code: function _enterFrame(timeStamp) { // @arg DOMHighResTimeStamp|undefined - requestAnimationFrame give us timeStamp.
                                    // @bind this
      if (!this._active) {
          return;
      }
      switch (this._mode) {
      case MODE_RAF_EMULATE: this._timerID = setTimeout(this._enterFrame, 4); break;
      case MODE_RAF:         this._timerID = global["requestAnimationFrame"](this._enterFrame);
      }
      if (!this._ticks.length) {
          return;
      }
      // setInterval and setTimeout does not give us the timeStamp.
      if (this._mode === MODE_INTERVAL ||
          this._mode === MODE_RAF_EMULATE) {
        //timeStamp = this["now"](); // inlining WMClock#now
          timeStamp = (Date.now() - this._baseTimeDiff);
      }
      // reduce the suspended time
      timeStamp -= this._suspend.total;
  
      var garbageFunctions = 0; // functions that are no longer needed.
      var deltaTime = 0;        // elapsed time since the last frame.
      var count = this._counter++;
  
      // --- adjust timeStamp and deltaTime ---
      if (this._timeStamp < 0) { // init
          if (this._pulse) { // overwrite timeStamp
              timeStamp = this._pulse;
          }
      } else {
          if (this._pulse) { // overwrite timeStamp
              timeStamp = this._pulse + this._timeStamp;
              deltaTime = this._pulse;
          } else {
              deltaTime = timeStamp - this._timeStamp;
          }
      }
      this._timeStamp = timeStamp; // update time stamp
  
      // --- callback tick function ---
      for (var i = 0, iz = this._ticks.length; i < iz; ++i) {
          var tick = this._ticks[i];
  
          if (tick) {
              tick(timeStamp,  // @arg Number - current time
                   deltaTime,  // @arg Number - delta time
                   count);     // @arg Integer - callback count
          } else {
              ++garbageFunctions;
          }
      }
      if (garbageFunctions) {
          _garbageCollection(this);
      }
  }
docs: --- implements ------------------------------------------