protected _emit_callback(): void {
   if (this.callback != null) {
     if (isFunction(this.callback))
       this.callback(this)
     else
       this.callback.execute(this, {})
   }
 }

 this.connect(this.properties.selected.change, () => {
   const {callback} = this
   if (callback != null) {
     if (isFunction(callback))
       callback(this)
     else
       callback.execute(this)
   }
 })

const _strftime = function(t, format) {
  if (isFunction(format)) {
    return format(t);
  } else {
    // Python's datetime library augments the microsecond directive %f, which is not
    // supported by the javascript library timezone: http://bigeasy.github.io/timezone/.
    // Use a regular expression to replace %f directive with microseconds.
    // TODO: what should we do for negative microsecond strings?
    const microsecond_replacement_string = sprintf("$1%06d", _us(t));
    format = format.replace(/((^|[^%])(%%)*)%f/, microsecond_replacement_string);

    if (format.indexOf("%") === -1) {
      // timezone seems to ignore any strings without any formatting directives,
      // and just return the time argument back instead of the string argument.
      // But we want the string argument, in case a user supplies a format string
      // which doesn't contain a formatting directive or is only using %f.
      return format;
    }
    return tz(t, format);
  }
};