Here we provide an ability of external min and max indexes setting to let the viewport know how many items do we have.
<li ui-scroll="item in datasource">{{item}}</li>
Then you can play with minIndex and maxIndex properties which are being accessible on your datasource after the ui-scroll directive is linked:
datasource.minIndex = -100;
datasource.maxIndex = 100;
Such setting does not not lead to data fetching but the scroll bar params do match datasource size defined this way.