How to use the @egjs/infinitegrid.ItemManager function in @egjs/infinitegrid

_mount(viewer) {
		if (!viewer || this._viewer) {
			return;
		}
		this._viewer = viewer;
		const {
			isOverflowScroll,
			isEqualSize,
			isConstantSize,
			horizontal,
			threshold,
		} = this.props;

		this._items = new ItemManager();
		this._renderer = new DOMRenderer(viewer, {
			container: isOverflowScroll ? this._container : null,
			isEqualSize,
			isConstantSize,
			horizontal,
		});
		this._watcher = new Watcher(
			this._renderer.view,
			{
				isOverflowScroll,
				horizontal,
				container: this._renderer.container,
				resize: () => setTimeout(() => { this.setState({ layout: true }); }),
				check: param => this._onCheck(param),
			});
		this._infinite = new Infinite(this._items, {