5 May

Watching for changes with Reactive Forms in Angular4


On a recent project working with Angular 4 and reactive forms a need came up to allow child components ( which were driven off off their own child form groups) be be able to detect value changes to other child components. From these changes we needed to apply rules and detect validity of the values.

Now I know your thinking

So your first response is probably duh thats what valueChange subscriptions are for, and you are totally correct. What started to happen though was a huge duplication of code across many components for not only detecting changes (valueChanges) but also detecting the validity of those changes (statusChanges). What we needed was a consolidated way to detect changes and validity across the entire parent form group and be able to deliver those changes to any sub components.

Enter the watcher service

With the help of RXjs Observables we were able to provide a single watch point that any component in the entire application could subscribe to to get any changes from any component. This service would deliver not only value changes but validity status changes as well. GoodBye code duplication I’m staying dry in this code storm.

Service Code

import {Injectable} from '@angular/core';
import {AbstractControl, FormControl, FormGroup} from '@angular/forms';
import {BehaviorSubject} from 'rxjs/BehaviorSubject';
import {Observable} from 'rxjs/Observable';
import {Subscription} from "rxjs/Rx";

declare let _;

export interface FormGroupChange {
  path: string;
  name: string;
  control: FormControl;

 Allows you to subscribe to value and status changes for fields in a FormGroup. Events are debounced to avoid too many
 simultaneous calls, and are emitted even when the validity of a field updates because of dependencies on other fields.


 let subscription = this.watchFormGroupService.watch(this.formGroup, ['personalInformation.firstName', 'personalInformation.lastName'])
   .subscribe((data: FormGroupChange) => {
      // ...

 // Don't forget to unsubscribe! (eg. in ngOnDestroy)

export class WatchFormGroupService {

  private MAX_CHECK_COUNT = 5;

    public watch(formGroup: FormGroup, paths: string[], debounce = 400): Observable<FormGroupChange> {
    return Observable.create(observer => {
      let internalSubs = [];
      paths.map(path => {
        let control = formGroup.root.get(path);
        if(!control) {
          let checkCount = 0
          // lets give angular some time to finalize the form groups and make them available. This happens due to race conditions with when different components can 
          // load and when watchers are setup from other components.
          let checkAgainInterval = setInterval(() => {
            let control = formGroup.root.get(path);
            if(control) {
              let eventData: FormGroupChange = {
                path: path,
                name: _.last(path.split('.')),
                control: control
              let subject = new BehaviorSubject(eventData);
              internalSubs.push(Observable.merge(...[control.valueChanges, control.statusChanges, subject]).debounceTime(debounce).map(data => {
            if(checkCount >= this.MAX_CHECK_COUNT) {
              console.warn("NO WATCHER PATH MATCH", path);
          }, 1000);
        } else {
          let eventData: FormGroupChange = {
            path: path,
            name: _.last(path.split('.')),
            control: control
          let subject = new BehaviorSubject(eventData);
          internalSubs.push(Observable.merge(...[control.valueChanges, control.statusChanges, subject]).debounceTime(debounce).map(data => {

      // Provide a way of canceling and disposing the interval resource
      return function unsubscribe() {
        _.forEach(internalSubs, sub => {


Now one caveat to this was we need to know which child component and form control to watch on. With no other real way around it we agreed that using the dot notation to a component and field as a string while is ‘hard coded’ was the best balance between ease of use and pseudo coupling of components. Coupling in this case i feel is loosely said, because if the component doesn’t exist in the view, but its subscribed to, nothing will be broadcasted anyway.

So how do we use this thing

In any of your components you simply inject the WatchFormGroupService and watch on an array of fields.

NOTE: registration must be in the AfterViewInit or later, so all the form groups have time to build and setup from the OnInit event. This is important because when listening across all components we do not know when the form group will be ready from initialization.

What is great about the WatchFormGroupService is that it can work with many first class form group citizens (meaning you can watch different un-related form groups from the same application).

 ngAfterViewInit() {
    this.watcherSubscription = this.watchFormGroupService.watch(this.formGroup, [
    ]).subscribe(data => {
         this.formData[data.name] = data.control.value;

In this example we are listening for address changes in the personalInformation component and then updating "my" component with the new values, but only if the value coming in is valid. While there would be more logic around when to update the value this code at least shows what is possible with the WatchFormGroupService


One thing to call out as with using any RSjs Observable, make sure you clean up your subscriptions. In OnDestroy of the watching component unsubscribe from the watcher service

  ngOnDestroy() {

I want to give a shout out to Alex Brombal who I collaborated with on the concept. He was the lucky one who got to write the WatchFormGroupService.

