This article is part of a series on creating multi step form wizards in Angular.

Part 1 – Basic Setup
Part 2 – Building Our Forms
Part 3 – Tying It All Together

I recently came across a question on social media asking what the best way to create a “multi stage” form wizard in Angular would be. Now multi stage might be one word for it, but I’ve also seen it called multi step, multi page, multi form, hell even just “form wizard”. But they all roughly mean the same thing – a big long form that should be done in one sitting, but is broken up (often into bite size sections) across multiple “pages”.

The solutions on social media mostly involved complex third party libraries, like piping results around using rxJS, or creative use of reactive forms in Angular etc. I might be showing my age here… But I’ve built multi step forms in the exact same way since Angular 1.6. Using a shared service to hold the form “state”, and simply sharing that service between different forms/components. I’m going to outline that solution here in a very simple way so that (hopefully), you can follow on and realize that while Angular can be powerful, it can often handle complex scenarios with very non-complex code.

Project Setup

The first thing we need to do is create a brand new project for the walkthrough. Using Angular CLI we can run our standard angular new project command :

ng new multistageform

Make sure to select “Yes” when asked if I wanted to add Angular Routing as we need this to route between different steps of the form.

Inside our app.component.html we want to replace the default content in here with :

<h1>My Multistep Form</h1>
<router-outlet></router-outlet>

This will be the starting point for our form. In a real world scenario we would probably go with some sort of nested routing strategy, but for our first cut, let’s make things simple! Fire up our angular serve command in the CLI :

ng serve

And we should be able to browse to localhost:4200 and be greeted with our project all nicely set up!

Shared Service Setup

The secret to making this multi step form work is a shared service that can hold “state” as we click next throughout the form. Go ahead and run the following CLI command  to generate a new service called “FormData”. I personally like throwing all my services into a services folder – it just makes things a bit easier to find.

ng generate service services/FormData

This should create a new folder called services in your app directory, and inside it should be form-data.service.ts. Also note that it’s decorated with the following attribute :

@Injectable({
  providedIn: 'root'
})

This just happens by default but it’s actually incredibly important to what we are doing. The providedIn : ‘root’ property tells Angular that the service should be created at the root level, and re-used for every component/service that requests it. In simple terms, our service will be a singleton. It will be created once, and that instance will be reused everywhere. This is super important for us because as we step through the form, we want to “share” data between steps and right at the end be able to grab all the data at once.

Let’s create a model to hold some data.

ng generate class models/PersonalDetails

This will generate a class to hold the “personal details” step of the form – just a first/last name for now. Replace the contents of your newly created class with the following :

export class PersonalDetails {
    firstName : string;
    lastName : string;
}

Go ahead and do the same and create an AddressDetails model

ng generate class models/AddressDetails

And fill it with the following class definition

export class AddressDetails {
    fullAddress : string;
}

Let’s head back to our FormDataService as we need to add references to our two new models.

There’s a few different ways we could go about this, we could do it with methods, properties, or just plain public variables. For now let’s just go with public variables. We will also want to “reset” our variables in the constructor which essentially means that on page load, we have an “empty” form.

So with all that in mind, our FormDataService should end up looking like :

@Injectable({
  providedIn: 'root'
})
export class FormDataService{

  personalDetails : PersonalDetails;
  addressDetails : AddressDetails;

  constructor() {
    this.personalDetails = new PersonalDetails();
    this.addressDetails = new AddressDetails();
   }
}

That’s all we will be doing with the service for now as we jump into actually building out our forms!

Following along so far? I’ve broken up the tutorial into bite sized chunks so it’s a little less daunting. Check out the next part of this series where we build out our multi step form with some very simple HTML and Angular components.

Let’s be honest, it’s pretty common that when your pipe isn’t running how you think it should in Angular, you can sometimes just tack on the “pure : false” tag like so

@Pipe({
  name: 'myPipe', 
  pure : false
})

And you just hope for the best. But this actually has severe consequences for your application, and in some cases can grind your entire application to a halt. It’s also extremely easy to end up with infinite loops or memory issues as an impure pipe can easily end up running far more often than you realize. But let’s rewind a bit, what’s this “pure” business about anyway?

Pure Pipes

A “pure” pipe (Which I have to say, I don’t like the naming.. It’s not that intuitive…), is an Angular Pipe that only runs when the underlying variable value changes. So for example if I had the following

{{ myVariable | myPipe }}

Then myPipe would only run when myVariable changes value. This typically means on load, and then anytime that variable is touched. But there is a small caveat. Let’s say for example I have a variable that looks like so :

myVariable : any = { propertyA : "foo"};

And then I am piping my variable as normal

{{ myVariable | myPipe }}

What happens when I do the following?

myVariable.propertyA = "bar";

Does the pipe run again? The answer is actually no. The easiest way to understand this is that it’s only looking if the top level value of the variable changes. It does not look at child properties at all. The only way for this pipe to be run again is to change the entire instance of myVariable like :

myVariable = { propertyA : "bar"}

But that’s somewhat impractical. You can’t be rebuilding the entire object every time just so a pipe can run. And you may even be trying to pipe an object you have no control over how it’s being set. So that’s where impure pipes come in.

