Is there a way to decorate/intercept a built-in directive in Angular 4 without monkey patching?

Question

Problem

I am trying to add functionality to the built-in NgForm directive by intercepting the onSubmit function in order to prevent double-submission and invalid submission, but I haven't been able to find a way to do so without monkey patching.

Failed Attempt 1: Decorator via Dependency Injection

I didn't really expect this to work with directives since they aren't really "providers", but I tried it anyway (to no avail).

import { Injectable } from '@angular/core';
import { NgForm } from '@angular/forms';

@Injectable()
export class NgFormDecorator extends NgForm {
    constructor() {
        super(null, null);
    }

    onSubmit($event: Event): boolean {
        // TODO: Prevent submission if in progress
        return super.onSubmit($event);
    }
}

// Module configuration
providers: [{
    provide: NgForm,
    useClass: NgFormDecorator
}]

Working Attempt 2: Monkey Patch with Secondary Directive

This works great but is obviously not ideal.

import { Directive, Output, EventEmitter } from '@angular/core';
import { NgForm } from '@angular/forms';
import { Observable } from 'rxjs/Observable';
import 'rxjs/add/operator/finally';
import { noop } from 'rxjs/util/noop';

@Directive({
    selector: 'form',
    exportAs: 'NgFormExtension'
})
export class NgFormExtensionDirective {
    private onSubmitBase: ($event: Event) => void;
    submitting: boolean;

    constructor(private ngForm: NgForm) {
        this.onSubmitBase = ngForm.onSubmit;
        ngForm.onSubmit = this.onSubmit.bind(this);
    }

    private onSubmit($event: FormSubmitEvent): boolean {
        if (this.submitting || this.ngForm.invalid) {
            return false;
        }
        this.submitting = true;
        const result = this.onSubmitBase.call(this.ngForm, $event);
        if ($event.submission) {
            $event.submission
                .finally(() => this.submitting = false)
                .subscribe(null, noop);
        } else {
            this.submitting = false;
        }
        return result;
    }
}

export class FormSubmitEvent extends Event {
     submission: Observable<any>;
}

Question

Is there a way to decorate/intercept a built-in directive in Angular 4 without monkey patching?

Answer 1

You can always just override the ngForm selector, and extend the NgForm class:

@Directive({
  selector: 'form:not([ngNoForm]):not([formGroup]),ng-form,[ngForm]',
})
export class CNgFormDirective extends NgForm {
 constructor(
      @Optional() @Self() @Inject(NG_VALIDATORS) validators: any[],
      @Optional() @Self() @Inject(NG_ASYNC_VALIDATORS) asyncValidators: any[]) {
    super(validators, asyncValidators);
  }

  onSubmit($event: Event): boolean {
    console.log(`I'm custom!`);

    return super.onSubmit($event);
  }
}

working stack