audio.js

/**
 * Module that takes care of audio playback
 */
var AudioEngine = {
    FADE_TIME: 1,
    AUDIO_BUFFER_CACHE: {},
    _audioContext: null,
    _master: null,
    _currentBackgroundMusic: null,
    _currentEventAudio: null,
    _currentSoundEffectAudio: null,
    init: function () {
        AudioEngine._initAudioContext();
    },
    _initAudioContext: function () {
        // for legacy browsers
        AudioEngine._audioContext = new (window.AudioContext || window.webkitAudioContext);
        if (AudioEngine._audioContext.state === 'suspended') {
            AudioEngine._audioContext.resume().then(function () {
                AudioEngine._createMasterChannel();
            });
        } else {
            AudioEngine._createMasterChannel();
        }
    },
    _createMasterChannel: function () {
        // create master
        AudioEngine._master = AudioEngine._audioContext.createGain();
        AudioEngine._master.gain.setValueAtTime(1.0, AudioEngine._audioContext.currentTime);
        AudioEngine._master.connect(AudioEngine._audioContext.destination);
    },
    _canPlayAudio: function () {
        if (AudioEngine._audioContext.state === 'suspended') {
            return false;
        }
        return true;
    },
    _getMissingAudioBuffer: function () {
        // plays beeping sound to indicate missing audio
        var buffer = AudioEngine._audioContext.createBuffer(
            1,
            AudioEngine._audioContext.sampleRate,
            AudioEngine._audioContext.sampleRate
        );
        // Fill the buffer
        var bufferData = buffer.getChannelData(0);
        for (var i = 0; i < buffer.length / 2; i++) {
            bufferData[i] = Math.sin(i * .05) / 4; // max .25 gain value
        }
        return buffer;
    },
    _playSound: function (buffer) {
        if (!AudioEngine._canPlayAudio()) return;

        var source = AudioEngine._audioContext.createBufferSource();
        source.buffer = buffer;
        source.connect(AudioEngine._master);
        source.start();

        AudioEngine._currentSoundEffectAudio = {
            source: source
        };
    },
    _playBackgroundMusic: function (buffer) {
        if (!AudioEngine._canPlayAudio()) return;

        var source = AudioEngine._audioContext.createBufferSource();
        source.buffer = buffer;
        source.loop = true;

        var envelope = AudioEngine._audioContext.createGain();
        envelope.gain.setValueAtTime(0.0, AudioEngine._audioContext.currentTime);
        
        var fadeTime = AudioEngine._audioContext.currentTime + AudioEngine.FADE_TIME;

        // fade out current background music
        if (AudioEngine._currentBackgroundMusic) {
            var currentBackgroundGainValue = AudioEngine._currentBackgroundMusic.envelope.gain.value;
            AudioEngine._currentBackgroundMusic.envelope.gain.cancelScheduledValues(AudioEngine._audioContext.currentTime);
            AudioEngine._currentBackgroundMusic.envelope.gain.setValueAtTime(currentBackgroundGainValue, AudioEngine._audioContext.currentTime);
            AudioEngine._currentBackgroundMusic.envelope.gain.linearRampToValueAtTime(0.0, fadeTime);
            AudioEngine._currentBackgroundMusic.source.stop(fadeTime + 0.3); // make sure fade has completed
        }

        // fade in new backgorund music
        source.connect(envelope);
        envelope.connect(AudioEngine._master);
        source.start();
        envelope.gain.linearRampToValueAtTime(1.0, fadeTime);

        // update current background music
        AudioEngine._currentBackgroundMusic = {
            source: source,
            envelope: envelope
        };
    },
    _playEventMusic: function (buffer) {
        if (!AudioEngine._canPlayAudio()) return;

        var source = AudioEngine._audioContext.createBufferSource();
        source.buffer = buffer;
        source.loop = true;

        var envelope = AudioEngine._audioContext.createGain();
        envelope.gain.setValueAtTime(0.0, AudioEngine._audioContext.currentTime);

        var fadeTime = AudioEngine._audioContext.currentTime + AudioEngine.FADE_TIME * 2;

