Laya 使list渲染支持分帧的思路

@author ixenos 2019-09-06

1.由于Laya的list渲染时没有做分帧处理，只做了延迟帧处理，所以当单页元素较多时，会有大量运算卡帧的情况，

结合之前 Laya 分帧加载优化 这篇文章，对其中renderItems做出优化

2.原renderItems：

         /**

          * @private

          * 渲染单元格列表。

          */

         protected function renderItems(from:int = 0, to:int = 0):void {

             for (var i:int = from, n:int = to || _cells.length; i < n; i++) {

                 renderItem(_cells[i], _startIndex + i);

             }

             changeSelectStatus();

         }

3.分帧方案：

         private var _latterFrom:int = 0;

         private var _latterTo:int = 0;

         protected function renderItems(from:int = 0, to:int = 0):void {

             _latterFrom = from;

             _latterTo = to;

             Laya.timer.clear(this, onLatterFrames);

             Laya.timer.frameLoop(1, this, onLatterFrames);

         }

         private var _renderSt:Number = 0;

         private var _renderTLimit:Number = 0;

         public var latterFrameRatio:Number = 0.8;//运行时脚本占帧时间比，这个用来调优 --ixenos

         protected function onLatterFrames():void {

             _renderTLimit = 1000/60*latterFrameRatio;

             _renderSt = new Date().getTime();

             _latterTo = _latterTo>0?_latterTo:_cells.length;

             for (var i:int = _latterFrom; i < _latterTo; i++) {

                 var curT:Number = new Date().getTime();

                 if(curT - _renderSt > _renderTLimit){

                     _latterFrom = i;//暂存进度

                     return;

                 }

                 renderItem(_cells[i], _startIndex + i);

                 changeCellState(_cells[i], _selectedIndex === _startIndex + i, 1);

             }

             Laya.timer.clear(this, onLatterFrames);

         }