Select second dropdown based on first dropdown using Angular 4

Question

I have two dropdowns that I populate with data from the server. The first dropdown contains a state, and the second one contains city. Like if I select one state name only cities in that state should show in 2nd drop down using Angular 4.

<mat-tab-group>
                <mat-tab label="Basic Info">
                    <ng-template mat-tab-label>
                        <div fxLayout="center center">
                            <mat-icon style="font-size: 16px; line-height: 18px">gps_fixed</mat-icon>
                           State
                        </div>                           
                    </ng-template><br /><br /><br /><br />
                    <div class="ui fluid container" fusePerfectScrollbar>
                        <div class="content">
                            <mat-form-field>
                                <mat-select placeholder="State" [(ngModel)]="selectedValue" name="state" (change)="dropdownchange($event.target.value)">
                                  <mat-option>None</mat-option>
                                  <mat-option *ngFor="let state of states" [value]="state">{{state}}</mat-option>
                                </mat-select>
                              </mat-form-field><br />
                              <h3>You selected: {{selectedValue}}</h3>
                        </div>
                    </div>
                </mat-tab>
                <mat-tab label="Location">
                    <ng-template mat-tab-label>
                        <div fxLayout="center center">
                            <mat-icon style="font-size: 16px; line-height: 18px">location_on</mat-icon>
                            City
                        </div>
                    </ng-template>
                    <div class="ui fluid container" fusePerfectScrollbar>
                        <div class="ui fluid container" fusePerfectScrollbar>
                            <div class="content">
                                <mat-form-field>
                                    <mat-select placeholder="City">
                                      <mat-option>None</mat-option>
                                      <mat-option *ngFor="let cities of city" [value]="state">{{cities}}</mat-option>
                                    </mat-select>
                                  </mat-form-field><br />

                            </div>
                        </div>
                    </div>
                </mat-tab>
   </mat-tab-group>

Answer 1

app.component.ts

import { Component } from '@angular/core';

@Component({
  selector: 'my-app',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css']
})
export class AppComponent {
  private map = new Map<string, string[]>([
    ['Poland', ['Warszawa', 'Krakow']],
    ['USA', ['New York', 'Austin']],
  ])

  country: string;
  city: string;

  get countries(): string[] {
    return Array.from(this.map.keys());
  }

  get cities(): string[] | undefined {
    return this.map.get(this.country);
  }

}

app.component.html

<select [(ngModel)]="country">
  <option *ngFor="let country of countries" [value]="country"> {{ country }} </option>
</select>

<select *ngIf="country" [(ngModel)]="city" [value]="city">
  <option *ngFor="let city of cities"> {{ city }} </option>
</select>

Live demo

In your case all you need to change from the example above is to populate the country/cities map with the data from your server.