Angular2 Component: Semantic UI Dropdown

dropdown.ts

import {Component, ElementRef, AfterViewInit, Output, EventEmitter, Input, Self} from '@angular/core';
import {ControlValueAccessor, NgModel} from '@angular/common';
declare var $: any;
@Component({
  selector: 'dropdown',
  template: `
        <select class="ui dropdown" [(ngModel)]="selectedOption">
          <option value="">Select one</option>
          <option *ngFor="let item of items" [value]="item[valueField]">{{item[textField]}}</option>
        </select>
`,
  providers: [NgModel]
})
export class DropdownComponent implements AfterViewInit, ControlValueAccessor {
  @Input() valueField: string;
  @Input() textField: string;
  @Input() items: any[] = [];
  @Output() change: EventEmitter<string> = new EventEmitter<string>();
  public onChange: any = Function.prototype;
  public onTouched: any = Function.prototype;

  private _selectedOption: string = '';
  get selectedOption(): string {
    return this._selectedOption;
  }
  set selectedOption(option: string){
    if(option === this._selectedOption) return;
    this._selectedOption = option;
    this.writeValue(option);
  }

  constructor(private parentElement: ElementRef, @Self() private self: NgModel){
    this.self.valueAccessor = this;
  }
  ngAfterViewInit(): void {
    let selectElement: HTMLElement = this.parentElement.nativeElement.children[0];
    console.log(this.parentElement, selectElement);
    $(selectElement).dropdown();
  }
  writeValue (value: string): void {
    this.self.viewToModelUpdate(value);
    this.change.emit(value);
  }
  registerOnChange(fn: (_: any) => void): void { this.onChange = fn; }
  registerOnTouched(fn: () => void): void { this.onTouched = fn; }
}

This is nice. I'm working on my own DropdownComponent and was able to steal some ideas from this. I'm not understanding some of the code here though (I'm new to Angular). Why are you passing the parentElement and self into the component?

It seems to me that there's custom code that angular handles on its own. For instance, writeValue(). I don't understand the full consequences of those choices. Could you explain why you made these choices? I'm sure I'm missing some valuable information.

Thank you!