[ngx-formly] Using field hooks to listening value changes

Every serious form in large apps has some dependent fields. For instance a dropdown field containing all nations and another dropdown field displaying the cities based on the currently selected nation. In this less we're looking into how we can dynamically load and filer our city dropdown based on the changes on our nation dropdown field.

import { Component } from '@angular/core';
import { FormGroup } from '@angular/forms';
import { FormlyFieldConfig } from '@ngx-formly/core';
import { DataService } from './core/data.service';
import { switchMap, startWith } from 'rxjs/operators';

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.scss'],
})
export class AppComponent {
  form: FormGroup = new FormGroup({});
  model = {
    firstname: 'Zhentian',
    age: 31,
    nationId: 2,
    cityId: 1,
  };
  fields: FormlyFieldConfig[] = [
    {
      key: 'firstname',
      type: 'input',
      templateOptions: {
        type: 'text',
        label: 'FirstName',
      },
    },
    {
      key: 'age',
      type: 'input',
      templateOptions: {
        type: 'number',
        label: 'Age',
      },
    },
    {
      key: 'nationId',
      type: 'select',
      templateOptions: {
        label: 'Nation',
        options: this.dataService.getNations(),
      },
    },
    {
      key: 'cityId',
      type: 'select',
      templateOptions: {
        label: 'Cities',
        options: [],
      },
      hooks: {
        onInit: (field: FormlyFieldConfig) => {
          field.templateOptions.options = field.form.get('nationId').valueChanges.pipe(
            startWith(this.model.nationId),
            switchMap(nationId => this.dataService.getCities(nationId)),
          );
        },
      },
    },
  ];

  constructor(private dataService: DataService) {}

  onSubmit({ value, valid }) {
    console.log(value, valid);
  }
}