Impure Pipes

Introducing Impure pipes, you can make *any* Pipe impure just by setting the pure flag to false in your declaration :

@Pipe({
  name: 'myPipe', 
  pure : false
})

Now instead of running every time the value changes, it runs often, *much* more often. Specifically it runs every digest cycle which is essentially Angular’s internal loop. Generally speaking, you may be running the pipe every time you move your mouse or use your keyboard. There’s a couple of issues this may cause :

• You are now running code irrespective of changes to the underlying data, meaning there is a lot of time wasted doing unneeded work.
• Depending on how heavy your pipe is, you could slow down your app trying to process work that doesn’t need to be done.

The second point is important. If your Pipe is doing something with a large piece of data, for example sorting a large array, it’s impractical to be doing this every digest cycle.

You’ll even notice this little tidbit from the Angular documentation around the lack or filter/orderBy pipes in the framework :

Angular doesn’t provide pipes for filtering or sorting lists. Developers familiar with AngularJS know these as filter and orderBy. There are no equivalents in Angular.

This isn’t an oversight. Angular doesn’t offer such pipes because they perform poorly and prevent aggressive minification. Both filter and orderBy require parameters that reference object properties. Earlier in this page, you learned that such pipes must be impure and that Angular calls impure pipes in almost every change-detection cycle.

Filtering and especially sorting are expensive operations. The user experience can degrade severely for even moderate-sized lists when Angular calls these pipe methods many times per second.

So as a framework, Angular has gone to pains to point out that even pure pipes should be lightweight and try and do as little heavy lifting as possible. Let alone if you are doing impure pipes.

A good trick to get into is to just add a simple console.log inside your pipe to see how often it’s being run, the results may blow your mind and make you think twice about switching to impure.

Good Examples Of Impure Pipes

For good examples of impure pipes, we can look at which pipes in the framework are marked as impure. These are :

• JsonPipe
• SlicePipe
• KeyValuePipe

Key to all 3 of these pipes are that they are working on objects, not primitive values (So require Impure pipes to detect “deep” changes), and all of them do a very simple process to output the data. For example the JsonPipe simply does a stringify on an object (Essentially a one liner). It doesn’t mean that these are particular performant and should be used everywhere, but they are a good example of “when you have to”.

As an example of the absolute worst thing you could do would be to make an HTTP Call from an Impure pipe. This will surely result in thousands of HTTP calls to an API and your page grinding to a halt.

Using Input Parameters Instead Of Impure Pipes

There may be times where you need to pipe a particular object, but within that object you are only using a couple of properties to derive the pipe output. You can get around having to use an impure pipe by instead using input parameters on your pipe. Using our object from earlier :

myVariable : any = { propertyA : "foo"};

We can then create a pipe that looks like so :

@Pipe({
  name: 'myPipe'
})
export class myPipe implements PipeTransform {
  transform(myVariable : any, inputParam : any): string {
    return inputParam;
  }
}

In my HTML I can then call the pipe like so :

{{ myVariable | myPipe:myVariable.propertyA }}

The pipe will run when myVariable changes *and* when propertyA changes. You don’t even have to use the parameter (Or the initial input variable) inside the pipe, but you can still use them to “trigger” the pipe. This can also be handy when you need the pipe to trigger based on another objects parameter. This can feel hacky at times and if your pipe is lightweight enough, it can be simpler to just to go impure, but it’s an option if you don’t want things firing multiple times per second.

There’s going to be a couple of assumptions for this guide on adding App Insights to an Angular app. The biggest of all is that you are familiar with the Application Insights product itself. Maybe you’ve used in an another app before, maybe in a C# backend application or maybe a React/Vanilla Javascript app. But the biggest thing is that you know what it does and how to create an instance in the Azure Portal.

So with that out of the way. Why use Application Insights at all in an Angular App? The biggest win of all is that it can be used to track every application error thrown by your application, both handled and unhandled. And the second is that it can be used to track pageviews of users as they move through your SPA, which can then be used to track navigation paths, where users drop off in your sign up process, how your pipelines perform etc.

With all that said and done. Let’s jump right into it.

