Angular2，Ionic2:当用户至少选择4个字段中的2个时，如何导航到下一页？

import { RouterModule, Routes } from '@angular/router';

const routes: Routes = [
  { path: '', redirectTo: 'home', pathMatch: 'full' },
  { path: 'home', component: HomePage },
  { path: 'next', component: NextPage }
];

@NgModule({
  imports: [
    RouterModule.forRoot(routes)
  ],
  ...
})

<form (ngSubmit)="onSubmit()">
  <label>
    <input type="checkbox" [(ngModel)]="field1" name="field1"> Field 1
  </label>
  <label>
    <input type="checkbox" [(ngModel)]="field2" name="field2"> Field 2
  </label>
  <label>
    <input type="checkbox" [(ngModel)]="field3" name="field3"> Field 3
  </label>
  <label>
    <input type="checkbox" [(ngModel)]="field4" name="field4"> Field 4
  </label>
  <button type="submit">Next</button>
</form>

import { Router } from '@angular/router';

export class HomePage {
  field1: boolean;
  field2: boolean;
  field3: boolean;
  field4: boolean;

  constructor(private router: Router) {}

  onSubmit() {
    if ((this.field1 && this.field2) || (this.field1 && this.field3) || (this.field1 && this.field4) || (this.field2 && this.field3) || (this.field2 && this.field4) || (this.field3 && this.field4)) {
      this.router.navigate(['/next']);
    } else {
      // 提示用户至少选择2个字段
    }
  }
}