        // turn down current background music
        if (AudioEngine._currentBackgroundMusic != null) {
            var currentBackgroundGainValue = AudioEngine._currentBackgroundMusic.envelope.gain.value;
            AudioEngine._currentBackgroundMusic.envelope.gain.cancelScheduledValues(AudioEngine._audioContext.currentTime);
            AudioEngine._currentBackgroundMusic.envelope.gain.setValueAtTime(currentBackgroundGainValue, AudioEngine._audioContext.currentTime);
            AudioEngine._currentBackgroundMusic.envelope.gain.linearRampToValueAtTime(0.2, fadeTime);
        }

        // fade in event music
        source.connect(envelope);
        envelope.connect(AudioEngine._master);
        source.start();
        envelope.gain.linearRampToValueAtTime(1.0, fadeTime);

        // update reference
        AudioEngine._currentEventAudio = {
            source: source,
            envelope: envelope
        };
    },
    _stopEventMusic: function () {
        var fadeTime = AudioEngine._audioContext.currentTime + AudioEngine.FADE_TIME * 2;

        // fade out event music and stop
        if (AudioEngine._currentEventAudio) {
            var currentEventGainValue = AudioEngine._currentEventAudio.envelope.gain.value;
            AudioEngine._currentEventAudio.envelope.gain.cancelScheduledValues(AudioEngine._audioContext.currentTime);
            AudioEngine._currentEventAudio.envelope.gain.setValueAtTime(currentEventGainValue, AudioEngine._audioContext.currentTime);
            AudioEngine._currentEventAudio.envelope.gain.linearRampToValueAtTime(0.0, fadeTime);
            AudioEngine._currentEventAudio.source.stop(fadeTime + 1); // make sure fade has completed
            AudioEngine._currentEventAudio = null;
        }

        // turn up background music
        var currentBackgroundGainValue = AudioEngine._currentBackgroundMusic.envelope.gain.value;
        AudioEngine._currentBackgroundMusic.envelope.gain.cancelScheduledValues(AudioEngine._audioContext.currentTime);
        AudioEngine._currentBackgroundMusic.envelope.gain.setValueAtTime(currentBackgroundGainValue, AudioEngine._audioContext.currentTime);
        AudioEngine._currentBackgroundMusic.envelope.gain.linearRampToValueAtTime(1.0, fadeTime);
    },
    playBackgroundMusic: function (src) {
        AudioEngine.loadAudioFile(src)
            .then(function (buffer) {
                AudioEngine._playBackgroundMusic(buffer);
            });
    },
    playEventMusic: function (src) {
        AudioEngine.loadAudioFile(src)
            .then(function (buffer) {
                AudioEngine._playEventMusic(buffer);
            });
    },
    stopEventMusic: function () {
        AudioEngine._stopEventMusic();
    },
    playSound: function (src) {
        AudioEngine.loadAudioFile(src)
            .then(function (buffer) {
                AudioEngine._playSound(buffer);
            });
    },
    loadAudioFile: function (src) {
        if (src.indexOf('http') === -1) {
            src = window.location + src;
        }
        if (AudioEngine.AUDIO_BUFFER_CACHE[src]) {
            return new Promise(function (resolve, reject) {
                resolve(AudioEngine.AUDIO_BUFFER_CACHE[src]);
            });
        } else {
            var request = new Request(src);
            return fetch(request).then(function (response) {
                return response.arrayBuffer();
            }).then(function (buffer) {
                if (buffer.byteLength === 0) {
                    console.error('cannot load audio from ' + src);
                    return AudioEngine._getMissingAudioBuffer();
                }

                return AudioEngine._audioContext.decodeAudioData(buffer, function (decodedData) {
                    AudioEngine.AUDIO_BUFFER_CACHE[src] = decodedData;
                    return AudioEngine.AUDIO_BUFFER_CACHE[src];
                });
            });
        }
    },
    setVolume: function (volume, s) {
        if (AudioEngine._master == null) return;  // master may not be ready yet
        if (volume === undefined) {
            volume = 1.0;
        }
        if (s === undefined) {
            s = 1.0;
        }

        // cancel any current schedules and then ramp
        var currentGainValue = AudioEngine._master.gain.value;
        AudioEngine._master.gain.cancelScheduledValues(AudioEngine._audioContext.currentTime);
        AudioEngine._master.gain.setValueAtTime(currentGainValue, AudioEngine._audioContext.currentTime);
        AudioEngine._master.gain.linearRampToValueAtTime(
            volume,
            AudioEngine._audioContext.currentTime + s
        );
    }
};