Skip to main content
Ben Nadel
On User Experience (UX) Design, JavaScript, ColdFusion, Node.js, Life, and Love.

NgModel Host Bindings Finally Fixed But OnPush Change Detection Breaking In Angular 2 Beta 17

By Ben Nadel on

Getting ngModel to work in Angular 2 has been a frustrating uphill battle. From the breaking of host bindings to the complexity of implementing value-accessors on top of change-detection, nothing about the ngModel experience is all that fun. But, the Angular 2 team is making some progress. In Beta 17 (or perhaps earlier), they seem to have fixed the host binding problem; but, unfortunately, there appears to be a regression in OnPush change detection. I'm told by Jesus Rodriguez that this is slated to be fixed in an upcoming Beta; but, I wanted to document the current state of things as I am quite emotionally invested in seeing ngModel work.

Run this demo in my JavaScript Demos project on GitHub.

I don't want to go into too much detail here as this is mostly documentation (and practice) for myself. But, the gist of the update is that when using ngModel in conjunction with the OnPush change detection strategy, the target component's view does not seem to update. The host bindings do update, which is one of the fixes I'm excited about; but, the view remains out-of-sync.

To demonstrate this, I've created a simple CounterInput component that uses the [value] input to drive host bindings and view interpolation bindings. When we use the default change detection strategy, everything works. But, as soon as we enable OnPush change detection, the view stops updating at the appropriate time (which you can see in the demo).

<!doctype html>
	<meta charset="utf-8" />

		NgModel Host Bindings Finally Fixed But OnPush Change Detection Breaking In Angular 2 Beta 17

	<link rel="stylesheet" type="text/css" href="./demo.css"></lin>

		NgModel Host Bindings Finally Fixed But OnPush Change Detection Breaking In Angular 2 Beta 17


	<!-- Load demo scripts. -->
	<script type="text/javascript" src="../../vendor/angularjs-2-beta/17/es6-shim.min.js"></script>
	<script type="text/javascript" src="../../vendor/angularjs-2-beta/17/Rx.umd.min.js"></script>
	<script type="text/javascript" src="../../vendor/angularjs-2-beta/17/angular2-polyfills.min.js"></script>
	<script type="text/javascript" src="../../vendor/angularjs-2-beta/17/angular2-all.umd.js"></script>
	<!-- AlmondJS - minimal implementation of RequireJS. -->
	<script type="text/javascript" src="../../vendor/angularjs-2-beta/17/almond.js"></script>
	<script type="text/javascript">

		// Defer bootstrapping until all of the components have been declared.
			[ /* Using require() for better readability. */ ],
			function run() {

				ng.platform.browser.bootstrap( require( "App" ) );


		// --------------------------------------------------------------------------- //
		// --------------------------------------------------------------------------- //

		// I provide the root application component.
			function registerApp() {

				// Configure the App component definition.
						selector: "my-app",
						directives: [ require( "COUNTER_INPUT_DIRECTIVES" ) ],

						// In this template, the CounterInput component is using the
						// ngModel two-way data binding directive; however, the
						// CounterInput component doesn't actually know about ngModel.
						// The ngModel "aspect" of this component is being implemented
						// by a sibling directive bound to the same component.
							<counter-input [(ngModel)]="count"></counter-input>

								Set Count:
								<a (click)="setCount( 1 )">1</a>
								<a (click)="setCount( 2 )">2</a>
								<a (click)="setCount( 3 )">3</a>
								<a (click)="setCount( 4 )">4</a>
								<a (click)="setCount( 5 )">5</a>
								<a (click)="setCount( 10 )">10</a>
								<a (click)="setCount( 20 )">20</a>
						constructor: AppController

				return( AppController );

				// I control the App component.
				function AppController() {

					var vm = this;

					// I hold the value of the counter input.
					vm.count = 0;

					// Expose the public methods.
					vm.setCount = setCount;

					// ---
					// ---

					// I set the current count.
					function setCount( newCount ) {

						vm.count = newCount;




		// --------------------------------------------------------------------------- //
		// --------------------------------------------------------------------------- //

		// I provide all of the directives needed to work with the CounterInput
		// component. This includes the core component plus the ngModel bridge, which
		// adds [(ngModel)] functionality.
			function registerCounterDirectives() {

					require( "CounterInput" ),
					require( "CounterInputForNgModel" )


