@ViewChild always returns undefined

问题:

I know this has been asked before, but none of the selected answers are working for me.

I am trying to use @ViewChild to get my ng-select from the dom. And it always returns undefined

here is the select inside the main.html

  <ng-select id="user-select" 
             #userSelect 
             [allowClear]="true"
             [items]="singleSignOnUsers"
             [disabled]="disabled"
             placeholder="No user selected">
            </ng-select>

here is my component

import { Component, AfterViewInit, ViewChild  } from '@angular/core';

@Component({
    moduleId: module.id,
    selector: 'app-main',
    templateUrl: '../app/main.html',
    providers: [ApiHttpService, UserAdministrationService]
})

export class AppComponent {
      @ViewChild('userSelect') userSelect;


        ngAfterViewInit() {
           alert(this.userSelect);
        }
}

what am i missing here?

Update: Oh My Soul! I figured out why this wasn't working when this should.. I have the whole view wrapped in a div with an ngSwitch. If i move it out... I can access them. Now I don't know how to access them while in the ngSwitch however. But I can don't have to use the ngSwitch

<div [ngSwitch]='loading'>
        <div *ngSwitchCase="false">
            ...
            <ng-select id="user-select"
                       #userSelect
                       [allowClear]="true"
                       [items]="singleSignOnUsers"
                       [disabled]="disabled"
                       placeholder="No city selected">
            </ng-select>
            ...
        </div>
    <div

回答1:

@twaldron Are you using some delayed data loading in ngOnInit?

Because in that case, in my experience, reading a @ViewChild as a ElementRef produces no results

If your component has the data already resolved (like the case when a parent passes a child data object to a sub component) it should work (at least for me it did).

In the case of asynchronous data loading, the way I was able to make it work is using a change notification

 @ViewChildren('userSelect') userSelect: QueryList<ElementRef>;

 ngAfterViewInit(): void {
    this.userSelect.changes.subscribe(item => {
        if (this.userSelect.length) {
            alert(this.userSelect.first.nativeElment.outerHTML)
        }
    })
 }

回答2:

I replaced all occurrences of

*ngIf="flag"

with

[style.display]="flag ? 'block' : 'none'"

to get this to work consistently in my case. Otherwise the ViewChild components that didn't exist when my view first loaded would potentially remain undefined for ever.

回答3:

In my case, I encountered the same problem. I console.log()-ed the content that is retrieved from the child component and it gives me 'undefined'.

And I solved it by merely refreshing the page. ctrl+shift+R may help. I guess it's because the cache of the browser that leads to this problem.