Installing Application Insights Libraries

The first thing we need to is install the following NPM package

npm install @microsoft/applicationinsights-web --save

Now this next piece of code is going to look big but it should be easy to understand. We want to create an “ApplicationInsightsService” that can handle logging exceptions, setting the logged in user, and tracking page views. The service contents will look like so :

import { Injectable } from '@angular/core';
import { ApplicationInsights, IExceptionTelemetry, DistributedTracingModes } from '@microsoft/applicationinsights-web';
import { Router, NavigationEnd } from '@angular/router';
import { filter } from 'rxjs/operators';

@Injectable({
  providedIn: 'root',
})
export class ApplicationInsightsService {
  private appInsights : ApplicationInsights;

  constructor(private router: Router) {

    this.appInsights = new ApplicationInsights({
      config: {
        instrumentationKey: "YourKeyHere"
      }
    });

    this.appInsights.loadAppInsights();
    this.loadCustomTelemetryProperties();
    this.createRouterSubscription();
  }

  setUserId(userId: string) {
    this.appInsights.setAuthenticatedUserContext(userId);
  }

  clearUserId() {
    this.appInsights.clearAuthenticatedUserContext();
  }

  logPageView(name?: string, uri?: string) {
    this.appInsights.trackPageView({ name, uri});
  }

  logException(error : Error){
    let exception : IExceptionTelemetry = {
      exception : error
    };
    this.appInsights.trackException(exception);
  }

  private loadCustomTelemetryProperties()
  {
    this.appInsights.addTelemetryInitializer(envelope => 
      {
        var item = envelope.baseData;
        item.properties = item.properties || {};
        item.properties["ApplicationPlatform"] = "WEB";
        item.properties["ApplicationName"] = "My.Application.Name";
      }
    );
  }

  private createRouterSubscription()
  {
    this.router.events.pipe(filter(event => event instanceof NavigationEnd)).subscribe((event: NavigationEnd) => {
      this.logPageView(null, event.urlAfterRedirects);
    });
  }
}

So a couple of notes :

• LoadCustomTelemetryProperties can be used to set “custom” properties inside ApplicationInsights. This is handy to name your app, or pass through specific information that isn’t normally captured by AppInsights.
• CreateRouterSubscription allows us to listen for navigation events and then log these as pageviews. By default AppInsights only logs full page refreshes so you will need this if you are using the internal Angular router.
• LogException can be used to log exceptions, but you need to inject this service manually for it to be of any use (We will look at that shortly).
• The @Injectable attribute has been told that the provider is for the root, this means there will only ever be one instance of this service for your entire app (Singleton).

Error Handling

So in theory the Application Insights javascript packages should catch all unhandled errors. But in reality Angular actually catches any internal errors (for example a bug in your code), and has it’s own way of handling them. This kicks in well before AppInsights as a chance to see the error, and so you won’t actually see your exceptions being logged to AppInsights at all!

Let’s fix that. We want to create an ErrorHandler implementation that does nothing but log the exception to AppInsights, and print out to the console (If we want to). The code looks like :

import { ErrorHandler, Injector, Injectable } from '@angular/core';
import { ApplicationInsightsService } from './application-insights.service';

@Injectable()
export class ApplicationInsightsErrorHandler implements ErrorHandler{

  constructor(private injector : Injector)
  {
  }

  handleError(error: any): void {
    this.injector.get<ApplicationInsightsService>(ApplicationInsightsService).logException(error);
    console.log(error);
  }
}

Now I know the use of Injector here is a bit flaky, but there is a bit of reasoning behind it. I found that when dealing with exceptions, if they were issues with the error handler or the AppInsightsService itself, I ended up in infinite loop territory when trying to inject things in normally. The use of Injector isn’t ideal, but it works and typically broke out of loops before hitting max callstack exceptions.

In our NGModule, we then want to create a provider for our error handler :

@NgModule({ 
  declarations: [
	...
  ],
  imports: [
	...
  ],
  providers: [
    ...
    { provide : ErrorHandler, useClass : ApplicationInsightsErrorHandler},
  ],
  bootstrap: [AppComponent]
})
export class AppModule { }

We use the token of ErrorHandler so Angular knows that we want to use this particular class for handling all errors. And that’s it!

Using Distributed Tracing With CORS

If you are attempting to setup distributed tracing with CORS (That is your API is something like api.mydomain.com and your front end is mydomain.com), then there is some more mucking about to do before you get there. It’s a headache, I can assure you.

The first is that CORS is not turned on within the App Insights Angular library by default. To do so, in our ApplicationInsightsService we want to pass in an extra flag when creating our ApplicationInsights object :

this.appInsights = new ApplicationInsights({
  config: {
	instrumentationKey: "YourKeyHere", 
	enableCorsCorrelation : true
  }
});

Next you need to set up your backend app for CORS. Now this will be different depending on what language you are using. My suggestion would be to google “YourLanguage Setup CORS” and follow the steps from there, then right at the end you need to look how to set up “ExposedHeaders”. We need to expose the header “request-context” otherwise the token AppInsights uses to link up requests is not passed back and forth.

As an example, in C# .NET Core, it would look like :

app.UseCors(x =>
    x.AllowAnyOrigin()
        .AllowAnyHeader()
        .AllowAnyMethod()
        .WithExposedHeaders("request-context")
);

Only then will your application pass the distributed tracing tokens back and forth.

Obviously you will also need to wire up Application Insights for your backend application which will completely depend on which backend language you are using. But almost all “support” distributed tracing out of the box, just not using CORS.

