@viewChild not working - cannot read property nativeElement of undefined

问题

I'm trying to access a native element in order to focus on it when another element is clicked (much like the html attribute "for" - for cannot be used on elements of this type.

However I get the error:

TypeError: Cannot read property 'nativeElement' of undefined

I try to console.log the nativeElement in ngAfterViewInit() so that it is loaded but it still throws the error.

I also access nativeElement in the click event handler, so that I can focus the element when another element is clicked - is this possibly what is mucking it up, because it compiles before the view has loaded?.

eg:

ngAfterViewInit() {
    console.log(this.keywordsInput.nativeElement); // throws an error
}

focusKeywordsInput(){
    this.keywordsInput.nativeElement.focus();
}

full Code:

relevant part of the html template being used:

<div id="keywords-button" class="form-group" (click)="focusKeywordsInput()">
    <input formControlName="keywords" id="keywords-input" placeholder="KEYWORDS (optional)"/>
    <div class="form-control-icon" id="keywords-icon"></div>
</div>

component.ts:

import { Component, OnInit, AfterViewInit, ViewChild, ElementRef } from '@angular/core';
import {  REACTIVE_FORM_DIRECTIVES, 
          FormGroup, 
          FormBuilder, 
          Validators,
          ControlValueAccessor
        } from '@angular/forms';
import { NumberPickerComponent } from './number-picker.component';
import { DistanceUnitsComponent } from './distance-units.component';
import { MapDemoComponent } from '../shared/map-demo.component';
import { AreaComponent } from './area-picker.component';
import { GoComponent } from './go.component';
import { HighlightDirective } from '../highlight.directive';

@Component({
   selector: 'find-form',
   templateUrl: 'app/find-page/find-form.component.html',
   styleUrls: ['app/find-page/find-form.component.css'],
   directives: [REACTIVE_FORM_DIRECTIVES, 
                NumberPickerComponent, 
                DistanceUnitsComponent, 
                MapDemoComponent, 
                AreaComponent, 
                GoComponent]
})
export class FindFormComponent implements OnInit, AfterViewInit {
   findForm: FormGroup;
   submitted: boolean; // keep track on whether form is submitted
   events: any[] = []; // use later to display form changes
   @ViewChild('keywords-input') keywordsInput;
//comment
   constructor(private formBuilder: FormBuilder, el: ElementRef) {}

   ngOnInit() {
      this.findForm = this.formBuilder.group({
         firstname: ['', [ Validators.required, Validators.minLength(5) ] ],
         lastname: ['', Validators.required],
         keywords: [],
         area: ['', Validators.required],
         address: this.formBuilder.group({
            street: [],
            zip: [],
            city: []
         })
      });

      this.findForm.valueChanges.subscribe(data => console.log('form changes', data));
   }

     ngAfterViewInit() {
    console.log(this.keywordsInput.nativeElement); // throws an error
  }

   focusKeywordsInput(){
      this.keywordsInput.nativeElement.focus();
   }

   save(isValid: boolean) {
      this.submitted = true;
      // check if model is valid
      // if valid, call API to save customer
      console.log(isValid);
   }
}

full html template (probably irrelevant):

