Angular 2 @ViewChild annotation returns undefined

Question

I am trying to learn Angular 2.

I would like to access to a child component from a parent component using the @ViewChild Annotation.

Here some lines of code:

In BodyContent.ts I have:

import { ViewChild, Component, Injectable } from 'angular2/core';
import { FilterTiles } from '../Components/FilterTiles/FilterTiles';

@Component({
    selector: 'ico-body-content',
    templateUrl: 'App/Pages/Filters/BodyContent/BodyContent.html',
    directives: [FilterTiles] 
})
export class BodyContent {
    @ViewChild(FilterTiles) ft: FilterTiles;

    public onClickSidebar(clickedElement: string) {
        console.log(this.ft);
        var startingFilter = {
            title: 'cognomi',
            values: [ 'griffin', 'simpson' ]
        }
        this.ft.tiles.push(startingFilter);
    } 
}

while in FilterTiles.ts:

 import { Component } from 'angular2/core';

 @Component({
     selector: 'ico-filter-tiles',
     templateUrl: 'App/Pages/Filters/Components/FilterTiles/FilterTiles.html'
 })
 export class FilterTiles {
     public tiles = [];

     public constructor(){};
 }

Finally here the templates (as suggested in comments):

BodyContent.html

<div (click)="onClickSidebar()" class="row" style="height:200px; background-color:red;">
    <ico-filter-tiles></ico-filter-tiles>
</div>

FilterTiles.html

<h1>Tiles loaded</h1>
<div *ngFor="#tile of tiles" class="col-md-4">
     ... stuff ...
</div>

FilterTiles.html template is correctly loaded into ico-filter-tiles tag (indeed I am able to see the header).

Note: the BodyContent class is injected inside another template (Body) using DynamicComponetLoader: dcl.loadAsRoot(BodyContent, '#ico-bodyContent', injector):

import { ViewChild, Component, DynamicComponentLoader, Injector } from 'angular2/core';
import { Body } from '../../Layout/Dashboard/Body/Body';
import { BodyContent } from './BodyContent/BodyContent';

@Component({
    selector: 'filters',
    templateUrl: 'App/Pages/Filters/Filters.html',
    directives: [Body, Sidebar, Navbar]
})
export class Filters {

    constructor(dcl: DynamicComponentLoader, injector: Injector) {
       dcl.loadAsRoot(BodyContent, '#ico-bodyContent', injector);
       dcl.loadAsRoot(SidebarContent, '#ico-sidebarContent', injector);
   } 
}

The problem is that when I try to write ft into the console log, I get undefined, and of course I get an exception when I try to push something inside the "tiles" array: 'no property tiles for "undefined"'.

One more thing: FilterTiles component seems to be correctly loaded, since I'm able to see the html template for it.

Any suggestions?

Answer 1

I had a similar issue and thought I'd post in case someone else made the same mistake. First, one thing to consider is AfterViewInit; you need to wait for the view to be initialized before you can access your @ViewChild. However, my @ViewChild was still returning null. The problem was my *ngIf. The *ngIf directive was killing my controls component so I couldn't reference it.

import { Component, ViewChild, OnInit, AfterViewInit } from 'angular2/core';
import { ControlsComponent } from './controls/controls.component';
import { SlideshowComponent } from './slideshow/slideshow.component';

@Component({
  selector: 'app',
  template: `
    <controls *ngIf="controlsOn"></controls>
    <slideshow (mousemove)="onMouseMove()"></slideshow>
  `,
  directives: [SlideshowComponent, ControlsComponent],
})
export class AppComponent {
  @ViewChild(ControlsComponent) controls: ControlsComponent;

  controlsOn: boolean = false;

  ngOnInit() {
    console.log('on init', this.controls);
    // this returns undefined
  }

  ngAfterViewInit() {
    console.log('on after view init', this.controls);
    // this returns null
  }

  onMouseMove(event) {
    this.controls.show();
    // throws an error because controls is null
  }
}

Hope that helps.

EDIT
As mentioned by @Ashg below, a solution is to use @ViewChildren instead of @ViewChild.

Answer 2

The issue as previously mentioned is the ngIf which is causing the view to be undefined. The answer is to use ViewChildren instead of ViewChild. I had similar issue where I didn't want a grid to be shown until all the reference data had been loaded.

html:

   <section class="well" *ngIf="LookupData != null">
       <h4 class="ra-well-title">Results</h4>
       <kendo-grid #searchGrid> </kendo-grid>
   </section>

Component Code

import { Component, ViewChildren, OnInit, AfterViewInit, QueryList  } from '@angular/core';
import { GridComponent } from '@progress/kendo-angular-grid';

export class SearchComponent implements OnInit, AfterViewInit
{
    //other code emitted for clarity

    @ViewChildren("searchGrid")
    public Grids: QueryList<GridComponent>

    private SearchGrid: GridComponent

    public ngAfterViewInit(): void
    {

        this.Grids.changes.subscribe((comps: QueryList <GridComponent>) =>
        {
            this.SearchGrid = comps.first;
        });


    }
}

Here we are using ViewChildren on which you can listen for changes. In this case any children with the reference #searchGrid. Hope this helps.

Answer 3

You could use a setter for @ViewChild()

@ViewChild(FilterTiles) set ft(tiles: FilterTiles) {
    console.log(tiles);
};

If you have an ngIf wrapper, the setter will be called with undefined, and then again with a reference once ngIf allows it to render.

My issue was something else though. I had not included the module containing my "FilterTiles" in my app.modules. The template didn't throw an error but the reference was always undefined.

Answer 4

What solved my problem was to make sure static was set to false.

@ViewChild(ClrForm, {static: false}) clrForm;

With static turned off, the @ViewChild reference gets updated by Angular when the *ngIf directive changes.