Angular use modal dialog in canDeactivate Guard service for unsubmitted changes (Form dirty)

I solved it using ngx-bootstrap Modals and RxJs Subjects.

First of all I created a Modal Component:

import { Component } from '@angular/core';
import { Subject } from 'rxjs/Subject';
import { BsModalRef } from 'ngx-bootstrap';

@Component({
  selector: 'app-confirm-leave',
  templateUrl: './confirm-leave.component.html',
  styleUrls: ['./confirm-leave.component.scss']
})
export class ConfirmLeaveComponent {

  subject: Subject<boolean>;

  constructor(public bsModalRef: BsModalRef) { }

  action(value: boolean) {
    this.bsModalRef.hide();
    this.subject.next(value);
    this.subject.complete();
  }
}

here's the template:

<div class="modal-header modal-block-primary">
  <button type="button" class="close" (click)="bsModalRef.hide()">
    <span aria-hidden="true">&times;</span><span class="sr-only">Close</span>
  </button>
  <h4 class="modal-title">Are you sure?</h4>
</div>
<div class="modal-body clearfix">
  <div class="modal-icon">
    <i class="fa fa-question-circle"></i>
  </div>
  <div class="modal-text">
    <p>The form has not been submitted yet, do you really want to leave page?</p>
  </div>
</div>
<div class="modal-footer">
  <button class="btn btn-default" (click)="action(false)">No</button>
  <button class="btn btn-primary right" (click)="action(true)">Yes</button>
</div>

Then I modified my guard using a Subject, now it look like this:

import { CanDeactivate } from '@angular/router';
import { FormGroup } from '@angular/forms';
import { Injectable } from '@angular/core';
import { Subject } from 'rxjs/Subject';
import { BsModalService } from 'ngx-bootstrap';

import { ConfirmLeaveComponent } from '.....';

export interface FormComponent {
  form: FormGroup;
}

@Injectable()
export class UnsavedChangesGuardService implements CanDeactivate<FormComponent> {

  constructor(private modalService: BsModalService) {}

  canDeactivate(component: FormComponent) {
    if (component.form.dirty) {
      const subject = new Subject<boolean>();

      const modal = this.modalService.show(ConfirmLeaveComponent, {'class': 'modal-dialog-primary'});
      modal.content.subject = subject;

      return subject.asObservable();
    }

    return true;
  }
}

In app.module.ts file go to the @NgModule section and add the ConfirmLeaveComponent component to entryComponents.

@NgModule({
  entryComponents: [
    ConfirmLeaveComponent,
  ]
})

In addition to ShinDarth's good solution, it seems worth mentioning that you will have to cover a dismissal of the modal as well, because the action() method might not be fired (e.g. if you allow the esc button or click outside of the modal). In that case the observable never completes and your app might get stuck if you use it for routing.

I achieved that by subscribing to the bsModalService onHide property and merging this and the action subject together:

confirmModal(text?: string): Observable<boolean> {
    const subject = new Subject<boolean>();
    const modal = this.modalService.show(ConfirmLeaveModalComponent);
    modal.content.subject = subject;
    modal.content.text = text ? text : 'Are you sure?';
    const onHideObservable = this.modalService.onHide.map(() => false);
    return merge(
      subject.asObservable(),
      onHideObservable
    );
  }

In my case I map the mentioned onHide observable to false because a dismissal is considered an abort in my case (only a 'yes' click will yield a positive outcome for my confirmation modal).

Just expanding on the additional info provided by mitschmidt regarding click outside / escape button, this canDeactivate method works with Francesco Borzi's code. I just add the subscribe to onHide() inline in the function:

canDeactivate(component: FormComponent) {
        if (component.form.dirty) {
            const subject = new Subject<boolean>();

            const modal = this.modalService.show(ConfirmLeaveComponent, { 'class': 'modal-dialog-primary' });
            modal.content.subject = subject;

            this.modalService.onHide.subscribe(hide => {
                subject.next(false);
                return subject.asObservable();
            });

            return subject.asObservable();
        }

        return true;
    }