ViewChildren以动态呈现的反应形式显示“未定义”

Question

I'm trying to figure out how to dynamically render a form, and then cycle through each of the elements using @ViewChildren. However, the query appears not to be finding anything as I'm getting a zero length array back in ngAfterViewInit at the bottom of the following code. (For reference, here's the Stackblitz: https://angular-4lqtap.stackblitz.io ). I've found several articles relating to this topic, but none have solved the issue I'm facing.

import { Directive, Component, Input, OnInit, AfterViewInit, AfterContentInit, ViewChild, ViewChildren, ContentChildren, QueryList } from '@angular/core';
import {FormGroup, FormControl} from '@angular/forms';

@Directive({selector: '[foo]'})
export class IdVar {
  // TODO(issue/24571): remove '!'.
  @Input() idString !: string;
}

@Component({
  selector: 'hello',
  template: `<h1>Hello {{name}}!</h1>
            <form #myForm="ngForm" [formGroup]="form">
              <ng-container *ngFor="let itm of testArr; let i = index"> 
              {{i}}: <input [foo] [id]="i" />
              <foo [idString]="i">{{i}}:&nbsp;</foo>
              </ng-container>
             </form>`,
  styles: [`h1 { font-family: Lato; }`]
})
export class HelloComponent implements OnInit, AfterViewInit  {

  @Input() name: string;
  form: FormGroup;
  formControls={};
  testArr = ["a", "b", "c"];
  @ViewChildren('foo') id: QueryList<IdVar>; 


  ngOnInit() {
    this.form = new FormGroup(this.formControls);
    this.testArr.forEach((itm)=>{
      this.formControls[itm] = new FormControl(itm);
      this.form.addControl(itm, this.formControls[itm]);
    })
  }

  ngAfterViewInit() {
    setTimeout(()=>console.log(this.id.length),1000);
  }

}

Any idea what I'm doing wrong? Any solutions or workarounds? Tks!

NEW ADDITION BELOW (got rid of earlier "undefined" error):

app.module.ts

import { NgModule, NO_ERRORS_SCHEMA, CUSTOM_ELEMENTS_SCHEMA } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
import { FormsModule, ReactiveFormsModule } from '@angular/forms';

import { AppComponent } from './app.component';
import { HelloComponent } from './hello.component';

@NgModule({
  imports:      [ BrowserModule, FormsModule, ReactiveFormsModule ],
  declarations: [ AppComponent, HelloComponent ],
  bootstrap:    [ AppComponent ],
  schemas: [NO_ERRORS_SCHEMA, CUSTOM_ELEMENTS_SCHEMA]
})
export class AppModule { }

Answer 1

There is some changes that you need to do:

You need to add the directive in the declarations in the app.module.ts .

import { NgModule, NO_ERRORS_SCHEMA, CUSTOM_ELEMENTS_SCHEMA } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
import { FormsModule, ReactiveFormsModule } from '@angular/forms';

import { AppComponent } from './app.component';
import { HelloComponent, IdVar } from './hello.component';

@NgModule({
  imports:      [ BrowserModule, FormsModule, ReactiveFormsModule ],
  declarations: [ AppComponent, HelloComponent, IdVar ],
  bootstrap:    [ AppComponent ],
  schemas: [NO_ERRORS_SCHEMA, CUSTOM_ELEMENTS_SCHEMA]
})
export class AppModule { }

You are using a directive as a component, I think you dont need this line:

<foo [idString]="i">{{i}}:&nbsp;</foo>

The ViewChildren works with classes not with the selector,so you should change the line:

@ViewChildren('foo') id: QueryList<IdVar>; 

to:

@ViewChildren(IdVar) id: QueryList<IdVar>;  

Another note, you don't really need the extra [] in the directive

 <input foo [id]="i" />

I forked your example on stackblitz and modified it a little to make it work. I also changed the name of the directive to make it clear that it is a directive.

Live example here:

https://stackblitz.com/edit/angular-directive-viewchildren?file=src%2Fapp%2Fhello.component.ts