<form class="text-uppercase" [formGroup]="findForm" (ngSubmit)="save(findForm.value, findForm.valid)">
    <div class="row is-heading">
        <div class="col-sm-8 offset-sm-2 col-md-6 offset-md-3 col-lg-4 offset-lg-4 input-group">
            <h2 class="search-filter-heading heading m-x-auto">find vegan</h2>
        </div>
    </div>
    <div class="row has-error-text">
        <div class="col-sm-8 offset-sm-2 col-md-6 offset-md-3 col-lg-4 offset-lg-4 input-group btn-group" style="height:64px;">
            <div style="position: relative; display: inline-block; width: 100%;">
                <multiselect #multiselect></multiselect>
            </div>
        </div>
    </div>
    <div class="row error-text"  [style.display]="multiselect.selectedCategories.length < 1 && submitted ? 'block' : 'none'">
        <div class="col-sm-8 offset-sm-2 col-md-6 offset-md-3 col-lg-4 offset-lg-4 form-group input-group btn-group">
            <small>Please select at least 1 category.</small>
        </div>
    </div>
    <div class="row is-heading">
        <div class="col-sm-8 offset-sm-2 col-md-6 offset-md-3 col-lg-4 offset-lg-4 input-group">
            <h2 class="search-filter-heading heading m-x-auto">within</h2>
        </div>
    </div>
    <div class="row">
        <div class="col-sm-8 offset-sm-2 col-md-6 offset-md-3 col-lg-4 offset-lg-4 input-group btn-group" style="height:64px;">
            <div style="position: relative; display: inline-block;">
                <number-picker #numberPicker></number-picker>
            </div>
            <distance-units></distance-units>
        </div>
    </div>
    <div class="row is-heading">
        <div class="col-sm-8 offset-sm-2 col-md-6 offset-md-3 col-lg-4 offset-lg-4 input-group">
            <h2 class="search-filter-heading heading m-x-auto">of</h2>
        </div>
    </div>
    <div class="row has-error-text">
        <div class="col-sm-8 offset-sm-2 col-md-6 offset-md-3 col-lg-4 offset-lg-4 input-group btn-group" style="height:64px;">
            <div style="position: relative; display: inline-block; width: 100%;">
                <my-area></my-area>
            </div>
        </div>
    </div>
    <div class="row error-text"  [style.display]="multiselect.selectedCategories.length < 1 && submitted ? 'block' : 'none'">
        <div class="col-sm-8 offset-sm-2 col-md-6 offset-md-3 col-lg-4 offset-lg-4 form-group input-group btn-group">
            <small [hidden]="findForm.controls.firstname.valid || (findForm.controls.firstname.pristine && !submitted)">Please enter an area.</small>
        </div>
    </div>
    <div class="row is-heading">
        <div class="col-sm-8 offset-sm-2 col-md-6 offset-md-3 col-lg-4 offset-lg-4 input-group">
            <h2 class="search-filter-heading heading m-x-auto">keywords</h2>
        </div>
    </div>
    <div class="row form-group">
        <div class="col-sm-8 offset-sm-2 col-md-6 offset-md-3 col-lg-4 offset-lg-4 input-group btn-group" style="height:64px;">
            <div style="position: relative; display: inline-block; width: 100%;">
                <div id="keywords-button" class="form-group" (click)="focusKeywordsInput()">
                    <input formControlName="keywords" id="keywords-input" placeholder="KEYWORDS (optional)"/>
                    <div class="form-control-icon" id="keywords-icon"></div>
                </div>
            </div>
        </div>
    </div>
    <div class="row">
        <div class="col-sm-8 offset-sm-2 col-md-6 offset-md-3 col-lg-4 offset-lg-4 input-group btn-group" style="height:64px;">
            <div style="position: relative; display: inline-block; width: 100%;">
                <go></go>
            </div>
        </div>
    </div>
</form>

回答1:

@ViewChild('keywords-input') keywordsInput; doesn't match id="keywords-input"

id="keywords-input"

should be instead a template variable:

#keywordsInput

Note that camel case should be used, since - is not allowed in template reference names.

@ViewChild() supports names of template variables as string:

@ViewChild('keywordsInput') keywordsInput;

or component or directive types:

@ViewChild(MyKeywordsInputComponent) keywordsInput;

See also https://stackoverflow.com/a/35209681/217408

Hint:
keywordsInput is not set before ngAfterViewInit() is called

回答2:

You'll also get this error if your target element is inside a hidden element. If this is your HTML:

<div *ngIf="false">
    <span #sp>Hello World</span>
</div>

Your @ViewChild('sp') sp will be undefined.

Solution

In such a case, then don't use *ngIf.

Instead use a class to show/hide your element being hidden.

<div [class.show]="shouldShow">...</div>

回答3:

it just simple :import this directory

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

回答4:

What happens is when these elements are called before the DOM is loaded these kind of errors come up. Always use:

 window.onload = function(){
     this.keywordsInput.nativeElement.focus();
 }

来源：https://stackoverflow.com/questions/39158922/viewchild-not-working-cannot-read-property-nativeelement-of-undefined