Angular 2 how to make child component wait for async data to be ready

You can use a setter :

export class child implements OnInit{
    itemChunks: Items[][];

    private _items ;

    //bellow  will get called when ever the items are passed in by the parent component
    @Input( 'items' ) set items ( items: Items[] ) {
       this._items = items;

       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;
    }
}

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Any by the way, I feel like your parent component is not right as well, it should be :

@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
    ) { 
         this.items = this.itemService.getItemss(); // if getItemss is returning an observable, which I think it does
     }    

}

There are three ways to do this:

1. Put an *ngIf in parent. Only render child when parent's items is ready.

<div *ngIf="items">
   <child [items]="items | async">
</div>

2. Separate your input getter setter in child. Then act whenever the value is set, you can use RxJS BehaviorSubject also.

private _items = new BehaviorSubject<Items[]>([]);

@Input() set items(value: Items[]) { 
    this._items.next(value); 
}

get items() {
   return this._items.getValue();
}

ngOnInit() {
    this._items.subscribe(x => {
       this.chunk(x);
    })
}

3. Do it during the ngOnChanges of the child. Refer to here for example. https://angular.io/docs/ts/latest/guide/lifecycle-hooks.html#!#onchanges

Simpler solution:

ngOnChanges(changes: SimpleChanges) {
    if (changes['items'].currentValue) {
        this.items = items
    }
}