Laya 使list渲染支持分帧的思路

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  这篇文章，对其中renderItems做出优化

2.原renderItems：

 1         /**
 2          * @private
 3          * 渲染单元格列表。
 4          */
 5         protected function renderItems(from:int = 0, to:int = 0):void {
 6             for (var i:int = from, n:int = to || _cells.length; i < n; i++) {
 7                 renderItem(_cells[i], _startIndex + i);
 8             }
 9             changeSelectStatus();
10         }

3.分帧方案：

 1         private var _latterFrom:int = 0;
 2         private var _latterTo:int = 0;
 3         protected function renderItems(from:int = 0, to:int = 0):void {
 4             _latterFrom = from;
 5             _latterTo = to;
 6             Laya.timer.clear(this, onLatterFrames);
 7             Laya.timer.frameLoop(1, this, onLatterFrames);
 8         }
 9         
10         private var _renderSt:Number = 0;
11         private var _renderTLimit:Number = 0;
12         public var latterFrameRatio:Number = 0.8;//运行时脚本占帧时间比，这个用来调优 --ixenos
13         protected function onLatterFrames():void {
14             _renderTLimit = 1000/60*latterFrameRatio;
15             _renderSt = new Date().getTime();
16             _latterTo = _latterTo>0?_latterTo:_cells.length;
17             for (var i:int = _latterFrom; i < _latterTo; i++) {
18                 var curT:Number = new Date().getTime();
19                 if(curT - _renderSt > _renderTLimit){
20                     _latterFrom = i;//暂存进度
21                     return;
22                 }
23                 renderItem(_cells[i], _startIndex + i);
24                 changeCellState(_cells[i], _selectedIndex === _startIndex + i, 1);
25             }
26             Laya.timer.clear(this, onLatterFrames);
27         }