		// I provide a counter input component that accepts a [value] input binding and
		// emits a (valueChange) output binding event.
		// --
		// NOTE: The CounterInput component doesn't know anything about ngModel. It only
		// knows about the [value] and (valueChange) bindings. All ngModel functionality
		// is provided by the sibling directive that implements a "value accessor" bridge.
			function registerCounterInput() {

				// Configure the CounterInput component definition.
						selector: "counter-input",
						inputs: [ "value" ],
						outputs: [ "valueChange" ],
						host: {
							"(click)": "handleClick()",
							"[]": "( value >= 0 )",
							"[]": "( value >= 1 )",
							"[]": "( value >= 2 )",
							"[]": "( value >= 3 )",
							"[]": "( value >= 4 )",
							"[]": "( value >= 5 )",
							"[]": "( value >= 10 )"
						changeDetection: ng.core.ChangeDetectionStrategy.OnPush,
							Counter: {{ value }}
						constructor: CounterInputController,

						// Define the life-cycle methods on the prototype to that they
						// will be picked up at runtime.
						ngOnChanges: function noop() {}

				return( CounterInputController );

				// I control the CounterInput component.
				function CounterInputController() {

					var vm = this;

					// I hold the value of the counter.
					// --
					// NOTE: Injected as an input binding.
					vm.value = 0;

					// I hold the event stream for the valueChange output binding.
					vm.valueChange = new ng.core.EventEmitter();

					// Expose the public methods.
					vm.handleClick = handleClick;
					vm.ngOnChanges = ngOnChanges;

					// ---
					// ---

					// I handle the internal click event on the component.
					function handleClick() {

						// In the one-way data flow philosophy, we can't change this
						// value directly. As such, we have to announce the desired
						// change and leave it up to the calling context to decide
						// whether or not the change is piped back into the component. vm.value + 1 );


					// I get called when any of the input bindings change.
					function ngOnChanges( changes ) {

							"ngOnChanges: [value] from %s to %s.",
							( changes.value.isFirstChange() ? 0 : changes.value.previousValue ),




		// I provide an ngModel-enabled bridge for the CounterInput component.
			function registerCounterInputForNgModel() {

				// Configure the CounterInputForNgModel directive definition.
						// As the ngModel bridge, we want to match on instances of the
						// CounterInput that are attempting to use ngModel.
						selector: "counter-input[ngModel]",

						// The bridge has to deal with both input and output bindings. As
						// such, we have to listen for (valueChange) events on the target
						// component and translate them into "change" events on ngModel.
						host: {
							"(valueChange)": "handleValueChange( $event )"

						// When ngModel is being used, we need to create a bridge between
						// the ngModel directive and the target component. This bridge
						// has to implement the "value accessor" interface. In this case,
						// we're telling Angular to use THIS DIRECTIVE INSTANCE as that
						// value accessor provider. This means that the following
						// controller needs to provide the value accessor methods:
						// --
						// * registerOnChange
						// * registerOnTouched
						// * writeValue
						// --
						// NOTE: You don't need the forwardRef() here because we are
						// using ES5 instead of TypeScript. Woot! ES5 for the win!
						providers: [
									useExisting: CounterInputForNgModelController,
									multi: true
						constructor: CounterInputForNgModelController

				CounterInputForNgModelController.parameters = [
					new ng.core.Inject( require( "CounterInput" ) ),
					new ng.core.Inject( ng.core.ChangeDetectorRef )

				return( CounterInputForNgModelController );

				// I control the CounterInputForNgModel directive. As part of the ngModel
				// contract, I implement the value accessor interface.
				function CounterInputForNgModelController( counterInput, changeDetectorRef ) {

					var vm = this;

					// As part of the value accessor "bridge" that this directive is
					// providing, we need to be able to manually trigger the ngOnChanges
					// life-cycle event on the target component. To do that properly, we
					// need to keep track of when the first value is written so that we
					// can announce it as the first SimpleChange instance.
					var isFirstChange = true;

					// I hold the change hander registered by ngModel. This is the method
					// we need to call when the target component value changes.
					var onTargetChange = null;

					// Expose the public methods.
					vm.handleValueChange = handleValueChange;
					vm.registerOnChange = registerOnChange; // Value accessor interface.
					vm.registerOnTouched = registerOnTouched; // Value accessor interface.
					vm.writeValue = writeValue; // Value accessor interface.

