speak 0.1.0

Text-to-speech for Flutter on iOS, macOS, Android and web, made for short spoken prompts over other audio. No native code to write or set up.

speak

Text-to-speech for Flutter on iOS, macOS, Android and the web, made for short spoken prompts over other audio: turn-by-turn directions, workout cues, timers.

final speak = Speak(audio: SpeakAudio.navigation);

await speak('In two hundred metres, turn left.');
await speak('Turn left now.', queue: SpeakQueue.interrupt);

• Awaitable. Every call completes when the text has been spoken, or tells you how it ended instead: completed, interrupted or expired.
• Queue or interrupt. Utterances wait their turn by default. An urgent one cuts in, and the ones it displaces complete as interrupted. A distance announcement that waited too long can expire unsaid.
• Plays well with other audio. Music is lowered (and podcasts paused) only while something is spoken, and comes back afterwards: iOS's audio session and Android's audio focus are claimed right before speaking and released shortly after. A short silent lead-in keeps Bluetooth headsets from eating the first syllable.
• Voices. List them with their language (BCP-47) and quality, pick one or let speak pick the best for a language, and check whether a language can be spoken before you try.
• Word progress, pause and resume on every platform.
• No native code to write or set up: no Podfile, no Swift package, no Kotlin. iOS and macOS call AVFoundation from Dart through FFI, Android calls TextToSpeech through JNI, the web uses the Web Speech API. See How it works for what is compiled for you.

Platform	Support
iOS 13+	AVSpeechSynthesizer, AVAudioSession
macOS 10.15+	AVSpeechSynthesizer
Android 7+ (API 24)	TextToSpeech, audio focus
Web	Web Speech API, also compiled to Wasm
Windows, Linux	Not supported: Speak.isSupported is false

Install

flutter pub add speak

This needs Flutter 3.38 or later, the first release where build hooks run for you.

• iOS: to keep speaking with the screen locked, add the audio background mode to Info.plist:

  <key>UIBackgroundModes</key>
  <array><string>audio</string></array>
  

• Android: nothing to add. The TTS_SERVICE query that Android 11+ needs to see the installed engines is merged into your manifest from the package.
• macOS and web: nothing to add.

Usage

Speak and wait

final speak = Speak();

final outcome = await speak('Your timer is done.');
// SpeakOutcome.completed, or .interrupted if stop() or an interrupting
// utterance cut it off.

A Speak is one speaker. Create it once for the feature that talks, and dispose() it when that feature goes away.

Queue, interrupt, expire

speak('In 500 metres, turn right.');            // waits its turn
speak('In 200 metres, turn right.',
    expireAfter: const Duration(seconds: 6));    // dropped if still waiting
speak('Turn right now.', queue: SpeakQueue.interrupt); // cuts in

await speak.stop(); // everything ends as interrupted

Voices and languages

final german = await speak.voices(language: 'de'); // best first
await speak('Links abbiegen.', voice: german.first);

// Or let speak choose the best installed voice for a language:
await speak('Tournez à gauche.', language: 'fr-FR');

// Defaults for every call:
speak
  ..language = 'en-GB'
  ..rate = 1.1; // 1 is normal speed

A Voice has an id (store this to remember a choice), a name, a language, a quality (low to premium), requiresNetwork and isInstalled. When speak chooses for a language, it prefers the asked region, voices that work offline, and better quality, in that order.

Is a language available?

switch (await speak.availability('de-DE')) {
  case VoiceAvailability.available:
    break;
  case VoiceAvailability.needsNetwork:
    // Only an online voice speaks it.
  case VoiceAvailability.notInstalled:
    // Android: the voice data has to be downloaded in the system settings.
  case VoiceAvailability.unsupported:
    // No voice for it at all.
}

Speaking a language no installed voice speaks fails with SpeakException(SpeakError.languageUnavailable).

Word progress

await speak(
  text,
  onProgress: (progress) => setState(() => highlighted = progress),
);
// progress.start, progress.end: offsets into text; progress.word

Pause and resume

await speak.pause();
await speak.resume();

Sharing audio with other apps

final guidance = Speak(audio: SpeakAudio.navigation);

final custom = Speak(
  audio: const SpeakAudio(
    otherAudio: OtherAudio.pauseSpoken, // music plays on, podcasts pause
    usage: AudioUsage.navigation,
    leadIn: Duration(milliseconds: 500),
    linger: Duration(seconds: 2),
  ),
);

The audio is claimed right before speaking and given back shortly after, as navigation apps do: on iOS the session is activated (category first, then setActive(true)) and later deactivated with notifyOthersOnDeactivation, so other apps raise their volume or resume; on Android transient audio focus is requested and then abandoned. Nothing is held while the app is quiet, which is also what costs the least battery.

