code: function(frame) {
                  if (this.status != "playing") {
                      return;
                  }
                  this.time += frame.dur;
                  this.setValue(this.getValue());
                  if (this.time >= this.duration) {
                      this.timeUp();
                  }
                  frame.next();
              }
docs: @private
  
  让时间线进入下一帧