import { ReadiumSpeechPlaybackEngine } from '../engine';
import { ReadiumSpeechEngineProvider } from '../provider';
import { ReadiumSpeechPlaybackEvent, ReadiumSpeechPlaybackState } from '../navigator';
import { ReadiumSpeechUtterance } from '../utterance';
import { ReadiumSpeechVoice } from '../voices/types';
export interface FallbackSpeechEngineOptions {
    primaryEngine: ReadiumSpeechPlaybackEngine;
    primaryProvider: ReadiumSpeechEngineProvider;
    fallbackProvider: ReadiumSpeechEngineProvider;
    onFailure?: "fallback" | "error" | "fallbackAndRecover";
    healthCheckIntervalMs?: number;
}
export declare class FallbackSpeechEngine implements ReadiumSpeechPlaybackEngine {
    private activeEngine;
    private readonly primaryProvider;
    private readonly fallbackProvider;
    private readonly onFailure;
    private readonly healthCheckIntervalMs;
    private hasFallenBack;
    private healthCheckTimer;
    private primaryReachable;
    private currentUtterances;
    private lastVoiceRequest;
    private desiredIndex;
    private desiredPlaying;
    private engineStarted;
    private swapInFlight;
    private teardownEpoch;
    private loadToken;
    private readonly events;
    private unbindActiveEngine;
    constructor(options: FallbackSpeechEngineOptions);
    initialize(): Promise<unknown>;
    loadUtterances(contents: ReadiumSpeechUtterance[], startIndex?: number): void;
    setVoice(voice: ReadiumSpeechVoice | string): void;
    getCurrentVoice(): ReadiumSpeechVoice | null;
    getAvailableVoices(): Promise<ReadiumSpeechVoice[]>;
    setSpeakInContentLanguage(enabled: boolean): void;
    getSpeakInContentLanguage(): boolean;
    speak(utteranceIndex?: number): void;
    pause(): void;
    resume(): void;
    stop(): void;
    setRate(rate: number): void;
    getRate(): number;
    setPitch(pitch: number): void;
    getPitch(): number;
    setVolume(volume: number): void;
    getVolume(): number;
    private isEngineTrusted;
    getState(): ReadiumSpeechPlaybackState;
    getCurrentUtteranceIndex(): number;
    setCurrentUtteranceIndex(index: number, onComplete?: (success: boolean) => void): void;
    getUtteranceCount(): number;
    on(event: ReadiumSpeechPlaybackEvent["type"], callback: (event: ReadiumSpeechPlaybackEvent) => void): () => void;
    private emitEvent;
    private bindActiveEngine;
    private handleError;
    private copyPlaybackParameters;
    private startEngineWhenReady;
    private startHealthCheck;
    private maybeRecoverNow;
    private swapToFallback;
    private recoverToPrimary;
    private performSwap;
    private pickBestFallbackVoice;
    destroy(): Promise<void>;
}