What happens to music and podcasts

OtherAudio	iOS: music	iOS: podcasts, audiobooks	Android (music and podcasts alike)
mix	plays on at full volume	plays on	no audio focus: plays on
duck (SpeakAudio.prompt, the default)	lowered	lowered	lowered (AUDIOFOCUS_GAIN_TRANSIENT_MAY_DUCK)
duckAndPauseSpoken (SpeakAudio.navigation)	lowered	paused, resumed after	lowered; see below
pauseSpoken	plays on at full volume	paused, resumed after	lowered; see below
pause (SpeakAudio.exclusive)	paused, resumed after	paused, resumed after	paused, resumed after (AUDIOFOCUS_GAIN_TRANSIENT)

On iOS the options are mixWithOthers, plus duckOthers for the modes that lower, plus interruptSpokenAudioAndMixWithOthers for those that pause spoken audio; pause sets no mixing option. Android has no "spoken audio" category to pause: an app can only ask for focus that may duck. A podcast player that prefers pausing to being lowered pauses itself (setWillPauseWhenDucked), and nothing can make another app do so; the rest are lowered. On macOS other apps are never affected. A web page cannot touch other audio at all.

AudioUsage	iOS mode	Android usage
navigation	voicePrompt	USAGE_ASSISTANCE_NAVIGATION_GUIDANCE
prompt (default)	voicePrompt	USAGE_ASSISTANT
spokenContent	spokenAudio	USAGE_MEDIA

On iOS speech uses the playback category, so it is heard with the Ring/Silent switch on silent. On Android the attributes are always CONTENT_TYPE_SPEECH.

Lead-in and linger: Bluetooth without lost syllables

A Bluetooth headset or car stereo mutes its stream while nothing plays and takes a moment to open it again, so speech that starts at once loses its first syllables. leadIn (350 ms by default) plays silence first, but only before the first utterance after the audio is claimed: utterances queued behind it, or arriving within linger, follow straight on. linger (1.5 s by default) keeps the audio claimed after the last utterance, so prompts close together are not each preceded by a lead-in, and music is not raised and lowered again in between. stop(), pause() and dispose() give the audio back at once. On iOS and macOS the lead-in is real silent samples played through the session; on Android it is playSilentUtterance; the web has none. It is not counted against an utterance's timeout.

This is the recommended setup for navigation: the radio path is only open while speaking, plus a second and a half.

Holding the audio

await speak.holdAudio() claims the audio at once and keeps it until releaseAudio(), however long the gaps. It costs battery: the audio path, and on Bluetooth the A2DP stream, stays active the whole time, and other audio stays lowered or paused the whole time. Use it only for a short stretch of continuous prompts; the lead-in and a short linger cover the rest.

Android: choose the engine

final engines = await Speak.androidEngines(); // the user's default first
final speak = Speak(android: AndroidSpeakOptions(engine: engines.last.name));

Speak.androidEngines() returns an empty list on every other platform.

Errors

Interruptions are outcomes, not errors. Failures complete the call with a SpeakException whose error says what went wrong: languageUnavailable, voiceUnavailable, engineUnavailable, notAllowed (web, see below), timeout or synthesisFailed. A failed utterance never blocks the ones queued after it. On Windows and Linux every call throws UnsupportedError.

Platform differences

	iOS / macOS	Android	Web
Word progress	every word	onRangeStart: Android 8+, and only from engines that report it	boundary events where the browser sends them; when a browser gives only where a word starts, speak finds its end
Pause	native, at the end of a word	stops, and resumes from the start of the last word reported (from the beginning when none was)	native; in Chrome on Android pause() acts as a stop
Voice quality	compact = normal, enhanced, premium; novelty and Eloquence voices are low	from the engine's quality grade	unknown
Online voices	none	requiresNetwork from the engine	requiresNetwork for non-local voices
Voices not installed	not listed	listed, isInstalled: false	not listed
Gender	reported	unknown	unknown
Other audio	lowered/paused (iOS); macOS has no session	lowered/paused through audio focus (API 26+ with attributes, older with the music stream)	not possible
Lead-in	silent samples through the session	playSilentUtterance	none

rate is relative everywhere: 1 is the platform's normal speed. On iOS and macOS it scales AVFoundation's default utterance rate, capped at its maximum (about twice the normal speed).