Check this out in action on Plunkr


Since I first published this a couple changes have been made to the WatchFormGroupService. We needed a way for subscribers to get values on the initial subscribe from from the service. This would be in the case the formGroup was loaded from database data, and a valueChange event has not fired yet. To accomplish this we added in a BehaviorSubject and loaded it with the initial value from the formControl.

  public watch(formGroup: FormGroup, paths: string[], debounce = 400): Observable<FormGroupChange> {
    return Observable.merge(...paths.map(path => {

      let control = formGroup.root.get(path);
      if (!control) {
        console.warn("NO WATCHER PATH MATCH", path);
        return Observable.empty();
      let eventData = {
        path: path,
        name: _.last(path.split('.')),
        control: control
      let subject = new BehaviorSubject(eventData);
      return Observable.merge(...[control.valueChanges, control.statusChanges, subject]).debounceTime(debounce).map(data => eventData);
1 Apr

Using ngDragula with ngPrime from Angular 4

Recently on a project I had the requirement to provided table row reordering. The challenge to that was getting access to the table rows that were created dynamically at runtime and within an ngPrime data table. To provide the drag and drop support I turned to ngDragula. This is a great plugin that provides plenty of events and html support. One thing it currently does not provide is the ability to override the container it uses for setting up the drag and drop. It currently only can use the element that the directive was placed on. As you can see with the use of 3rd party components placing the directive in the proper place can be a challenge.

To address these short comings in my project I created my own version of the ngDragula Directive. Lets walk through what you need to do to add support for drag and drop to your ngPrime data tables.

Creating your own ngDragula Directive

To add support for integration with ngPrime a couple of things need to happen. First we need to override the container that dragula will attach to. In regards to drag and drop with tables the container needs to be the closest parent to the items you want to drag. In our case that ends up being the tbody element. Unfortunately ngPrime data tables do not allow access to this element, so we have no way to place the dragula directive in the correct location. Secondly we need to allow for delayed binding to the data table. Since we provide a collection of rows to the ngPrime data table the tbody tag will not be available onInit of the component. To make this work we need to bind the ngDragula directive after the view is available from Angular. Luckily Angular provides us with the AfterViewInit lifecycle hook. Lets take a look at the code.

The first thing we need to do is create our own version of the ngDragula directive. ensure you give it a unique name that wont clash with existing libs. Also add AfterViewInit to the implements of the class

import { Directive, OnChanges, AfterViewInit, OnInit, Input, ElementRef, SimpleChange } from '@angular/core';
import { DragulaService, dragula } from 'ng2-dragula';

@Directive({ selector: '[primeDragula]' })
export class PrimeDragulaDirective implements OnChanges, OnInit, AfterViewInit { }

Next we change the container to protected so we can provide extension later

   protected container: any;

Now we can implement the Angular events OnInit, AfterViewInit. In OnInit we wire up the options, and set the initial container element. New options that can be provided to the directive are ‘initAfterView’ and ‘childContainerSelector’. Here we check to see if late binding is needed and if not initialize the directive like usual. If late binding is needed AfterViewInit handles that check.

    this.options = Object.assign({}, this.dragulaOptions);
    this.container = this.el.nativeElement;


  ngAfterViewInit() {

Lets move the initialization code to its own method for reuse. Here the only new code is the ‘childContainerSelector’ check this is what gives us the ability to use a different container then the one the ngDragula directive was placed on. Notice how we set the mirrorContainer as well. This is because since the container is a sub element of the parent we want to ensure mirror object (the drag object visual that shows the movement) is positioned relative to the correct parent.

