Angular manually update ngModel and set form to dirty or invalid?

Why not use Reactive forms (FormGroup),

let testForm = new FormGroup({
    myText: new FormControl('initial value')
})

<form [formGroup]="testForm">
    <input type="text" formControlName="myText">
</form>

<button (click)="updateMyTextModel()">Update myTextModel</button>
<div *ngIf="testForm.dirty">testForm diry</div>
<div *ngIf="testForm.touched">testForm touched</div>

In your function, you can use markAsDirty() based on whatever condition you want.

updateMyTextModel(): void {
    this.myTextModel = "updated model value";
    if ( // some condition ) {
        this.testForm.markAsDirty();
    }
}

To set individual form controls as dirty/touched, you can use

this.testForm.get('myText').markAsDirty();
this.testForm.get('myText').markAsTouched();

Solution:

//our root app component
import {Component, NgModule, VERSION} from '@angular/core'
import {BrowserModule} from '@angular/platform-browser'
import { Component, ViewChild } from '@angular/core';
import { FormsModule }   from '@angular/forms';

@Component({
  selector: 'my-app',
  template: `
    <form #testForm="ngForm" id="testForm">
        <input type="text" id="myText" [(ngModel)]="myTextModel" name="myText" #myText="ngModel">
    </form>
    <button (click)="updateMyTextModel()">Update myTextModel</button>
    <div *ngIf="testForm.dirty">testForm diry</div>
    <div *ngIf="testForm.touched">testForm touched</div>
  `,
})
export class App {

  @ViewChild('testForm') test: any;

  updateMyTextModel(){
    this.test.control.markAsTouched();
    this.test.control.markAsDirty();

  }

  constructor() {
    console.log(this.test);
  }
}

@NgModule({
  imports: [ BrowserModule,FormsModule ],
  declarations: [ App ],
  bootstrap: [ App ]
})
export class AppModule {}

Plunkr working:

https://plnkr.co/edit/YthHCEp6iTfGPVcNr0JF?p=preview

This should work:

@ViewChild('testForm') testForm;


updateMyTextModel(): void {
    this.myTextModel = "updated model value";
    this.myForm.form.markAsDirty();
}