Yes, we should always be doing unit tests, E2E tests, integration tests, and whatever other sort of tests are the flavor of the month. But you might also find yourself working on a proof of concept that tests are just going to be overkill on.

Even so, each time you run an ng generate command, you end up with an annoying spec file that isn’t too hard to delete, but is just hella annoying to have to do each time. Luckily Angular provides a way to turn off spec files via the CLI. Let’s take a look.

When Creating A New Project

When creating a new project, there is a way to skip the entire rest of this tutorial by one simple flag.

ng new --skipTests

When creating your new project, simply pass the –skip-tests flag and “theoretically” it should skip *ALL* tests right?

Well wrong (As of Angular 8). You will probably see the following fly by :

CREATE SkipTestsFromStart/e2e/protractor.conf.js (808 bytes)
CREATE SkipTestsFromStart/e2e/tsconfig.json (214 bytes)
CREATE SkipTestsFromStart/e2e/src/app.e2e-spec.ts (651 bytes)
CREATE SkipTestsFromStart/e2e/src/app.po.ts (262 bytes)

So as it turns out there is an open bug on the Angular Github Repo for this exact issue : https://github.com/angular/angular-cli/issues/9160. Personally I think it makes sense that when you say “Hey, I’m not doing tests”, that you actually mean it, but Angular gonna Angular.

So if it still creates the e2e folders, what does it actually do when you pass –skipTests? Well it adds the following into your angular.json file :

{
  ...
  
  "projects": {
    "SkipTestsFromStart": {
      "projectType": "application",
      "schematics": {
        "@schematics/angular:component": {
          "style": "scss",
          "skipTests": true
        },
        "@schematics/angular:class": {
          "skipTests": true
        },
        "@schematics/angular:directive": {
          "skipTests": true
        },
        "@schematics/angular:guard": {
          "skipTests": true
        },
        "@schematics/angular:module": {
          "skipTests": true
        },
        "@schematics/angular:pipe": {
          "skipTests": true
        },
        "@schematics/angular:service": {
          "skipTests": true
        }
      }
	  
	 ...
}

What it’s essentially doing is saying “Hey, when I create these things, don’t create tests”. So for example if we run the following command :

ng generate component MyNewFeature

It won’t generate the spec file along with it. Awesome!

When Working On An Existing Project

You may have already created your project so passing the –skipTests flag to the ng new command isn’t going to cut it. So you have two ways you can work around this (Or really one if I’m being honest).

The first way is less than ideal because 9 times out of 10 you forget to pass the flag. That is, when you run ng generate , you can pass skipTests in there :

ng generate component MyNewFeature --skipTests

Note that in older versions of Angular this was a spec flag that you passed true or false in. This has now been deprecated but if you are on an old version of Angular it will look like so :

ng generate component MyNewFeature --spec=false

It’s quickly going to become a headache to always pass this into your generate commands. So a better way is to take the above schematics JSON that we talked about in the “When Create A New Project” section, and put that into your angular.json.

For example, if I don’t want new components generating a spec file, we can add the following to our angular.json file :

{
  "$schema": "./node_modules/@angular/cli/lib/config/schema.json",
  "version": 1,
  "newProjectRoot": "projects",
  "projects": {
    "MyProject": {
      "projectType": "application",
      "schematics": {
        "@schematics/angular:component": {
          "style": "scss",
          "skipTests": true
        }
.....
}

I’ll also note that you can pick and choose where you do this. For example you could disable tests for components only, but still generate tests for your services, pipes and guards.

The best way I’ve found to get the JSON just right is to create a brand new project in another folder using

ng new --skipTests

And then copy and paste over the JSON you need. This will also give you a good idea of what how fine grain you can go when turning off tests (Or not turning them off as it might be).

In almost all of my Angular projects, one of the first things I add is a simple AppConfig service that allows me to swap out configurations on the fly for different environments. In most cases, this is so that I can set a different base API endpoint for my different environments, and have them swapped in at release time. Let’s jump into it!

Why Not Just Use Angular Environments?

So Angular actually has an inbuilt concept of “environments” (more info here) that is pretty similar to a typical appconfig setup. But the main issue is that this environment is also tied to individual builds. In modern day scenarios we want to build *once* and release multiple times. If our app settings are swapped out/configured at build time, this doesn’t afford us the ability to only build once and deploy multiple times. Instead we are building a different distribution for each environment.

Instead, I would highly recommend keeping Angular Environments for settings that you want to have at build time (e.g. Turn off Debug), rather than setting up individual settings between Dev,  Test,  Production etc.

App Config Is Not A Secret Store

The next thing I want to touch on is that no matter what we do in Angular, it’s still a front end web technology. That means that, even if we obfuscate things a bit, any user can still view the source code. In this tutorial, we are using a publicly available JSON file as our application configuration file, so even more so, it’s not a secret store. Do not put anything in there that you don’t want shared to the public.

Really, in most cases we are just going to be putting in there a API base endpoint and maybe a couple of other small config params that change markup. But nothing like like DB Connection Strings, authentication tokens etc.