Known issues

• macOS under flutter run / flutter test. When the app is started as a child of the flutter tool, AVSpeechSynthesizer cannot load its speech audio unit (OSStatus -3000, invalidComponentID), and every utterance ends at once as SpeakException(synthesisFailed). Started normally (from the Finder, or with open), the same app speaks. To run the integration tests on macOS, use tool/macos_integration_test.sh, which builds the test app and starts it with open.
• Web: a user gesture first. Browsers refuse to speak until the user has interacted with the page. Before that, a call fails with SpeakError.notAllowed.
• Web: word progress depends on the browser's voice. onProgress is only called for the boundary events the browser sends. Chrome on Linux sends none, so there the callback is never called (the text is still spoken, and pause still works).
• Web: Chrome's online voices may stop after about 15 seconds of continuous speech. Short prompts are not affected; split long text into sentences.
• iOS Simulator has only compact voices installed.

How it works

There is no plugin code of the usual kind: no method channel and no Swift, Kotlin or Objective-C to maintain. What does ship natively:

• iOS and macOS: the Dart bindings to AVFAudio are generated by ffigen. For the synthesizer's delegate, ffigen also generates a small Objective-C trampoline file (darwin/av_bindings.m). A build hook (hook/build.dart) compiles it for you during the app's build, as package:objective_c does for its own glue.
• Android: the bindings to TextToSpeech and AudioManager are generated by jnigen. One hand-written Java file (android/src/main/java/com/orkitec/speak/SpeakPlugin.java) subclasses UtteranceProgressListener. Dart can implement Java interfaces but not extend abstract classes, so this class forwards each callback to an interface that Dart implements. Gradle compiles it with your app.
• Web: package:web, behind conditional imports, so dart:ffi and JNI never reach a web build.

Design notes

• Speak is an instance, not a static API. A speaker owns state: its queue, its engine (on Android, a bound TextToSpeech service, possibly a non-default engine), its audio configuration and its defaults. Two features can each have their own speaker. Facts about the platform, which are the same for every speaker, are static: Speak.isSupported and Speak.androidEngines().
• Calling the speaker speaks. speak('text') reads as what it does, and everything else is a named method on the same object.
• Outcomes are values, failures are exceptions. Being cut off by stop() or by an urgent prompt is normal flow in a navigation app, and should not need a try. An engine that fails is not normal flow, so it throws a SpeakException.
• One utterance on the engine at a time. speak keeps the queue itself and gives the engine the next utterance only when the previous one has ended. Interrupting, expiring and pausing then work the same on every platform.
• "Interrupted" is decided in Dart, from the speaker's own stop() or interrupting call, never from which callback the platform sends. After a stop, macOS reports "finished", not "cancelled".
• Nothing waits forever. Each utterance has a timeout scaled to its length and rate (10 s plus 150 ms per character at normal speed). stop() gives the engine one second to confirm, then ends the utterance anyway.
• The audio is held only while speaking, plus a short linger, unless you ask for more with holdAudio(). Whether a lead-in is due is decided in Dart from when the audio was claimed, so it is the same on every platform.

Testing

flutter test                         # the core, against a fake engine
tool/web_test.sh                     # the same, plus the real engine, in Chrome
tool/web_test.sh --wasm              # ... compiled to Wasm
cd example && flutter test integration_test -d <iOS simulator or Android emulator>
tool/macos_integration_test.sh       # macOS, see Known issues

The device and browser tests run the same scenarios (test/support/scenarios.dart) against each platform's real engine. To regenerate the bindings, run tool/generate.sh (Xcode for ffigen; a JDK and ANDROID_SDK_ROOT with platform 36 for jnigen).

Releasing

Tagged versions are published by GitHub Actions; see RELEASING.md.

Contributing

Issues and pull requests are welcome at github.com/orkitec/speak. See CONTRIBUTING.md.

License

BSD-3-Clause. See LICENSE.

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