code: function(){
          this._context = context;
          this._gainNode = context.createGain ? context.createGain() : context.createGainNode();
          this._gainNode.connect(context.destination);
  
          this._onAudioEvent = this._onAudioEvent.bind(this);
          this._onDecodeComplete = this._onDecodeComplete.bind(this);
          this._onDecodeError = this._onDecodeError.bind(this);
      }
docs: 播放偏移量 the offset of current playing audio 
  @language=en
  @private Initialize.