Create Our Config Files

The first thing to do is to create a couple of configuration files in a safe directory. By safe I mean that Angular will just take the directory as a whole and output it when building. We don’t want Angular to do anything with our config files, just copy them when we build.

For that reason I typically choose the assets  folder which is created when you create a new Angular project from the CLI.

So in a folder at /assets/config  I create two files. The first is just called app-settings.json with the following :

{
    "apiBaseUrl": "https://localhost:5001"
}

And then I create another called app-settings.production.json with the following :

{
    "apiBaseUrl": "https://myapi.com"
}

So that’s our config files ready to go!

Creating The AppConfig Service

Next create a new service using the Angular CLI called AppConfig. Then replace all the code with the following :

import { Injectable } from '@angular/core';
import { HttpClient } from '@angular/common/http';

@Injectable({
  providedIn: 'root'
})
export class AppConfigService {

  private appConfig: any;
  private http : HttpClient;
  
  constructor(http: HttpClient) {
	this.http = http;
  }

  loadAppConfig() {
    return this.http.get('/assets/config/app-settings.json')
      .toPromise()
      .then(config => {
        this.appConfig = config;
      });
  }

  get apiBaseUrl() : string {
    return this.appConfig.apiBaseUrl;
  }
}

Let’s walk through this a little, even though it’s pretty simple.

We have a method called loadAppConfig that essentially calls our config files we created earlier, then loads them into a local appConfig object that is basically a dynamic type.

We then have a property called apiBaseUrl that will return the apiBaseUrl that we just retrieved from our json file.

But there’s a problem. Before we can retrieve the apiBaseUrl, we first need to call loadAppConfig. And we only really want to do this a single time to load the file into memory, after that any further calls to load the json file are a complete waste. We could write some code in here with a boolean to check if we have loaded the config already, and if we haven’t to load it, and then continue. But that’s naff.

Luckily Angular already has a feature to help us!

Using APP_INITIALIZER To Load Configuration

In a previous post we talked about how to use APP_INITIALIZER in Angular to do a “one time” activity when the Angular app bootstraps for the first time. If you haven’t come across this feature before, I highly recommend going to read it because the following might not make much sense otherwise : Using The APP_INITIALIZER Token To Bootstrap Your Application. 

All we need to do is head to our main app.module.ts, and add a line to our providers array that looks like so :

providers: [
	{ 
	  provide : APP_INITIALIZER, 
		multi : true, 
		 deps : [AppConfigService], 
		 useFactory : (appConfigService : AppConfigService) =>  () => appConfigService.loadAppConfig()
	}
]

What this does is say, when our app is started, can you please call the following method. Our loadAppConfig method actually returns a promise anyway, but our application will wait until this promise completes before continuing on.

It may seem backward to wait until our configuration file loads to continue loading our application. Won’t the startup time be noticeable? Maybe. But we also can’t be sure what will be calling the config service and, more importantly, when. So it’s better for us to just load everything up front and be ready when things hit us.

Swapping Configurations At Release

Now comes the easy, but… different… part. By different I mean this next step will totally depend on how you release your application. The nuts and bolts of it is that when you release to say, production, you will need to rename the file app-settings.production.json to app-settings.json. That way when your service loads your configuration file, it’s the right one for that specific environment.

For me, because I’m doing releases from Azure Devops, I wrote a quick piece of powershell to simply rename the file. But you can use any sort of script, or maybe even an inbuilt step in your release pipeline can do it. Either way, renaming the file is the last step and you are ready to go with your brand new app config service in Angular!

Angular has a great CSS feature called “View Encapsulation”. It means that each component can have it’s own CSS that is encapsulated and applied to only that particular component/view. This does away with having to have great big long CSS declarations to try and narrow down the element you want to style. For example you probably have seen gnarly things like :

.main-page #header-wrapper #header-div #header-left-panel h1 span{

}

With Angular’s View Encapsulation that is no more (Well… Atleast most of the time).

But you may come across some situations where the view encapsulation gets in your way. Where you know the element you want to style, but the additions in the shadow dom are making things a headache. Luckily Angular adds in a couple of CSS Pseudo Elements that help you “break out” when you need to. These include :host, :host-context and ::ng-deep. Today we’ll do a dive on :host.

:host In A Nutshell

Imagine I create a component called “my-component” that’s pretty darn simple :

import { Component, OnInit } from '@angular/core';

@Component({
  selector: 'app-my-component',
  template : `<p>This is my component</p>`,
  styleUrls: ['./my-component.component.scss']
})
export class MyComponentComponent implements OnInit {

  constructor() { }

  ngOnInit() {
  }

}

Now if I want to style the component itself? How do I do that? How can I style the <app-my-component>  tag? A beginners take might be to try some CSS/SASS like so :

app-my-component
{
    display:block;
    background-color:blue;
}

After all, I’m using the name of my tag inside itself so hopefully it should be recognized. Wrong. Angular view encapsulation means I can only style things “inside” the component, but not the component itself. Unless I use the :host pseudo element of course :

:host
{
    display:block;
    background-color:blue;
}

Remember this goes *inside* the components style file, and not in the root etc. Using :host I am able to self style the component! It’s almost like using the “this” keyword in Angular CSS syntax.