NOTE: an upgrade to this directive would be to use another property to dictate overriding the defauly mirrorContainer = ‘document.body’

 protected initialize(){    
        //find the element starting at the directive element and search down
        this.container = this.el.nativeElement.querySelector(this.options.childContainerSelector);
        this.options.mirrorContainer = this.container;

    let bag = this.dragulaService.find(this.primeDragula);
    let checkModel = () => {
      if (this.dragulaModel) {
        if (this.drake.models) {
        } else {
          this.drake.models = [this.dragulaModel];
    if (bag) {
      this.drake = bag.drake;
    } else {
      this.drake = dragula([this.container], this.options);
      this.dragulaService.add(this.primeDragula, this.drake);

Finally, add in the pre-existing OnChanges method from the ngDragula directive

public ngOnChanges(changes: { dragulaModel?: SimpleChange }): void {
    if (changes && changes.dragulaModel) {
      if (this.drake) {
        if (this.drake.models) {
          let modelIndex = this.drake.models.indexOf(changes.dragulaModel.previousValue);
          this.drake.models.splice(modelIndex, 1, changes.dragulaModel.currentValue);
        } else {
          this.drake.models = [changes.dragulaModel.currentValue];

Using the new directive

So now the directive is create we are ready to implement it.

In our component template where we define our ngPrime data table lets add the dragula directive.

 <p-dataTable [value]="rows" [primeDragula]="bag" [dragulaModel]="rows" 
  [dragulaOptions]="{ childContainerSelector: 'tbody', initAfterView: true }">
  <p-column header="Move">
    <ng-template pTemplate="body" let-rowData="rowData">
      <i class="fa fa-bars"></i>
  <p-column field="name" header="Name">          

That is all there is to it. Pretty simple changes that allow a greater user experience.


  • Dont forget to add the javascript dragula version to your package.json and angular CLI styles and scripts sections. This is a requirement for ngDragula to work as expected.

Versions used

"@angular/animations": "4.0.0",
"@angular/common": "4.0.0",
"@angular/compiler": "4.0.0",
"@angular/core": "4.0.0",
"@angular/forms": "4.0.0",
"@angular/http": "4.0.0",
"@angular/platform-browser": "4.0.0",
"@angular/platform-browser-dynamic": "4.0.0",
"@angular/router": "4.0.0",
"dragula": "^3.7.2",
"lodash": "4.17.4",
"ng2-dragula": "^1.3.0",
"primeng": "2.0.5",
"rxjs": "5.1.0",
"zone.js": "0.8.4"

Hopefully you found this helpful. You can see a working example here on Plunkr. Also these changes have been submitted for review to the guys over at valor-software. With a little luck I can just use the official version of ngDragula one day!

10 Jul

Sticky Validation – Angular 1.5.7

Why was this created

Sticky validation is meant to fill the gap in Angular when you only want to show errors on submit,

and leave those errors displayed even after the user has started changing the input that had the errors.

Typically in Angular they operate on the concept of dynamic validation.

This validation evaluates the model in real time and adjusts the error messages accordingly based on user input.

Once the user has typed something else the original error that caused the problem has been removed automatically by Angular (provided they corrected that original error)

and the model value change has either became valid or possibly still invalid with another validation error occurring.

With the current implementation of Angular 1.5.7 there was no way to keep the original error and invalid state set once the user fixed

the problem. From a UX perspective consistency is vital and since we want to operate in the context of only show errors on submit,

the existing errors should be visible until a submit was again performed.

What is sticky validation

Sticky validation allows you to work with the pre-existing ng-messages/ng-message directives provided by Angular.

This injects new properties on the form and $error objects to allow for a constant state to be available when displaying error messages

to the user. Sticky validation also operates on the premise that you want to implement "submit" only validation (although nothing is prohibiting sticky state properties from being used with regular validation).

Sticky validation will allow you to preserve the original error message displayed and have a consistent property available indicating

that "submits" are happening (currently in Angular if you try and use $submitted and $invalid together to show the messages, once $invalid becomes false the messages will hide).

How to implement

Implementing can be done in 3 easy steps, once the directive has been added to your project and registered with your Angular app.

  1. Replace any existing ng-submit with se-submit

    <form name="cntrl.form" se-submit="cntrl.submit()" novalidate>

  2. To ensure ng-messages only display when the form is submitted and invalid. Add the following in the ng-if.

    $fieldInvalid is the custom property added to each element that is kept in sync with Angulars’ $invalid property

    <div ng-messages="cntrl.form.badgenumber.$error" ng-if="cntrl.form.$submitted && cntrl.form.badgenumber.$fieldInvalid">
        all the ng-message directives

  3. In your ng-message elements switch the current validation name “required” (Angular implementation) with “stickyrequired”

    <div ng-message="stickyrequired" class="has-error">
        Badge Number must be provided


Currently this directive only works for a single level form. If your form has child forms nested within, this will NOT inject

the custom properties. I have not come across many cases where usage of sub forms is needed, but there are always outliers.

