating the support status for the locale. One of TextToSpeech.LANG_AVAILABLE, TextToSpeech.LANG_COUNTRY_AVAILABLE, TextToSpeech.LANG_COUNTRY_VAR_AVAILABLE, TextToSpeech.LANG_MISSING_DATA TextToSpeech.LANG_NOT_SUPPORTED.
onLoadLanguage
added in API level 14
protected abstract int onLoadLanguage (String lang, String country, String variant)

Notifies the engine that it should load a speech synthesis language. There is no guarantee that this method is always called before the language is used for synthesis. It is merely a hint to the engine that it will probably get some synthesis requests for this language at some point in the future. Can be called on multiple threads. In <= Android 4.2 (<= API 17) can be called on main and service binder threads. In > Android 4.2 (> API 17) can be called on main and synthesis threads.

Parameters
lang String: ISO-3 language code.
country String: ISO-3 country code. May be empty or null.
variant String: Language variant. May be empty or null.
Returns
int Code indicating the support status for the locale. One of TextToSpeech.LANG_AVAILABLE, TextToSpeech.LANG_COUNTRY_AVAILABLE, TextToSpeech.LANG_COUNTRY_VAR_AVAILABLE, TextToSpeech.LANG_MISSING_DATA TextToSpeech.LANG_NOT_SUPPORTED.
onStop
added in API level 14
protected abstract void onStop ()
Notifies the service that it should stop any in-progress speech synthesis. This method can be called even if no speech synthesis is currently in progress. Can be called on multiple threads, but not on the synthesis thread.

onSynthesizeText
added in API level 14
protected abstract void onSynthesizeText (SynthesisRequest request, SynthesisCallback callback)
Tells the service to synthesize speech from the given text. This method should block until the synthesis is finished. Called on the synthesis thread.

Parameters
request SynthesisRequest: The synthesis request.
callback SynthesisCallback: The callback that the engine must use to make data available for playback or for writing to a file.

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