What Goes On Under The Hood?

When your view is compiled down, your components are given unique attributes to encapsulate styles from one other. So in my example, app-my-component actually gets output looking like :

<app-my-component _ngcontent-lsw-c0="" _nghost-lsw-c1="">
	<p _ngcontent-lsw-c1="">This is my component</p>
</app-my-component>

The way it works (in a simple way) is that each component is given an _nghost-unique-id . Each element that lives inside that component is then given an _ngcontent-unique-id  . Where the unique-id on the ngcontent label matches that of the parent nghost. Pretty smart stuff!

So that goes a ways to explain why when we try our beginners attempt of just using the component tag :

app-my-component
{
    display:block;
    background-color:blue;
}

It doesn’t work because the generated CSS actually looks like :

app-my-component[_ngcontent-lsw-c1] {
  display: block;
  background-color: blue;
}

So notice that it’s saying our tag should have an _ngcontent tag with a specific id but we are actually looking for the same tag with an _nghost.

When we check how it generates and outputs the :host tag, it looks like :

[_nghost-lsw-c1] 
{ 
	display: block; 
	background-color: blue; 
}

Makes sense! It outputs the exact tag it knows our component will have, allowing us to style the component itself, not just it’s children.

If you’ve ever tried to put a placeholder attribute on a date input like so :

<input type="text" name="myDateField" placeholder="Enter the date here" />

You know it doesn’t work.

Instead what you end up with is the field being pre-populated with “dd/MM/yyyy” which is incredibly annoying! But there is actually a way to get the placeholder working by instead making the input function as a textbox until clicked, and then quickly swapping it to act like a date input.

First we need to change out input to be of type “text” like so :

<input type="text" name="myDateField" placeholder="Enter the date here" />

We are now going to create a new directive. In my case I named it DynamicDateInputDirective. The contents of which is :

import { Directive, ElementRef, HostListener } from '@angular/core';

@Directive({
  selector: 'input[dynamic-date-input]'
})
export class DynamicDateInputDirective {

  private element : ElementRef;

  constructor(element : ElementRef) { 
    this.element = element;
  }

  @HostListener('focus')
  handleFocus()
  {
    this.element.nativeElement.type = 'date';
  }

  
  @HostListener('blur')
  handleBlur()
  {
    if(this.element.nativeElement.value == "")
    {
      this.element.nativeElement.type = 'text';
    }
  }

}

What does this do?

• We use the HostListener to bind to the focus event, on focus, we ensure that the field is of type “date”.
• We then use the HostListener to bind to the blur event (So focus is moving away from the field). We check to see if the field is empty, if it is, return it to the “text” type. What this means is if someone clicks into the field but doesn’t enter anything and moves away, we keep our placeholder.

You can then add the directive to your text input :

<input type="text" name="myDateField" dynamic-date-input placeholder="Enter the date here" />

Not using Angular? The paradigm is the same if you are using React, jQuery or even Vanilla JS. On focus you need to change your text type to date, and then on blur you need to do the reverse (Remembering to only do it if the value is empty).

Every “control” in Angular has EventEmitters called statusChanges  and valueChanges . The former is whether a control is “valid” so will emit an event every time a control changes from invalid to valid or vice versa. valueChanges  is when the actual control’s “value” changes. In most cases this is going to be a form element so it will be the value of the textbox, checkbox, input etc. In a custom component’s case, if you implement ControlValueAccessor, then it’s going to be the “value” of your control as bound by the ngModel.

In anycase, I came across an interesting scenario in which I needed to “subscribe” to changes when a control was “touched”. To my surprise, there is currently no event that is emitted when a control is touched – or when it’s status of pristine/dirty is changed. It’s actually pretty bonkers because typically when showing error messages on a form in Angular, you will make liberal use of the “IsDirty” or “IsTouched” properties. So being able to “listen” for them seems pretty important. Ugh. Time to Monkey Patch.

What Is Monkey Patching?

Monkey Patching is a technique to change or modify the default behaviour of code at runtime when you don’t have access to the original code. In our case, we want to be “notified” when “markAsTouched” is called. To do so, we need to modify the actual method of “markAsTouched” to emit an event/run a custom piece of code *as well* as do it’s regular method.