					// ---
					// ---

					// I handle the valueChange event coming out of the CounterInput,
					// bridging the output-gap between the target component and ngModel.
					function handleValueChange( newValue ) {

						// When we invoke the onChange() value accessor method, ngModel
						// already assumes that the DOM (Document Object Model) is in the
						// correct state. As such, we have ensure that the CounterInput
						// component reflects the change that it just emitted (by piping
						// the emitted value right back into the CounterInput component).
						// --
						// NOTE: At this point, we are disregarding the one-way data flow
						// paradigm. But, that's the WHOLE POINT OF NG-MODEL.
						applyChangesToTarget( counterInput.value, newValue );

						// Tell ngModel so that it can synchronize its own internal model.
						// --
						// NOTE: If we wanted to, we could use this as an opportunity to
						// "parse" the value into something more "model" oriented (such
						// as parsing string values into numbers). But, that is not
						// needed for this component since it deals with numbers.
						onTargetChange( newValue );


					// I register the onChange handler provided by ngModel.
					function registerOnChange( newOnChange ) {

						onTargetChange = newOnChange;


					// I register the onTouched handler provided by ngModel.
					function registerOnTouched() {

						// console.log( "registerOnTouched" );


					// I implement the value input changed by ngModel. When ngModel
					// wants to update the value of the target component, it doesn't
					// know what property to use (or how to transform that value into
					// something meaningful for the target component). As such, we have
					// to bridge the gap between ngModel and the input property of the
					// CounterInput component.
					function writeValue( newValue ) {

						applyChangesToTarget( counterInput.value, newValue );


					// ---
					// ---

					// I apply the given previous and next values to the underlying
					// component. Part of that process involves making sure all of the
					// right work-flows (such as change-detection) are initiated.
					function applyChangesToTarget( previousValue, nextValue ) {

						// Write the ngModel value to the CounterInput component.
						counterInput.value = nextValue;

						// When we update the component's Inputs programmatically,
						// Angular doesn't actually register this as an "Input change"
						// (since change detection is part of the component's template
						// bindings, not its state). This creates several complications.
						// First, it means that the OnChanges() life-cycle event method
						// won't be triggered implicitly on the target component. Second,
						// it means that OnPush change detection won't be triggered. As
						// such, we have to take care of fulfilling both of those
						// responsibilities explicitly here within this value accessor
						// bridge.

						// Ensure that the entire component path is marked for change
						// detection so that Angular will know to perform a check on the
						// CounterInput's View template.
						// --
						// CAUTION: This is the part that seems to be broken in Beta 17.
						// CAUTION: This is the part that seems to be broken in Beta 17.
						// CAUTION: This is the part that seems to be broken in Beta 17.
						// CAUTION: This is the part that seems to be broken in Beta 17.
						// CAUTION: This is the part that seems to be broken in Beta 17.
						// CAUTION: This is the part that seems to be broken in Beta 17.

						// If the CounterInput component doesn't provide a hook for the
						// life-cycle event, there's nothing we need to do.
						if ( ! counterInput.ngOnChanges ) {



						// If we made it this far, the CounterInput component is exposing
						// an ngOnChanges() method. As such, we have to prepare the
						// changes collection.
						var changes = {
							value: new ng.core.SimpleChange( previousValue, nextValue )

						// Unfortunately, the Angular API doesn't expose the necessary
						// utility library that is used to denote the "first" simple
						// change. As such, we have to hack this by overwriting the
						// isFirstChange() instance method when we know that this is the
						// first change we are sending to the CounterInput.
						if ( isFirstChange ) {

							isFirstChange = false;

							changes.value.isFirstChange = function() {

								return( true );



						counterInput.ngOnChanges( changes );






As you can see, the CounterInput is using the [value] property to drive host CSS bindings and view interpolation bindings. However, if we run this code and change the counter value externally to the component, we can see that the component's CSS and the interpolation bindings fall out of sync:

ngModel host bindings finally working in Angular 2 Beta 17; but, there is a regression in OnPush change detection.

As you can see, the host CSS bindings updated as the value changed. But, the internal view never updated.

This is getting close to working! I am not so sure that using ngModel with custom controls will ever be simple. But, I am confident that the Angular 2 team is quite close to getting this workflow operational.

Reader Comments