If child form support is needed please feel free to submit a pull request for review.

Example Walkthrough

  1. User comes to page and sees the form to fill out

    alt text

  2. User types invalid characters into the form field and clicks “Continue”

    alt text

    The error is displayed in standard Angular fashion

  3. With use of sticky validation, once the field is cleared (which sets its state back to valid). the error message and error highlighting remain intact

    alt text

  4. User corrects the issue and clicks “Continue” again. That validation issue is then re-evaluated and cleared accordingly.

    alt text

Final Words

I hope this helps in overcoming the pitfalls and gaps with Angular validation when it comes to not wanting dynamic validation on all the time.
One of the great things about this is it still allows you to use Angulars’ built in validation for dynamic error messaging. Want the source code? Visit my GitHub

For more information view


angular forms

9 Jul

Scroll watcher directive for Angular 1.5.7

Here is a directive I came up with to help with keeping track of page scroll position and when scrolling has started and stopped. I had a need for this in trying to hide page content while the user was scrolling up/down a page, and then re-showing the content once the scrolling had stopped. Currently this is only setup up to work at the document level, but and easy modification could be made to allow a new property to drive what scroll area is being monitored. I hope this helps others in case they need a way to tell if page scrolling has started or stopped.

How to Implement

Simply add the directive to the page you want to monitor scrolling on. Next add the scroll-callback function you want to be called from directive when scrolling starts and stops

<div page-scroll-watcher scroll-callback="cntl.scrollStop($event, isEndEvent, isScrollingEvent)">

Callback Function
Note: sample code is in ES6 format. This is an excerpt from a angular controller

 //$event is the standard scroll event from the browser. This contains the X,Y information
 //isEndEvent signals when scrolling has stopped
 //isScrollingEvent signals when scrolling has started
 scrollStop($event, isEndEvent, isScrollingEvent) {
    if (isEndEvent) {
      this.showBottomBar = true;
      this.showBottomBar = false;

Now that we have the how to implement lets get to the good stuff. The code that makes this all work
Page Scroll Directive
Note: code is in ES6 format

//this would just need to be registered with your Angular app
import angular from "angular";
import * as _ from "lodash";

const directivesModule = angular.module("MyDirectives", [])
  .directive("pageScrollWatcher", ["$window", "$document", pageScrollWatcher]);

function pageScrollWatcher($document) {
  return {
    restrict: "A",
    scope: {
      scrollCallback: "&"
    link: function (scope) {
      //here could be updated to use the element this directive is attached to if needed to watch a scrollable div container
      const el = angular.element($document); 

      //here we delay evaluating the scrolling events until they have stopped
      const dbnce = _.debounce(function (e) {
        //send event that scrolling stopped
        scope.$apply(function () {
          //execute the provided callback
          scope.scrollCallback({ $event: e, isEndEvent: true, isScrollingEvent: false });

        //register first scroll interceptor. Since scrolling has stopped we now need to register a start scrolling event binding
        el.bind("scroll", firstScrollFunc);

      }, 200);

      const firstScrollFunc = function (e) {
        //so we have detected the scrolling needs to start. Since this is a one time event between starts/stops we need to
        //unregister the start scrolling event
        el.unbind("scroll", firstScrollFunc);
        scope.$apply(function () {
          //execute the provided callback
          scope.scrollCallback({ $event: e, isEndEvent: false, isScrollingEvent: true });
          //We do this incase angular removes dom parts causing the scroll bar to disappear or change.
          //we need to trigger the end event again 

      //on first load of directive register the start and stop events
      el.bind("scroll", firstScrollFunc);
      el.bind("scroll", dbnce);

      scope.$on("$destroy", function handleDestroyEvent() {
        //when switching pages remove event
        el.unbind("scroll", dbnce);
        el.unbind("scroll", firstScrollFunc);


Want the source? Visit my GitHub

7 Jan



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