If you are interested in reading more about Monkey Patching, there is a great article (In Javascript no less) on the subject here : audero.it/blog/2016/12/05/monkey-patching-javascript/

Monkey Patching MaskAsTouched/Pristine/Dirty

To make this work, the first thing you need is a reference to the *control* that you are trying to listen for the touched event on. This could be done using ViewChild if you are looking for a child control on the page. To get the “self” control (e.g. in a custom component), a common pattern is to modify your constructor to inject in the “Injector” class which you can use to get the NGControl instance of yourself. You need to do this in the ngAfterViewInit method.

constructor(private injector: Injector) { }

ngAfterViewInit()
{
  let ngControl : NgControl = this.injector.get(NgControl, null);
}

OK so we have the reference to the control. Time to patch things up!

let self = this;
let originalMethod = this.control.markAsTouched;
this.control.markAsTouched = function () {
  originalMethod.apply(this, arguments);
  //Extra code needed here. 
}

Let’s walk through this code.

• First we get a reference to ourselves. This isn’t necessarily required, but it’s highly likely we may want to refer to local variables/properties, and inside our function we lose scope of “this”.
• We store the original “markAsTouched” method from our control reference. This line will obviously differ slightly on how you are referencing the control you want to patch.
• We then set the markAsTouched method to a new function, that then itself runs the original method (e.g. Run Angular’s standard “markAsTouched”).
• We can then run our extra code as required. This may be to raise an event on an EventEmitter, set another variable, run a method, whatever!

In my case, when my custom component was touched, I wanted to then manually “touch” another child component. So my full code looked like :

let self = this;
let originalMethod = this.control.markAsTouched;
this.control.markAsTouched = function () {
  originalMethod.apply(this, arguments);
  self.bankDetailsForm.form.markAllAsTouched();
}

We can use this method to do the same for markAsPristine, markAsDirty, or actually any other method that Angular for unknown reasons doesn’t give us an event for.

Beware Infinite Loops

I would suggest when you do your first monkey patch, be liberal with your console.log statements. I found pretty quickly when trying to touch another control from within my monkey patched markAsTouched method, that I created an infinite chain of touching. It’s super easy to do so you’ll need to be extra careful.

When it comes to running “one off” startup methods when your Angular Application starts, everyone will tell you to use the “APP_INITIALIZER” token. That’s all well and good to be told that, but when you actually check the documentation, it’s a little sparse…

Admittedly there is a bit more on the token scattered around in other documentation pieces, but they are surprisingly hard to find and again, typically only lightly touch on actual real world usage. So consider this a beginners guide on how to use APP_INITIALIZER in your own app.

The Basics

The first step is to create our startup function. For the purposes of this article, I’m going to create a simple function that itself, returns a function to run.

export function startmeup()
{
  return () => console.log('Started!');
}

It’s important that the function itself returns a function!

In our main module (typically AppModule), we want to add a provider to our NGModule decorator. For example :

@NgModule({
  declarations: [
  ],
  imports: [
  ],
  providers: [
    { provide : APP_INITIALIZER, multi : true, useFactory : startmeup}
  ],
  bootstrap: [AppComponent]
})
export class AppModule { }

Ignore the fact I don’t have any declarations, I’ve removed them so it doesn’t clutter things up. Now a couple of things to note here.

• We are adding a provider with the key of APP_INITIALIZER
• We are setting the “multi” flag to true as we may have multiple APP_INITIALIZER’s to run.
• And finally we are telling it to use the startmeup method to create the method we want to run.

Running our application now, we should see a console message of “Started!” as soon as the page loads.

UseValue vs Use Factory

It may look strange for our startup method to itself return a method. It seems a bit silly to box things up like that. Why not just run the startup code itself? Well you can do that (With some caveats).

For example, we could change our startmeup function to look like so :

export function startmeup()
{
  console.log('Started!');
}

Then we can change our provider line to “useValue” instead of “useFactory”

{ provide : APP_INITIALIZER, multi : true, useValue : startmeup }

The main problem you run into with this method is that it’s much harder to use dependencies. We’ll talk about that a little later on. For now, just know you can do either but there are certainly pros and cons for each. Typically I find factories provide me the most versatility.

Inline Methods

It’s obviously worth noting that you don’t need to create an entirely new function variable/object, and you can instead do it all inline.

For the useValue route, you can use :

{ provide : APP_INITIALIZER, multi : true, useValue : () => { console.log('Started!'); }}

To do the same thing with factories does look a little messier because you need to return a method that returns a method… So the nesting can look a bit extreme but in general it looks like :

{ provide : APP_INITIALIZER, multi : true, useFactory : () => { return () => console.log('Started!'); }},

Using Dependencies

It’s possible, but unlikely, that your startup method can run on it’s own with no dependencies. But it’s probably a whole lot more likely that you are going to require either outside services of your own, or Angular’s own helper services. HttpClient for example.

You can inject these into your startup factories. Note that you cannot (easily) inject dependencies when using useValue. Your application won’t blow up, but your dependency will be undefined. There are ways around this but it’s the primary reason to stick with useFactory.

As an example of using dependencies, I can rewrite my startup factory like so :

export function startmeup(http : HttpClient)
{
  return () => 
  {
    console.log(http);
    console.log('Started!');
  }
}

And I can modify my provider line adding in the “deps” field :

{ provide : APP_INITIALIZER, multi : true, deps : [HttpClient], useFactory : startmeup}

This allows Angular’s built in DI to inject in HttpClient to my startup method which is extremely handy!

