Angular 2如何让子组件等待异步数据准备好

Question

我正在将异步数据从父组件传递到子组件。并且子组件需要知道数据的长度才能做某事。

问题是子组件无法使用“Oninit”挂钩来完成工作，因为此时数据不可用。那我该怎么做呢？

父组件代码如下：

@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
            );
    }

}

子组件看起来像：

@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;
    }
}

处理此问题的最佳做法是什么？

Answer 1

有三种方法可以做到这一点：

1. 将*ngIf 放在父级中。仅当父母的items 准备好时才渲染孩子。

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

1. 将您的输入 getter setter 分开。然后在设置值时执行操作，您也可以使用 RxJS BehaviorSubject。

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);
    })
}

1. 在孩子的ngOnChanges 期间进行。例如，请参阅此处。 https://angular.io/docs/ts/latest/guide/lifecycle-hooks.html#!#onchanges

Answer 2

你可以使用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;
    }
}

---

---

---

顺便说一句，我觉得你的父组件也不对，应该是：

@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
     }    

}

Answer 3

更简单的解决方案：

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