Angular 2如何让子组件等待异步数据准备好

我将异步数据从父组件传递到子组件.子组件需要知道数据的长度才能执行某些操作.

问题是子组件无法使用"Oninit"钩子进行工作，因为此时数据不可用.那我该怎么做呢？

父组件代码如下所示:

@Component({
    moduleId: module.id,
    selector: 'parent',
    template: `<div>
                    <child [items]="items | async">
                </div>`
})

export class Parent implements OnInit {

    items: Items[];

    constructor(
        private itemService: ItemService,
        private router: Router
    ) { }    

    ngOnInit() {
        this.itemService.getItemss()
            .subscribe(
                items => this.items = items,
                error => this.errorMessage = <any>error
            );
    }

}

子组件如下所示:

@Component({
    moduleId: module.id,
    selector: 'child',    
    template: `<div>
                    <div *ngFor="let itemChunk of itemChunks"></div>
                    content here
                </div>`
})
export class child implements OnInit{
    @Input() items: Items[];
    itemChunks: Items[][];

    ngOnInit() {
        this.itemChunks = this.chunk(this.Items);
    }

    chunk(items: Items[]) {
        let result = [];
        for (var i = 0, len = items.length; i < len; i += 6) { // this line causes the problem since 'items' is undefined
            result.push(items.slice(i, i + 6));
        }
        return result;
    }
}

处理此问题的最佳实践是什么？