Calling Startup Methods Of Services

So, exporting functions all over the place is sort of rough. And it’s pretty likely that we want to bootstrap an actual service (Maybe to reach out and grab configuration values etc). To do that, we can create a class that holds our startmeup factory. We also need to mark it as Injectable.

@Injectable({
  providedIn: 'root'
})
export class StartupClass
{
  constructor(private http:HttpClient)
  {

  }

  startmeup()
  {
    return () => 
    {
      console.log(this.http);
      console.log('Started!');
    }
  }
}

When adding our provider line, we now instead set our dependency to be the StartupClass (The additional dependency of HttpClient will be resolved behind the scenes when requesting the StartupClass). We still call useFactory as we still have to tell it which particular method we want to run on startup.

{ provide : APP_INITIALIZER, multi : true, deps : [StartupClass], useFactory : (startupClass : StartupClass) => startupClass.startmeup()}

Note that our method is actually still returning a function itself. This can be a bit off putting to read in this way so we can change it to instead simply run it’s startup calls :

startmeup()
{
  console.log(this.http);
  console.log('Started!');
}

And then modify the provider line to instead create a factory in of itself :

{ provide : APP_INITIALIZER, multi : true, deps : [StartupClass], useFactory : (startupClass : StartupClass) => () => startupClass.startmeup()}

Notice the additional () to create a method within the useFactory line.

Using Promises

A pretty common scenario for startup methods is reaching for configuration values. That could be from a file, local storage, a cookie etc. And it’s highly possible that these methods are async in nature. As an example below, I’ve modified my startup method to read a settings file.

startmeup()
{
    this.http.get('/assets/config/app-settings.json').subscribe(async x => 
    {
      await new Promise(resolve => setTimeout(resolve, 5000));
      console.log('Configuration loaded');
    });
}

Note that the await promise line is simply to make it “seem” like it’s taking a long time to complete it’s job. e.g. It’s a slow startup task. In a real world example this wouldn’t be there.

Let’s also go to our main app component and write an NgOnInit method that simple writes to the console log.

export class AppComponent implements OnInit{
  ngOnInit(){
    console.log('On Init');
  }
}

Now let’s pause for a second and think. If we run our app right now, what will be written to the console first? The “Configuration loaded” message or the “On Init” message. Well logic would tell us that surely our message that our configuration loaded will run first because we’ve told Angular that the startmeup method should be loaded before everything else.

Well that is wrong.

The problem is that the http get method returns an observable to tell us when it’s complete. And we aren’t waiting for that to finish at all.

The general pattern is to use promises and return that promise to the APP_INITIALIZER. So as an example :

startmeup()
{
    return this.http.get('/assets/config/app-settings.json').toPromise().then(async x => 
    {
      await new Promise(resolve => setTimeout(resolve, 5000));
      console.log('Configuration loaded');
    });
}

Here we are returning a promise from our method. The App Initializer will actually wait until this promise is resolved before continuing on. If we refresh our page now, we will notice that not only does the “On Init” message come after the “Configuration loaded” message. But the page itself is blank for 5 seconds (Because of our fake sleep to emulate a long http call) because all progress is halted until startup is complete.

This may sound bogus to halt an application like this. But the general idea is that the “majority” of work from your Angular application would not actually be able to take place until the startup method is resolved. For example if you are using telemetry like New Relic, Raygun, Application Insights etc. You really should have the token loaded and App Insights running before attempting to do other work. Otherwise if your application crashes, there is now a race condition as to whether you have all the necessary information to log the error or not.

Usage Outside Of Single Page Apps

As we saw above, the use of APP_INITIALIZER essentially halts the loading of a page until complete. In a true Single Page App (SPA), this is fine because it’s only really going to happen once on the initial load. If you are making use of Angular’s router for instance, then the App Initializer won’t be called again until a proper browser refresh occurs.

Obviously this isn’t the case if you are doing full page reloads. Every page load will re-load the angular app, and therefore kick off the startup process again. Be wary of using the APP_INITIALIZER token in these situations as it can severely slow down your application on every single page load.

I get this little error almost daily when working in Angular (Particularly in VS Code).

Type 'EventEmitter' is not generic.

The first time I got the issue, it drove me mad as I was copy and pasting code from another file where it was working just fine, yet in my new code file everything was blowing up.

As it turns out, NodeJS has it’s own version of “EventEmitter” that is not generic. When using VS Code or other IDE’s that have auto completion, they will sometimes pick this EventEmitter to import rather than the one from Angular. The way to check is to see what your import statement looks like. It should be coming from @angular/core :

import { EventEmitter } from '@angular/core';

In my (broken) case, it was :

import { EventEmitter } from 'events';

In actuality, there are a couple of EventEmitter objects. For example another half broken example would be :

import { EventEmitter } from 'stream';

So just check exactly where you are importing EventEmitter from to get around this pesky error!